Bacteriological spectrum, antimicrobial susceptibility, and year-to-year variation among referred culture-positive presumed chronic suppurative otitis media specimens in a Syrian reference laboratory (2022–2025)

Abstract

Background

Chronic suppurative otitis media (CSOM) remains an important cause of chronic otorrhea and preventable hearing impairment, particularly in resource-limited settings. Because bacterial profiles and antimicrobial susceptibility patterns vary across settings, local laboratory surveillance may support culture-guided management, especially in persistent or previously treated cases.

Methods

We conducted a retrospective laboratory-based study at a private reference laboratory in Damascus, Syria. The analyzed cohort primarily represents referred laboratory specimens rather than a population-based CSOM sample. Culture-positive aerobic bacterial isolates recovered from purulent ear-discharge specimens classified as presumed CSOM were included between January 1, 2022, and December 31, 2025. Presumed CSOM was operationally defined as culture-positive purulent otorrhea associated with documented tympanic membrane perforation in the referral/laboratory context. Isolates were deduplicated across the study period by retaining only the first isolate of a given bacterial species per patient. Antimicrobial susceptibility testing was performed as part of routine laboratory workflow and interpreted according to CLSI M100, 30th edition, 2020. Susceptibility was reported as percent susceptible with corresponding denominators.

Results

A total of 638 culture-positive bacterial isolates were analyzed. The mean age was 34.75 ± 24.42 years, and 322 patients were male (50.5%). Gram-positive organisms slightly predominated over Gram-negative organisms (56.0% vs 44.0%). Methicillin-susceptible Staphylococcus aureus was the most frequent isolate (311/638, 48.7%), followed by Pseudomonas aeruginosa (233/638, 36.5%). MRSA accounted for 26 isolates (4.1%). MSSA showed high in vitro susceptibility to vancomycin, linezolid, and teicoplanin, but low susceptibility to ciprofloxacin, azithromycin, clindamycin, and trimethoprim-sulfamethoxazole. P. aeruginosa showed highest in vitro susceptibility to colistin, meropenem, amikacin, imipenem, and piperacillin-tazobactam, while ciprofloxacin susceptibility was low. Exploratory annual analyses showed year-to-year variation in susceptibility for several agents among MSSA and P. aeruginosa.

Conclusions

This Damascus reference-laboratory study provides contemporary bacteriological and susceptibility data for referred culture-positive purulent otorrhea specimens classified as presumed CSOM. The findings should be interpreted as local laboratory surveillance data rather than nationally representative CSOM epidemiology or direct treatment guidance for uncomplicated community cases. The low fluoroquinolone susceptibility observed in this referred cohort supports culture and susceptibility testing in recurrent, persistent, or previously treated otorrhea, while preserved activity of reserve systemic agents should be interpreted within antimicrobial stewardship principles.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-026-05238-7.

Introduction

Chronic suppurative otitis media (CSOM) is a chronic inflammatory and infectious disorder of the middle ear and mastoid mucosa, classically characterized by recurrent or persistent otorrhea through a perforated tympanic membrane [1]. The minimum duration used to define chronicity varies across sources from more than 2 weeks to more than 6 weeks [1, 2].

CSOM remains a major public health problem, because it is closely linked to preventable hearing loss, persistent morbidity, and potentially serious extracranial and intracranial complications, particularly in underserved populations [3–5]. WHO estimates have suggested that 65–330 million people worldwide may be affected by CSOM, with the greatest burden concentrated in low-income countries, especially in Sub-Saharan Africa, South-East Asia, and the Western Pacific [1, 3]. A systematic global review further estimated an annual CSOM incidence of approximately 31 million cases, with 22.6% occurring in children under 5 years of age, and about 21,000 deaths each year from otitis media-related complications [4]. More broadly, the 2021 Global Burden of Disease analysis showed that otitis media affected 391.3 million people worldwide, with persistent regional disparities and a disproportionate burden in low and low-middle regions [6].

The pathogenesis of chronic middle-ear infection is multifactorial and involves Eustachian tube dysfunction, bacterial and viral load, host immune response, and genetic as well as environmental determinants [2]. From a microbiologic standpoint, CSOM is predominantly bacterial, and the organisms most frequently implicated across the literature include Pseudomonas aeruginosa, Staphylococcus aureus, Proteus spp., Klebsiella spp., and Escherichia coli [7, 8]. However, the relative predominance of these pathogens varies markedly across settings; some studies have reported staphylococcal predominance as in Iran and Shanghai, whereas others identified P. aeruginosa as the leading isolate, including reports from Somalia, Iraq, and Western Rajasthan [5, 7–10]. This heterogeneity becomes even more complex in some low-resource environments, where CSOM may be highly polymicrobial and include substantial contributions from Enterobacteriaceae and other Gram-negative organisms [1]. The therapeutic challenge has intensified because empirical antibiotic use has been accompanied by rising antimicrobial resistance, including fluoroquinolone resistance, methicillin-resistant S. aureus (MRSA), and extended-spectrum beta-lactamase-producing Gram-negative pathogens [5, 11, 12]. Accordingly, repeated local surveillance of bacteriological profiles and susceptibility patterns has been consistently emphasized as essential for effective treatment selection and antimicrobial stewardship in CSOM [2, 13]. Yet, because pathogen distribution and resistance patterns vary considerably across regions, contemporary local data remain essential to support antimicrobial stewardship and culture-guided management, particularly in persistent, recurrent, or previously treated cases.

In Syria, the available literature on CSOM and otologic infections remains limited. Previous Syrian work has described the clinical picture and hearing impairment associated with CSOM in Damascus Hospital, but did not address bacterial distribution or antimicrobial susceptibility patterns [14]. Older local studies have also reported bacterial isolates and resistance patterns from ear infections or mixed clinical specimens [15]. However, these data were not designed as cumulative antibiogram analyses and do not capture recent post-2020 antimicrobial susceptibility trends. Therefore, there is a need for updated local microbiological data, particularly for referred culture-positive cases in which prior empirical treatment and recurrent disease may enrich for resistant organisms.

Against this background, the present study aimed to characterize the bacterial spectrum, antimicrobial susceptibility profile, and year-to-year susceptibility patterns among culture-positive presumed CSOM specimens referred to a Damascus reference laboratory between 2022 and 2025.

Materials and methods

Study design and setting

This retrospective laboratory-based study was conducted at Alkhatib Medical Laboratory, a private reference laboratory located in Damascus, Syria. The analysis included eligible culture-positive ear-discharge specimens classified as presumed CSOM and processed at the laboratory between January 1, 2022, and December 31, 2025. During routine practice, patients are referred to the laboratory from otolaryngology clinics, healthcare facilities, and external laboratories for bacterial culture and antimicrobial susceptibility testing. Accordingly, the dataset reflects the referral stream of patients at this Damascus reference laboratory during the study period.

Data source

The study relied exclusively on de-identified laboratory records retrieved from the laboratory information system. Available data included patient age and sex, year of specimen processing, specimen type/source, bacterial culture results, organism identification, and antimicrobial susceptibility testing results. No direct patient contact, medical chart review, clinical examination, imaging review, or follow-up outcome assessment was performed.

Ethical statement

The study protocol was approved by the Ethics Committee of the Syrian Private University (SPU) (Approval No. MD-24112025–110).

Study population and eligibility criteria

The study included patients of all ages with culture-positive aerobic bacterial isolates recovered from ear-discharge specimens processed at the laboratory between January 1, 2022, and December 31, 2025. Eligible records were restricted to patients with purulent ear discharge through a documented perforated tympanic membrane, as indicated in the referral/laboratory request, and submitted for bacterial culture and antimicrobial susceptibility testing. Patients without documented tympanic membrane perforation were excluded, as were records in which the available referral/laboratory information indicated isolated otitis externa or a non-ear source of infection. Duplicate isolates were excluded according to the deduplication rule described below. Fungal-only and anaerobic-only results were not included because the present analysis focused on aerobic bacterial isolates recovered through the routine laboratory workflow.

Operational case definition

Because the laboratory information system did not include complete clinical records or physician-adjudicated final diagnoses, cases were classified as presumed CSOM using an operational laboratory-referral definition. The included cohort was restricted to culture-positive purulent otorrhea associated with documented tympanic membrane perforation. The term “presumed CSOM” was used throughout the manuscript to reflect this laboratory-based classification rather than a fully validated clinical diagnosis supported by complete clinical records.

Deduplication rule

Isolates were deduplicated across the entire 2022–2025 study period. For each patient, only the first isolate of a given bacterial species recovered during the study period was included in the analysis. Therefore, the same patient could contribute more than one isolate only if different bacterial species were recovered, but repeated isolates of the same species from the same patient were excluded regardless of the year of recovery. The culture year of the retained isolate was used for annual susceptibility analyses.

Sample collection and microbiological processing

Referred patients attended Alkhatib Medical Laboratory, where ear-discharge specimens were collected directly using sterile cotton swabs under aseptic conditions. No interfacility specimen transport was involved for the included samples. All included specimens consisted of purulent ear discharge associated with documented tympanic membrane perforation. The specimens were inoculated onto blood agar, chocolate agar, and MacConkey agar following routine standard operating procedures for processing ear swabs, and the inoculated media were incubated at 37 °C for 24–48 h, after which bacterial growth was evaluated according to routine laboratory procedures. All culture and susceptibility testing procedures were performed as part of the routine laboratory workflow. Organism identification was performed using standard phenotypic and biochemical methods according to the laboratory’s standard operating procedures.

Antimicrobial susceptibility testing

Antimicrobial susceptibility testing was performed using the Kirby–Bauer disk diffusion method as part of the routine laboratory workflow. MIC-based testing was used for selected agents, including oxacillin, vancomycin, and colistin, when applicable according to the organism and routine testing panel. Susceptibility results were interpreted according to the Clinical and Laboratory Standards Institute criteria, using CLSI M100, 30th edition, 2020 [16]. To maintain comparability across the 2022–2025 study period, the same CLSI M100 2020 interpretive framework was applied consistently during data extraction and analysis. Results were categorized as susceptible (S), intermediate (I), or resistant (R). For Staphylococcus aureus isolates, methicillin susceptibility was inferred using oxacillin testing according to CLSI criteria.

The dataset included results for all antimicrobials tested for each isolate rather than being restricted to selectively reported agents. Because antimicrobial testing panels varied according to organism and routine workflow, the number of isolates tested for each antimicrobial was reported to document denominator differences. Cumulative antibiogram construction and isolate deduplication followed CLSI M39-A4 guidance [17]. Quality-control procedures were performed according to the laboratory’s standard operating procedures using standard reference strains, including Staphylococcus aureus ATCC 25923, Escherichia coli ATCC 25922, and Pseudomonas aeruginosa ATCC 27853.

Construction of susceptibility summaries

Overall antimicrobial susceptibility profiles were summarized for the main organism groups identified in the cohort. Staphylococcus aureus isolates were stratified according to methicillin susceptibility into methicillin-susceptible S. aureus (MSSA) and methicillin-resistant S. aureus (MRSA), while Gram-negative isolates were summarized primarily for Pseudomonas aeruginosa and Enterobacterales. For each organism–antimicrobial combination, susceptibility was reported as the percentage of susceptible isolates together with the corresponding numerator and denominator. Percent susceptibility was calculated as follows: (number of susceptible isolates/number of isolates tested for that antimicrobial) × 100. Because routine antimicrobial testing panels varied by organism and laboratory workflow, denominators differed across antimicrobials and were explicitly reported. Results based on small denominators, particularly MRSA and less frequent Gram-negative subgroups, were interpreted cautiously in accordance with cumulative antibiogram reporting principles.

Annual susceptibility analysis

To evaluate year-to-year variation in susceptibility, annual susceptibility analyses were performed for MSSA and Pseudomonas aeruginosa, the two most frequent pathogens in the cohort. For each antimicrobial and year, susceptibility was reported as %S with the corresponding numerator and denominator [n/N tested], allowing annual differences in testing coverage to be documented. Annual comparisons were considered exploratory and were interpreted as year-to-year variation rather than definitive evidence of validated temporal resistance trends.

Statistical analysis

Continuous variables were summarized as mean ± standard deviation (SD) and range, whereas categorical variables were presented as frequencies and percentages. Descriptive analyses were used to summarize patient characteristics, organism distribution, and antimicrobial susceptibility patterns. Percent susceptibility was calculated as the number of susceptible isolates divided by the number of isolates tested for the corresponding antimicrobial agent. Where appropriate, 95% confidence intervals (CIs) were calculated for susceptibility percentages.

Associations between categorical variables were assessed using Pearson’s chi-square test when assumptions were met, and Fisher’s exact test was used when expected cell counts were small. For annual susceptibility analyses in MSSA and Pseudomonas aeruginosa, yearly differences were evaluated using susceptible (S) versus non-susceptible categories, with intermediate and resistant results grouped as non-susceptible. Cramer’s V was reported as a measure of effect size for annual comparisons and was interpreted descriptively, with values around 0.10 considered small, around 0.30 moderate, and around 0.50 large. These effect-size interpretations were applied because several comparisons included unequal or small annual denominators.

All statistical tests were two-sided, and a p value of < 0.05 was considered statistically significant. Statistical comparisons were not performed when the variable was constant or when available data were insufficient for valid analysis. Analyses were performed using IBM SPSS Statistics version 26.0.

Results

A total of 638 culture-positive bacterial isolates recovered from ear-discharge specimens classified as presumed CSOM were included in the analysis. The study population showed an almost equal sex distribution, with 322 males (50.5%) and 316 females (49.5%), and a mean age of 34.75 ± 24.42 years (range, 0–94 years). Adults aged 26–60 years constituted the largest age category (37.9%), followed by patients older than 60 years (26.5%), children aged 5–14 years (14.7%), young adults aged 15–25 years (11.9%), and children younger than 5 years (8.9%). Most isolates were recovered in 2024 (35.9%), followed by 2025 (24.6%), 2023 (24.5%), and 2022 (15.0%). Overall, Gram-positive bacteria slightly predominated over Gram-negative bacteria (56.0% vs 44.0%) (Table 1).

Table 1.

Characteristics of culture-positive bacterial chronic suppurative otitis media isolates (N = 638)

Characteristic	Total (N = 638)
Patient characteristics
Male, n (%)	322 (50.5%)
Female, n (%)	316 (49.5%)
Age, Mean ± SD (range)	34.75 ± 24.42 (0–94)
< 5, n (%)	57 (8.9%)
5–14, n (%)	94 (14.7%)
15–25, n (%)	76 (11.9%)
26–60, n (%)	242 (37.9%)
> 60, n (%)	169 (26.5%)
Year of Culture
2022, n (%)	96 (15.0%)
2023, n (%)	156 (24.5%)
2024, n (%)	229 (35.9%)
2025, n (%)	157 (24.6%)
Organism group
Gram-positive bacteria, n (%)	357 (56)
Gram-negative bacteria, n (%)	281 (44)
Organisms identified
MSSA, n (%)	311 (48.7%)
Escherichia coli*, n (%)	19 (3.0%)
MRSA*, n (%)	26 (4.1%)
Klebsiella pneumoniae*, n (%)	20 (3.1%)
Pseudomonas aeruginosa, n (%)	233 (36.5%)
Other organisms*, n (%)	29 (4.6%)

Values are n (%) unless otherwise specified. Percentages are calculated out of the total number of unique isolates

Abbreviations: MSSA Methicillin-susceptible Staphylococcus aureus, MRSA Methicillin-resistant Staphylococcus aureus

^*Indicates fewer than 30 isolates for that organism category

^*Other organisms includes: Bacillus cereus (N = 1), Streptococcus pyogenes (N = 2), Staphylococcus epidermidis (N = 16), Enterococcus (N = 1), Enterobacter (N = 4), Proteus vulgaris (N = 5)

Regarding organism distribution, methicillin-susceptible Staphylococcus aureus (MSSA) was the most frequent isolate, accounting for 311 isolates (48.7%), followed by Pseudomonas aeruginosa with 233 isolates (36.5%). Less common organisms included methicillin-resistant S. aureus (MRSA; 26 isolates, 4.1%), Klebsiella pneumoniae (20 isolates, 3.1%), Escherichia coli (19 isolates, 3.0%), and a heterogeneous group of other organisms (29 isolates, 4.6%) (Table 1). In the age-stratified distribution, S. aureus remained the leading organism across all age groups, accounting for 56.1%, 44.7%, 46.1%, 53.7%, and 58.0% of isolates in the < 5, 5–14, 15–25, 26–60, and > 60 years groups, respectively. In contrast, P. aeruginosa reached its highest relative proportion in the 5–14-year group (47.9%) and remained a substantial contributor in the 15–25-year (40.8%) and 26–60-year (38.0%) groups. Enterobacterales represented a smaller proportion across all age strata, ranging from 5.8% to 10.5%, while other organisms consistently accounted for only a minor fraction of isolates (Fig. 1).

Fig. 1.

Age-stratified distribution of major bacterial groups. Stacked bar chart showing the proportional distribution of major bacterial groups across age categories. Numbers above the bars indicate the total number of isolates in each age group

Among S. aureus isolates, MSSA showed high in vitro susceptibility to vancomycin (100.0%), linezolid (98.0%), and teicoplanin (95.8%). By contrast, low susceptibility was observed for ciprofloxacin (22.8%), azithromycin (21.2%), clindamycin (35.1%), and trimethoprim-sulfamethoxazole (9.5%). MRSA accounted for 26 isolates only; therefore, MRSA susceptibility estimates were considered descriptive. Within this limited subgroup, susceptibility was observed mainly for vancomycin (87.0%), minocycline (83.3%), tigecycline (79.2%), and linezolid (61.5%), whereas no susceptible isolates were observed among the small numbers tested for ampicillin/sulbactam, azithromycin, clindamycin, or ciprofloxacin (Table 2).

Table 2.

Antimicrobial susceptibility profile of Staphylococcus aureus isolates with 95% confidence intervals

Antimicrobial agent	S. aureus overall (N = 337)	MSSA (N = 311)	MRSA (N = 26)
Beta-lactam/beta-lactamase inhibitor combinations
Ampicillin/sulbactam	18.3% (51/279; 95% CI, 14.2–23.2)	18.8% (51/271; 95% CI, 14.6–23.9)	0.0% (0/8; 95% CI, 0.0–32.4)
Piperacillin/tazobactam	72.8% (244/335; 95% CI, 67.8–77.3)	77.0% (238/309; 95% CI, 72.0–81.4)	23.1% (6/26; 95% CI, 11.0–42.1)
Cephalosporins
Cefixime	0.3% (1/288; 95% CI, 0.1–1.9)	0.4% (1/279; 95% CI, 0.1–2.0)	0.0% (0/9; 95% CI, 0.0–29.9)
Cefotaxime	8.5% (24/282; 95% CI, 5.8–12.4)	8.8% (24/274; 95% CI, 6.0–12.7)	0.0% (0/8; 95% CI, 0.0–32.4)
Ceftriaxone	5.1% (17/332; 95% CI, 3.2–8.0)	5.5% (17/307; 95% CI, 3.5–8.7)	0.0% (0/25; 95% CI, 0.0–13.3)
Cefuroxime	58.4% (174/298; 95% CI, 52.7–63.8)	60.6% (174/287; 95% CI, 54.9–66.1)	0.0% (0/11; 95% CI, 0.0–25.9)
Carbapenems
Imipenem	99.6% (281/282; 95% CI, 98.0–99.9)	99.6% (273/274; 95% CI, 98.0–99.9)	100.0% (8/8; 95% CI, 67.6–100.0)
Meropenem	90.3% (289/320; 95% CI, 86.6–93.1)	94.2% (278/295; 95% CI, 91.0–96.4)	44.0% (11/25; 95% CI, 26.7–62.9)
Macrolides
Azithromycin	19.7% (63/319; 95% CI, 15.8–24.5)	21.2% (63/297; 95% CI, 16.9–26.2)	0.0% (0/22; 95% CI, 0.0–14.9)
Lincosamides
Clindamycin	31.8% (27/85; 95% CI, 22.8–42.3)	35.1% (27/77; 95% CI, 25.3–46.2)	0.0% (0/8; 95% CI, 0.0–32.4)
Aminoglycosides
Amikacin	99.6% (281/282; 95% CI, 98.0–99.9)	99.6% (273/274; 95% CI, 98.0–99.9)	100.0% (8/8; 95% CI, 67.6–100.0)
Gentamicin	49.1% (140/285; 95% CI, 43.4–54.9)	49.8% (137/275; 95% CI, 44.0–55.7)	30.0% (3/10; 95% CI, 10.8–60.3)
Tobramycin	50.0% (145/290; 95% CI, 44.3–55.7)	51.1% (143/280; 95% CI, 45.2–56.9)	20.0% (2/10; 95% CI, 5.7–51.0)
Fluoroquinolones
Ciprofloxacin	22.0% (62/282; 95% CI, 17.5–27.2)	22.8% (62/272; 95% CI, 18.2–28.1)	0.0% (0/10; 95% CI, 0.0–27.8)
Levofloxacin	51.4% (147/286; 95% CI, 45.6–57.1)	52.9% (146/276; 95% CI, 47.0–58.7)	10.0% (1/10; 95% CI, 1.8–40.4)
Glycopeptides
Vancomycin	99.1% (328/331; 95% CI, 97.4–99.7)	100.0% (308/308; 95% CI, 98.8–100.0)	87.0% (20/23; 95% CI, 67.9–95.5)
Teicoplanin	92.5% (309/334; 95% CI, 89.2–94.9)	95.8% (295/308; 95% CI, 92.9–97.5)	53.8% (14/26; 95% CI, 35.5–71.2)
Oxazolidinones
Linezolid	95.1% (313/329; 95% CI, 92.2–97.0)	98.0% (297/303; 95% CI, 95.7–99.1)	61.5% (16/26; 95% CI, 42.5–77.6)
Tetracyclines
Minocycline	55.6% (179/322; 95% CI, 50.1–60.9)	53.4% (159/298; 95% CI, 47.7–58.9)	83.3% (20/24; 95% CI, 64.1–93.3)
Glycylcyclines
Tigecycline	81.8% (99/121; 95% CI, 74.0–87.7)	82.5% (80/97; 95% CI, 73.7–88.8)	79.2% (19/24; 95% CI, 59.5–90.8)
Folate pathway inhibitors
Trimethoprim/sulfamethoxazole	9.9% (28/283; 95% CI, 6.9–13.9)	9.5% (26/275; 95% CI, 6.5–13.5)	25.0% (2/8; 95% CI, 7.1–59.1)

Values are presented as % susceptible (%S) (n/N; 95% CI) for S. aureus overall, MSSA, and MRSA. Confidence intervals were calculated using the Wilson score method

MRSA susceptibility estimates are descriptive and should be interpreted cautiously because of the small number of MRSA isolates and variable tested denominators

Denominators vary across antimicrobial agents and organism groups, particularly for MRSA, because not all isolates were tested against every antimicrobial agent

Abbreviations: CI Confidence interval, MSSA Methicillin-susceptible Staphylococcus aureus, MRSA Methicillin-resistant Staphylococcus aureus. Values are shown as the proportion susceptible (%S), with n indicating the number of susceptible isolates and N the number tested. Intermediate and resistant categories are not displayed in this summary table

Among Gram-negative isolates, P. aeruginosa showed the highest in vitro susceptibility among tested agents to colistin (96.5%), meropenem (91.8%), amikacin (90.9%), imipenem (83.5%), and piperacillin-tazobactam (75.9%). Lower susceptibility was observed for ciprofloxacin (32.5%), levofloxacin (45.9%), gentamicin (31.3%), tobramycin (32.3%), cefotaxime (5.2%), and ceftriaxone (5.6%). Enterobacterales showed highest susceptibility to amikacin (87.5%), meropenem (82.9%), imipenem (82.6%), and colistin (82.1%), while susceptibility was limited for ampicillin-sulbactam (14.8%), cefotaxime (22.9%), ceftriaxone (20.8%), and trimethoprim-sulfamethoxazole (12.5%) (Table 3).

Table 3.

Antimicrobial susceptibility profile of Gram-negative isolates from ear-discharge specimens classified as presumed CSOM

Antimicrobial agent	P. aeruginosa %S (n/N; 95% CI)	Enterobacterales %S (n/N; 95% CI)
Aminoglycosides
Amikacin	90.9% (211/232; 95% CI, 86.6–94.0)	87.5% (42/48; 95% CI, 75.3–94.1)
Gentamicin	31.3% (72/230; 95% CI, 25.7–37.6)	60.4% (29/48; 95% CI, 46.3–73.0)
Tobramycin	32.3% (65/201; 95% CI, 26.3–39.1)	37.9% (11/29; 95% CI, 22.7–56.0)
Penicillins/beta-lactamase inhibitor combinations
Ampicillin-sulbactam	0.5% (1/198; 95% CI, 0.1–2.8)	14.8% (4/27; 95% CI, 5.9–32.5)
Piperacillin-tazobactam	75.9% (173/228; 95% CI, 69.9–81.0)	54.5% (24/44; 95% CI, 40.1–68.3)
Cephalosporins
Cefotaxime	5.2% (12/231; 95% CI, 3.0–8.9)	22.9% (11/48; 95% CI, 13.3–36.5)
Ceftriaxone	5.6% (13/232; 95% CI, 3.3–9.3)	20.8% (10/48; 95% CI, 11.7–34.3)
Fluoroquinolones
Ciprofloxacin	32.5% (75/231; 95% CI, 26.8–38.8)	45.8% (22/48; 95% CI, 32.6–59.7)
Levofloxacin	45.9% (105/229; 95% CI, 39.5–52.3)	51.1% (24/47; 95% CI, 37.2–64.7)
Carbapenems
Imipenem	83.5% (193/231; 95% CI, 78.2–87.8)	82.6% (38/46; 95% CI, 69.3–90.9)
Meropenem	91.8% (212/231; 95% CI, 87.5–94.7)	82.9% (34/41; 95% CI, 68.7–91.5)
Polymyxins
Colistin	96.5% (223/231; 95% CI, 93.3–98.2)	82.1% (32/39; 95% CI, 67.3–91.0)
Tetracyclines/glycylcyclines
Tigecycline	4.7% (5/106; 95% CI, 2.0–10.6)	52.9% (9/17; 95% CI, 31.0–73.8)
Folate pathway inhibitors
Trimethoprim-sulfamethoxazole	0.0% (0/231; 95% CI, 0.0–1.6)	12.5% (6/48; 95% CI, 5.9–24.7)

Values are shown as %S (n/N; 95% CI), where n represents the number of susceptible isolates and N represents the total number of isolates tested against the corresponding antimicrobial agent in each group. 95% confidence intervals were calculated using the Wilson score method

Denominators vary across antimicrobial agents and organism groups, particularly for Enterobacterales, because not all isolates were tested against every antimicrobial agent

Abbreviations: CSOM Chronic suppurative otitis media, CI Confidence interval, %S percentage susceptible

Annual susceptibility analysis showed statistically significant year-to-year variation for several tested agents among MSSA and P. aeruginosa isolates. Among MSSA isolates, significant annual differences were observed for azithromycin, cefuroxime, gentamicin, teicoplanin, tobramycin, trimethoprim-sulfamethoxazole, minocycline, levofloxacin, and linezolid. Teicoplanin susceptibility was lower in 2025 than in 2022–2024, whereas minocycline susceptibility was higher in later study years (Supplementary Table 1, Fig. 2). Among P. aeruginosa isolates, the most pronounced 2025 decreases were observed for piperacillin-tazobactam, which declined to 36.7% (18/49) from 84.7% (83/98) in 2024, amikacin, which declined to 63.5% (33/52) from 99.0% (97/98), meropenem, which declined to 64.7% (33/51) from 99.0% (97/98), and colistin, which declined to 86.5% (45/52) from 99.0% (96/97). Ciprofloxacin and ceftriaxone remained low across years without statistically significant annual variation (Supplementary Table 2, Fig. 3). Because these analyses were exploratory and annual denominators differed across antimicrobial agents, these findings were interpreted as year-to-year variation rather than validated temporal resistance trends.

Fig. 2.

Year-to-year variation in antimicrobial susceptibility among MSSA isolates. Line chart showing annual susceptibility variation for six selected clinically informative antimicrobial agents among MSSA isolates. Asterisks indicate statistically significant overall annual variation in susceptibility categories (S vs non-S, p < 0.05). Antimicrobials without asterisks did not show statistically significant annual variation or had constant values. Annual denominators varied by antimicrobial agent and are provided in Supplementary Table 1

Fig. 3.

Year-to-year variation in antimicrobial susceptibility among Pseudomonas aeruginosa isolates. Line chart showing annual susceptibility variation for six selected clinically informative antimicrobial agents among P. aeruginosa isolates. Asterisks indicate statistically significant overall annual variation in susceptibility categories (S vs non-S, p < 0.05). Antimicrobials without asterisks did not show statistically significant annual variation. Annual denominators varied by antimicrobial agent and are provided in Supplementary Table 2

Discussion

In this retrospective reference-laboratory cohort of culture-positive ear-discharge specimens classified as presumed CSOM, the bacteriological spectrum was dominated by MSSA and Pseudomonas aeruginosa, with Gram-positive organisms slightly outnumbering Gram-negative organisms overall. The most important interpretive point is that these findings should be understood within the study setting: referred, culture-positive purulent otorrhea specimens obtained in the context of documented tympanic membrane perforation, rather than a population-based estimate of CSOM microbiology in Syria. Within that scope, the study provides contemporary local susceptibility data from 2022 to 2025, a period for which Syrian CSOM-specific bacteriology and antibiogram data have been scarce.

The predominance of MSSA in our cohort is notable because many CSOM series report P. aeruginosa as the leading isolate. In Yemen, P. aeruginosa was the most frequent isolate in active CSOM, followed by S. aureus and Klebsiella spp [18] Similarly, in Somalia, Gram-negative organisms predominated, and Pseudomonas spp. were the leading isolates among monomicrobial CSOM cultures [8]. Studies from Western Rajasthan and Iraq also found P. aeruginosa to be the leading organism, accounting for 55.8% and 57.5% of isolates, respectively [9, 10]. A 2025 Pakistani study likewise identified Pseudomonas as the most common CSOM isolate, followed by S. aureus [19]. These findings contrast with our MSSA-predominant pattern, but they also highlight the strong geographic and setting-specific variability of CSOM microbiology.

At the same time, a staphylococcal-predominant profile is not without precedent. In Iran, staphylococci were the most prevalent isolates in clinically diagnosed CSOM, accounting for 64.9% of bacteria, whereas P. aeruginosa represented 10.3% [7]. A tertiary-care study from Shanghai similarly found S. aureus to be the most common organism, followed by P. aeruginosa, with MRSA representing 18.9% of S. aureus isolates [5]. A Bangladeshi tertiary-care study also found S. aureus to be the leading isolate, followed by P. aeruginosa [13]. These comparisons suggest that the high MSSA proportion in our cohort is best interpreted as a local reference-laboratory pattern rather than as an implausible deviation from the CSOM literature. Differences in case definition, sampling technique, clinical severity, prior treatment exposure, referral pathway, inclusion or exclusion of cholesteatoma, and local antimicrobial pressure may all influence the relative proportions of S. aureus and P. aeruginosa [1, 5, 10, 20]. In addition, routine swab-based culture of otorrhea may not fully represent the microbiology of the middle-ear cleft, as culture-based methods can underestimate pathogen detection compared with PCR-based approaches; this is particularly relevant in biofilm-associated CSOM, where P. aeruginosa may persist within polymicrobial communities [1, 9, 21].

The age-stratified distribution in our study adds further nuance. S. aureus remained the leading organism across all age strata, whereas P. aeruginosa reached its highest relative proportion in children aged 5–14 years and remained frequent in adolescents and adults. Recent pediatric CSOM data also support the central role of aerobic pathogens, particularly P. aeruginosa and S. aureus [22]. The same study also found a significant association between biofilm formation and antibiotic resistance, suggesting that resistant phenotypes in pediatric CSOM may reflect not only antibiotic exposure but also biofilm-associated persistence. However, our study was not designed as a pediatric-only analysis, and age-stratified susceptibility testing was limited by denominator size for many organism–antibiotic combinations. Pediatric and adult CSOM cohorts may differ in disease duration, recurrence pattern, prior treatment exposure, and biofilm-associated resistance [22].

The antimicrobial susceptibility results have several clinically relevant implications, but they must be interpreted as in vitro data rather than direct treatment recommendations. Among MSSA isolates, preserved susceptibility to vancomycin, linezolid, and teicoplanin indicates retained in vitro activity of anti-staphylococcal reserve agents, whereas susceptibility to ciprofloxacin, azithromycin, clindamycin, and trimethoprim-sulfamethoxazole was low. In the CSOM literature, S. aureus susceptibility varies widely across settings; Shanghai reported full susceptibility of S. aureus to vancomycin and high susceptibility to gentamicin and rifampicin, but substantial resistance to penicillin and erythromycin [5]. The Korean multicenter study highlighted the emergence and persistence of MRSA in CSOM and emphasized that antibiotic sensitivity changes over time, requiring continued surveillance [23]. In Western Rajasthan, one-third of S. aureus isolates were methicillin-resistant, whereas our MRSA subgroup was smaller with only 26 MRSA isolates [10]. This small MRSA subgroup showed two findings that merit specific comment: apparent meropenem susceptibility of 44.0% and linezolid susceptibility of 61.5%. The meropenem finding is difficult to translate into clinical relevance because methicillin-resistant staphylococci are considered resistant to most β-lactam agents, including carbapenems, according to CLSI interpretive principles [16]. Therefore, meropenem should not be inferred as a reliable anti-MRSA option from this in vitro subgroup result. The linezolid finding also differs from published otitis-media MRSA data; in a multicenter Korean study of MRSA infections across OME, AOM, COM, and CCOM, all tested MRSA isolates remained susceptible to linezolid, teicoplanin, and vancomycin [24]. Linezolid resistance in S. aureus is biologically plausible but remains uncommon and is usually associated with defined mechanisms, including 23S rRNA mutations, ribosomal protein mutations, cfr-mediated methylation, optrA/poxtA-mediated ribosomal protection, or LmrS efflux [25]. In the absence of molecular confirmation or repeat susceptibility testing, these MRSA-specific meropenem and linezolid findings should be regarded as subgroup observations rather than a stable MRSA treatment profile.

Topical quinolones, usually combined with aural toilet, are commonly used in the conservative management of active CSOM because they provide high local antimicrobial concentrations and avoid the ototoxicity concerns associated with some non-quinolone topical agents [3, 26]. This therapeutic framework is supported by several microbiological studies in which P. aeruginosa retained relatively high fluoroquinolone susceptibility. In Yemen, ciprofloxacin showed preserved activity against P. aeruginosa isolates from active CSOM, and in Iraq, ciprofloxacin was also reported among the effective agents against P. aeruginosa [9, 18]. Similarly, the Angolan CSOM study concluded that topical quinolones would be a preferred antibiotic option based on local susceptibility findings, although the authors also discussed topical antiseptics as potentially feasible alternatives in the context of emerging resistance [1]. These findings contrast with our Damascus reference-laboratory cohort, in which P. aeruginosa showed low in vitro susceptibility to ciprofloxacin. However, reduced fluoroquinolone activity has also been reported in other contemporary CSOM and aural-infection studies. In Uttarakhand, P. aeruginosa was least sensitive to levofloxacin and ciprofloxacin, and S. aureus also showed high resistance to these agents [20]. In Shanghai CSOM isolates, P. aeruginosa showed substantial resistance to levofloxacin and ciprofloxacin, leading the authors to caution against routine empirical reliance on fluoroquinolones and cephalosporins [7]. Molecular data from Shanghai aural P. aeruginosa isolates provide a plausible biological explanation for this pattern, showing high ciprofloxacin and levofloxacin resistance associated with QRDR mutations in gyrA and parC, together with strong biofilm-forming capacity [27]. Taken together, these comparisons suggest that the low fluoroquinolone susceptibility observed in our cohort may reflect local antimicrobial pressure, referral of persistent or previously treated cases, and possible geographic clustering within the laboratory referral stream. Therefore, this finding should not be generalized to all first-line community CSOM cases in Syria, but it supports culture-guided therapy for recurrent, persistent, or previously treated otorrhea.

The therapeutic implications of our findings should be interpreted within a CSOM management framework in which aural toilet and topical therapy remain central, whereas systemic therapy is generally reserved for selected, refractory, or complicated situations. WHO guidance and CSOM management scenarios emphasize the importance of aural toilet and topical approaches, while parenteral antipseudomonal therapy, although effective in selected studies, is expensive and not a routine first-line strategy [3]. Cochrane evidence on topical antibiotics indicates that topical therapy is commonly used in CSOM, but the certainty of evidence remains low or very low for several comparisons between different antimicrobial agents [26]. Evidence comparing topical versus systemic antibiotics is also limited and should not be used to justify routine systemic therapy solely on the basis of in vitro susceptibility [28]. The Iraqi CSOM study similarly noted that treatment commonly involves aural cleansing and topical antibiotic drops, whereas systemic antibiotics are generally reserved for selected situations such as mixed infection or failure of initial therapy [9]. Therefore, the preserved in vitro activity of reserve systemic agents in our cohort, including carbapenems, glycopeptides, linezolid, and colistin, should be viewed as microbiological information for severe, refractory, or complicated cases rather than as support for routine use in uncomplicated CSOM.

The Gram-negative findings also raise concern about beta-lactam resistance mechanisms, although our study did not perform molecular confirmation. Enterobacterales represented a smaller proportion of isolates in our cohort, but susceptibility to third-generation cephalosporins and ampicillin-sulbactam was limited. A molecular CSOM study reported Extended-Spectrum β-Lactamase (ESBL)-producing Gram-negative pathogens and identified clinically relevant resistance genes including blaSHV, blaTEM, and blaCTX-M [11]. A Tanzanian ear-infection study also reported ESBL-producing Enterobacterales and MRSA among ear isolates, although that study was not limited to CSOM [12]. These data suggest that low cephalosporin susceptibility in our Gram-negative isolates may be compatible with beta-lactamase-mediated resistance, but this remains inferential because phenotypic ESBL confirmation and molecular resistance testing were not included in the present study [12].

The temporal susceptibility analysis should be interpreted as exploratory year-to-year variation rather than validated resistance-trend modeling. In our dataset, statistically significant annual variation was observed for several agents among MSSA and P. aeruginosa, including lower teicoplanin susceptibility among MSSA in 2025 and lower 2025 susceptibility of P. aeruginosa to several agents. Periodic change in CSOM bacteriology and susceptibility has been described previously; the Korean multicenter study concluded that bacterial predominance and antibiotic sensitivity changed over time and that continuous surveillance was necessary [23]. Nevertheless, our analysis included only four calendar years, annual denominators differed by antibiotic, and some organism–antibiotic–year combinations had sparse counts. Therefore, the annual results should be considered surveillance signals that justify continued cumulative antibiogram monitoring rather than proof of stable temporal resistance trends.

The biofilm-associated and polymicrobial nature of CSOM provides an additional framework for interpreting the discrepancy between in vitro susceptibility and clinical persistence. CSOM is increasingly understood as a disorder involving disrupted host–microbial interactions, dysbiosis, immune dysregulation, polymicrobial communities, and biofilm-associated persistence rather than a simple single-pathogen infection [21]. In Angola, CSOM ear-discharge cultures were highly polymicrobial, and nasopharyngeal isolates did not correspond well with ear-discharge isolates, underscoring the complexity of the microbial ecology and the limitations of relying on one culture source to represent the entire disease process [1]. A 2025 CSOM-specific study found P. aeruginosa to be the predominant organism and explored acetic acid as an in vitro antibiofilm strategy, reinforcing the concept that biofilm-directed approaches may be relevant in chronic otorrhea [29]. Molecular and biofilm data from aural P. aeruginosa isolates showed that nearly all isolates formed biofilms and that biofilm-related genes were highly prevalent [27]. Pediatric CSOM data also showed a significant association between biofilm formation and antibiotic resistance [22]. Therefore, routine AST should be viewed as essential microbiological guidance, but not as a complete predictor of clinical response in chronic or recurrent disease.

Low-frequency organisms such as Staphylococcus epidermidis are best interpreted within the broader microbiological and sampling context. Some CSOM studies report coagulase-negative staphylococci and S. epidermidis among recovered organisms, whereas others emphasize careful sampling to minimize external auditory canal contamination [5, 10]. In highly polymicrobial CSOM cohorts, recovered organisms may also reflect a complex microbial community rather than a single causative pathogen [1]. For this reason, low-frequency organisms in our dataset were grouped as “other organisms” and were not interpreted as major pathogens in the absence of quantitative culture, repeat isolation, microscopy correlation, or detailed clinical correlation.

The main strength of this study is that it provides contemporary, organism-specific susceptibility data from a large referred culture-positive cohort over a four-year period in Damascus. The use of denominator-specific susceptibility reporting, deduplication across the study period, and confidence intervals improves transparency and reduces overinterpretation of low-count susceptibility estimates. Another strength is that the cohort reflects a clinically relevant referral-laboratory population in which cultures are often requested for persistent, recurrent, or previously treated otorrhea, which is precisely the group in which empirical treatment failure and resistance are most concerning.

Limitations

Several limitations should be considered when interpreting this study. This was a retrospective laboratory-based study from a single private reference laboratory in Damascus; therefore, the cohort reflects the laboratory referral stream rather than a population-based or nationally representative sample of CSOM in Syria. Because cultures are more likely to be requested for persistent, recurrent, previously treated, or clinically refractory otorrhea, the observed resistance rates may overestimate those expected in unselected community CSOM. Geographic and referral clustering may also have occurred, with overrepresentation of patients from areas, clinics, providers, or external laboratories linked to this reference laboratory and underrepresentation of other Syrian regions. Cases were classified using an operational definition of presumed CSOM based on purulent ear discharge associated with documented tympanic membrane perforation and referral context; however, full clinical records and physician-adjudicated final diagnoses were unavailable, so misclassification with other causes of otorrhea, such as acute otitis media with perforation or post-surgical otorrhea, cannot be completely excluded. The dataset also included only culture-positive specimens, meaning that culture-negative cases were not captured and the findings do not represent the full clinical spectrum of CSOM. Important patient-level clinical variables were unavailable, including symptom duration, laterality, safe versus unsafe CSOM, cholesteatoma status, prior otologic surgery, comorbidities, disease severity, complications, treatment history, treatment response, and clinical outcomes, limiting clinical correlation between susceptibility patterns and disease phenotype, previous antimicrobial exposure, recurrence, or treatment response. Individual-level data on prior topical or systemic antibiotic exposure were unavailable; therefore, any interpretation linking the observed resistance patterns to previous antimicrobial use remains speculative. Although specimens were collected directly at the laboratory, the study relied on routine swab culture rather than quantitative culture, repeat cultures, or microscopy-correlated pathogen assessment; therefore, low-frequency organisms such as coagulase-negative staphylococci could not be definitively distinguished as pathogens, colonizers, or contaminants and were not interpreted as major causative organisms. The routine workflow focused on aerobic bacterial culture; anaerobic cultures, fungal cultures, and molecular microbiome profiling were not systematically performed, limiting assessment of the full polymicrobial, anaerobic, fungal, or dysbiotic ecology of CSOM. Antimicrobial testing was performed as part of routine clinical laboratory practice, and testing panels varied by organism and workflow; although denominators were reported for each organism–antimicrobial combination, unequal testing coverage may have affected the precision and comparability of susceptibility estimates. Subgroup analyses, particularly for MRSA, Enterobacterales, and less common organisms, were based on small denominators and should be interpreted descriptively. Finally, annual susceptibility analysis covered only four years, with variable denominators and sparse counts for some organism–antimicrobial–year combinations; therefore, year-to-year differences should be regarded as exploratory susceptibility variation rather than validated temporal resistance trends. Molecular resistance testing, clonal typing, ESBL confirmation, carbapenemase testing, quinolone-resistance determinant analysis, and biofilm assays were not performed, so resistance phenotypes could not be linked to specific mechanisms such as ESBL genes, QRDR mutations, carbapenemase production, clonal expansion, or biofilm-forming capacity.

Conclusion

This reference-laboratory study provides contemporary bacteriological and antimicrobial susceptibility data for culture-positive purulent ear-discharge specimens classified as presumed CSOM and processed in Damascus between 2022 and 2025. MSSA and Pseudomonas aeruginosa were the predominant organisms, and the susceptibility patterns showed limited in vitro activity of several commonly used agents, particularly fluoroquinolones, within this referred cohort. These findings should be interpreted as local laboratory surveillance data rather than as nationally representative CSOM epidemiology or direct treatment guidelines for uncomplicated community cases. The results support the value of culture and susceptibility testing in recurrent, persistent, or previously treated otorrhea, while reinforcing the need to interpret reserve-agent activity within antimicrobial stewardship principles and the clinical framework of CSOM management. Future multicenter Syrian studies integrating standardized clinical definitions, treatment-exposure data, geographic source information, clinical outcomes, and molecular or biofilm analyses are needed to clarify the generalizability, mechanisms, and clinical implications of these resistance patterns.

Authors’ contributions

**AS (Alaa Senjab)** conceived and designed the study, coordinated data acquisition, performed data curation, conducted the statistical analysis, contributed to interpretation of findings, drafted the manuscript, and critically revised the final version. **HA (Hussein Aljbawi)** contributed to data collection and verification, assisted with data interpretation, and participated in manuscript drafting and revision. **MAA (Mohammad Alaa Aldakak)** assisted with study design, contributed to data validation and interpretation, and revised the manuscript for important intellectual content. **IA (Ibrahim Alkhatib)** provided clinical oversight and contextual interpretation of the microbiology results. **OA (Omran Alkhatib)** contributed to methodological refinement, and critically revised the manuscript. **AA (Adnan Alkhatib) and LN (Louei Nahas)** supervised the study, provided overall scientific oversight and methodological guidance. All authors reviewed and approved the final manuscript and agree to be accountable for all aspects of the work, including the accuracy and integrity of the data and analyses.

Funding

No funding was received for this research.

Data availability

De-identified data and analysis outputs are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

The study protocol was approved by the Ethics Committee of the Syrian Private University, Damascus (Approval No.: MD-24112025–110). The study was conducted in accordance with the ethical principles of the Declaration of Helsinki, and all methods/study procedures were carried out in accordance with relevant guidelines and regulations. The requirement for informed consent to participate was waived by the Ethics Committee of the Syrian Private University because of the retrospective design and the use of de-identified routine laboratory data.

Consent for publication

Not applicable. The manuscript contains no identifiable personal data from any participant.

Competing interests

The authors declare no competing interests.