Patterns of Bacterial Infection and Antibiotic Resistance in Canine Corneal Ulcers: A Retrospective Analysis (Berlin, Germany, 2021–2024)

ABSTRACT

Objective

To investigate the bacterial spectrum and antimicrobial susceptibility patterns of isolates obtained from canine corneal ulcers, assessing possible associations with breed, season, and multidrug resistance.

Methods

A retrospective review was conducted on corneal ulcers from 112 dogs between May 2021 and June 2024 at a specialized veterinary ophthalmology clinic in Berlin, Germany. Bacteriological cultures and antimicrobial susceptibility tests (ASTs) were analyzed according to the Clinical and Laboratory Standards Institute (CLSI).

Results

Brachycephalic breeds represented 79.5% of affected dogs. Positive bacterial growth was obtained in 79.5% (89/112) of samples, yielding 107 isolates. Gram‐positive organisms predominated (72.2%), mainly Staphylococcus pseudintermedius (27.8%) and β‐hemolytic Streptococcus (19.4%). Among 100 isolates tested, 59% were resistant to penicillins, while 71% were susceptible to aminoglycosides and fluoroquinolones, 73% to amoxicillin–clavulanate, and 78% to tetracyclines. Multidrug resistance (MDR) occurred in 27% of isolates.

Conclusions

Staphylococcus pseudintermedius and β‐hemolytic Streptococcus were the main bacterial agents associated with infected canine corneal ulcers. The relatively high frequency of MDR isolates emphasizes the need for culture‐based therapy and ongoing regional surveillance to guide empirical antibiotic use.

1. Introduction

Corneal ulceration represents one of the most frequent and clinically significant ocular conditions in dogs and, when uncontrolled, remains a major cause of vision loss in veterinary ophthalmology. The ulcers are characterized by loss of corneal tissue, including the epithelium and variable amounts of stroma, and are often complicated by secondary infection. Opportunistic bacteria can readily colonize the damaged cornea, leading to rapid stromal degradation, corneal melting, and, in severe cases, perforation [1, 2].

Brachycephalic breeds are particularly predisposed to corneal ulceration due to a combination of anatomical and physiological factors, including reduced corneal sensitivity, lagophthalmos, and increased exposure of the ocular surface [3, 4, 5]. The high prevalence of ulcerative keratitis in these breeds has been widely recognized. It is considered part of the so‐called Brachycephalic Ocular Syndrome (BOS), now viewed by some authors as a pandemic in canine ophthalmology [6].

Since infected corneal ulcers can deteriorate quickly, initial antibiotic treatment is often empirical and guided by the most commonly isolated microorganisms in that geographical region [7, 8]. Early administration of appropriate topical antibiotics is crucial for preserving vision. Definitive therapy should ideally be based on bacterial culture and antimicrobial susceptibility testing (AST), which enable targeted and effective antibiotic selection [7, 9].

However, cultures are not always obtained in clinical practice, often due to economic limitations, the lack of established routines, or even due to the lag time to obtain the culture result, with possible ulcer improvement or deterioration by this time. Consequently, treatment frequently relies on empirical antibiotic use. This practice, combined with the increasing prevalence of antimicrobial resistance in both human and veterinary medicine, poses a growing concern [10, 11]. Under the One Health framework [12], resistant bacterial strains shared between pets and humans may have implications beyond animal health, contributing to the broader issue of multidrug resistance. Nevertheless, appropriate selection of empiric antibiotic therapy, based on local predominant microorganisms while waiting for the culture and susceptibility test results, could enhance therapeutic outcome and reduce antibacterial resistance [8].

Given the clinical importance of bacterial keratitis in dogs and the potential for antibiotic resistance to compromise therapeutic success, continuous regional surveillance is essential. This retrospective study aimed to describe the bacterial isolates obtained from canine corneal ulcers diagnosed at a specialized Veterinary Ophthalmology clinic in Germany to determine their antimicrobial susceptibility patterns and to assess possible seasonal and breed‐related variations.

2. Materials and Methods

2.1. Study Design and Sample Characterization

A retrospective study was performed using bacteriological data from canine corneal ulcer cases diagnosed and treated at the Augen‐Tierarztpraxis Dr. Allgoewer (Berlin, Germany) between May 2021 and June 2024. A total of 112 dogs presenting with corneal ulceration were included. The study period covered 38 months, corresponding to 11 months of spring, 9 months of summer, 9 months of autumn, and 9 months of winter conditions.

2.2. Inclusion and Exclusion Criteria

To be eligible, dogs were required to have complete clinical records in the clinic's management system, including a full ophthalmic examination and documentation of bacteriological sampling for culture and antimicrobial susceptibility testing (AST).

No restrictions were applied regarding breed, age, or sex. Cases were included regardless of ulcer type—superficial (recurrent or not), non‐melting stromal (shallow or deep), melting, or perforated. Data collected for each case included breed, sex, age, weight, previous treatments, and clinical outcome.

2.3. Sample Collection

Samples were collected after topical anesthesia using 0.5% proparacaine hydrochloride (Proparakain‐POS 0.5%, Ursapharm, Saarbrücken, Germany). Sterile rayon mini swabs with aluminum shafts (Cliniswab TS 301/AL/SG, Aptaca SpA, Italy) were gently rubbed over the corneal surface and the conjunctival sac of the affected eye, then placed in Amies transport medium until processing.

2.4. Culture and Antimicrobial Susceptibility Testing (AST)

All samples were submitted to a certified microbiology laboratory (Laboklin, Berlin, Germany; DIN EN ISO 17025:2018) within 24 to 48 h after collection. Specimens were plated on Columbia agar with 5% sheep blood, Endo agar, and MacConkey agar (Becton Dickinson GmbH, Heidelberg, Germany), while residual material was inoculated into tryptic soy broth for enrichment. Plates were incubated aerobically at 36°C for 24 h, and if no growth was observed, incubation was extended for an additional 24 h. Positive samples were subcultured for isolation and identification.

Bacterial identification was performed using biochemical test panels (MAST Diagnostica GmbH, Reinfeld, Germany) and, when necessary, matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF MS; Bruker Corporation, Bremen, Germany). Antimicrobial susceptibility testing was performed via broth microdilution Micronaut plates (MERLIN Diagnostik GmbH, Germany), following Clinical and Laboratory Standards Institute (CLSI) guidelines valid at the time of testing.

The antibiotic panel tested included representatives from major antimicrobial classes, such as:

• Penicillins: Penicillin G, Ampicillin, Amoxicillin;

• Penicillin associated with beta‐lactamase inhibitors: Amoxicillin–Clavulanate;

• Cephalosporins: Cephalexin, Cefoxitin, Cefoperazone, Cefovecin, Cefquinome;

• Amphenicols: Florfenicol, Chloramphenicol;

• Aminoglycosides: Gentamicin, Neomycin, Kanamycin, Tobramycin;

• Macrolides: Erythromycin, Spiramycin;

• Tetracyclines: Tetracycline, Doxycycline;

• Lincosamides: Clindamycin, Lincomycin;

• Fluoroquinolones: Difloxacin, Enrofloxacin, Marbofloxacin, Ofloxacin, Pradofloxacin, Orbifloxacin, and

• Others: Fusidic acid, Trimethoprim–Sulfamethoxazole, Polymyxin B.

2.5. Classification of Resistance and Multidrug Resistance (MDR)

Susceptibility categories were interpreted according to the most recent CLSI criteria. For MDR classification, isolates showing ‘intermediate’ susceptibility were considered resistant. Classification into multidrug‐resistant (MDR—acquired non‐susceptibility to at least one agent in three or more antimicrobial categories), extensively drug‐resistant (XDR—non‐susceptibility to at least one agent in all but two or fewer antimicrobial categories), and pandrug‐resistant (PDR—non‐susceptibility to all agents in all antimicrobial categories) followed the definitions proposed by Magiorakos et al. [13], excluding intrinsic resistance mechanisms.

2.6. Data Processing and Statistical Analysis

Data were entered into Microsoft Excel spreadsheets and analyzed for descriptive and comparative statistics. Associations between categorical variables were assessed using Pearson's chi‐square test or Fisher's exact test when appropriate. Statistical significance was set at p < 0.05. All analyses were performed using R statistical software (version 4.5.3).

3. Results

3.1. Animal Population

A total of 112 dogs with corneal ulcers were included in the study. Brachycephalic breeds represented the majority of affected animals (79.5%, 89/112). Among these, French Bulldogs were most frequent (50.9%, 57/112), followed by Pugs (8.9%, 10/112), Shih Tzus (8.9%, 10/112), English Bulldogs (5.4%, 6/112), Boxers (3.6%, 4/112), and others. Non‐brachycephalic breeds accounted for 20.5% (23/112) of the cases.

Of the 112 dogs, 55% (62) were male (28 neutered), while 45% (50) were female (29 spayed). The mean body weight was 14.3 kg (range 1.9–45 kg, ±8.3), and the mean age was 8.6 years (range 4 months–16 years, ±3.2).

3.2. Previous Treatments

The study evaluated treatments administered before ophthalmology referral. While 20.5% received no prior care, 79.5% had been treated, primarily with antibiotics (67%). These were mostly topical, with gentamicin, chloramphenicol, and tetracyclines being the most common drugs, while systemic treatments mainly utilized amoxicillin/clavulanate. Additionally, 16.1% of cases involved topical tacrolimus or cyclosporine for keratoconjunctivitis sicca, along with corticosteroids and non‐steroidal anti‐inflammatory drugs.

3.3. Ulcer Classification and Clinical Outcomes

A total of 112 ulcers were included in the study: Non‐melting stromal ulcers were most common (48.2%, 54/112), followed by superficial ulcers (30.4%, 34/112), melting ulcers (20.5%, 23/112), and perforated ulcers (0.9%, 1/112). Three eyes (2.6%) required enucleation due to severe melting ulcers, with all animals being over 10 years of age, and the microorganisms involved were β‐hemolytic Streptococcus, Pseudomonas aeruginosa , and MDR Escherichia coli , respectively. The mean healing time was 32 ± 25 days (range 2–180 days). For MDR microorganisms, it was 34.5 ± 21.2 days (range 7–69 days).

Although seasonal distribution of ulcers showed distinct patterns (Figure 1), no significant association was found between season and ulcer type (p = 0.50).

FIGURE 1.

Seasonal distribution of canine corneal ulcers. Stacked bar charts show the distribution of the different ulcer types among the four seasons.

3.4. Culture Results

Of the 112 samples analyzed, 89 (79.5%) yielded positive microbial growth and 23 (20.5%) were culture‐negative. In 69.6% (16/23) of the negative cultures, topical antibiotics had been used, mainly aminoglycosides (7/16), tetracyclines (5/16), and chloramphenicol (3/16). Two or more microorganisms were isolated from 17 samples, resulting in a total of 107 isolates. Gram‐positive bacteria predominated (72.9%, 78/107), followed by Gram‐negative species (27.1%, 29/107). The most frequent isolates were Staphylococcus spp. (43.9%, 47/107), primarily S. pseudintermedius (28%), S. hemolyticus (6.5%), S. epidermidis (4.6%), and S. aureus (1.9%). β‐hemolytic Streptococcus accounted for 19.6% (21/107), Enterobacteriaceae for 15.8% (17/107), and Pseudomonas spp. for 8.4% (9/107). Less common isolates included Pasteurella multocida , Bacillus spp., Enterococcus sp., and Acinetobacter dispersus, among others (Figure 2).

FIGURE 2.

General distribution of microorganisms isolated from canine corneal ulcers.

3.5. Ulcers and Microorganisms

Non‐melting stromal ulcers were associated mainly with Staphylococcus spp. (24%), β‐hemolytic Streptococcus (22%), Enterobacteriaceae (18.5%), and Pseudomonas spp. and Pasteurella sp. (7.4% each); negative cultures were 11%. Of these, 25.9% were MDR. From superficial ulcers, Staphylococcus spp. (50%), β‐hemolytic Streptococcus (8.8%), Pseudomonas sp. and others (5.9% each), and Enterobacteriaceae (2.9%) were isolated. Negative results were 26%, and MDR was 23.5%. Melting ulcers were linked to Staphylococcus spp. (39.1%), Enterobacteriaceae (13%), and Pseudomonas sp. (13%), with 21.7% MDR microorganisms and 30% negative cultures.

Staphylococcus spp. was significantly more frequent in superficial ulcers than in stromal or melting ulcers (p = 0.041). β‐hemolytic Streptococcus was significantly associated with non‐melting stromal ulcers (p = 0.019). Enterobacteriaceae were significantly more common in stromal and melting ulcers than in superficial ulcers (p = 0.048). No significant associations were detected between type of ulcer and Pseudomonas spp., Pasteurella sp., negative cultures, or multidrug‐resistant isolates (p > 0.05) (Table 1).

TABLE 1.

Distribution of bacterial isolates according to corneal ulcer type.

Bacterial group/outcome	Non‐melting stromal ulcers (n = 54)	Superficial ulcers (n = 34)	Melting ulcers (n = 23)	p *
Staphylococcus spp.	13 (24.0)	17 (50.0)	9 (39.1)	0.044
β‐hemolytic Streptococcus	12 (22.0)	3 (8.8)	0 (0.0)	0.018
Enterobacteriaceae	10 (18.5)	1 (2.9)	3 (13.0)	0.047
Pseudomonas spp.	4 (7.4)	2 (5.9)	3 (13.0)	0.623
Pasteurella sp.	4 (7.4)	0 (0.0)	0 (0.0)	0.109
Other microorganisms	0 (0.0)	2 (5.9)	0 (0.0)	0.241
Negative cultures	6 (11.0)	9 (26.0)	7 (30.0)	0.081
Multidrug‐resistant (MDR) isolates	14 (25.9)	8 (23.5)	5 (21.7)	0.931

Note: Significant associations are presented in bold.

^*Fisher–Freeman–Halton exact test.

3.6. Seasonal Distribution of Isolates

Staphylococcus spp. was the most frequently isolated microorganism across all seasons. Although seasonal variations were observed in the prevalence of the isolated microorganisms, these differences were not significant (p = 0.75).

In autumn, 21 isolates were detected, corresponding to a positivity rate of 85.7%. Staphylococcus spp. (52.4%) and β‐hemolytic Streptococcus (14.3%) were the predominant microorganisms, followed by Enterobacteriaceae, Pseudomonas spp., and Pasteurella spp. (9.5% each), while other microorganisms were detected at lower frequencies.

In winter, 24 samples were detected (positivity rate 77.4%). Staphylococcus spp. remained the most prevalent microorganism (41.6%), followed by β‐hemolytic Streptococcus (29.2%), Enterobacteriaceae (12.5%), and Pseudomonas spp. (8.3%).

In spring, 38 isolates were detected, corresponding to a positivity rate of 80.8%. Staphylococcus spp. continued to be the most frequently isolated microorganism (39.5%), followed by Enterobacteriaceae (21.1%), β‐hemolytic Streptococcus (18.3%), and Pseudomonas spp. (13.1%), whereas the remaining microorganisms were identified less frequently.

In summer, 24 isolates were detected (positivity rate 85.7%). Staphylococcus spp. was again the most frequently isolated microorganism (45.8%), followed by β‐hemolytic Streptococcus (16.6%) and Enterobacteriaceae (12.5%), while the remaining microorganisms were isolated less frequently (Figure 3).

FIGURE 3.

Seasonal distribution of bacterial isolates. Bar charts show the proportion of the different bacterial isolates during the four seasons of the year.

3.7. Antimicrobial Susceptibility Testing (AST) (Figure 4)

FIGURE 4.

Antimicrobial susceptibility profiles of the most common bacterial isolates recovered from canine corneal ulcers. Stacked bar charts show the proportions of resistant, intermediate, and susceptible isolates for each antimicrobial agent: Staphylococcus spp., β‐hemolytic Streptococcus, Enterobacteriaceae, and Pseudomonas spp. PEN, penicillins; AMC, amoxicillin/clavulanate; CEP, cephalosporins; CEP1, first‐generation cephalosporins; ≥ CEP2, second‐generation or higher cephalosporins; FQ, fluoroquinolones; LIN, lincosamides; MAC, macrolides; AMG, aminoglycosides; TET, tetracyclines; PHEN, phenicols; SXT, sulfamethoxazole/trimethoprim; POL, polymyxin B/colistin; FUS, fusidic acid.

AST was performed on 100 of the 107 isolates, as seven were considered potential contaminants. Overall, 59% of isolates were resistant to penicillins. Susceptibility rates were highest for amoxicillin–clavulanate (73%), fluoroquinolones and aminoglycosides (71% each), and tetracyclines (78%). Resistance to fusidic acid (64%) and polymyxin B (76%) was common.

3.7.1. Staphylococcus spp. (n = 45)

Resistance to the penicillins was observed in 73.3% of isolates, and 8.8% (4/45) were methicillin‐resistant S. pseudintermedius (MRSP). High susceptibility was recorded for amoxicillin–clavulanate, cephalosporins, fluoroquinolones (86.7% each), trimethoprim–sulfamethoxazole (84.4%), and also tetracyclines and amphenicols (82.2%).

3.7.2. β‐hemolytic Streptococcus (n = 21)

All isolates were susceptible to penicillins, cephalosporins, lincosamides, trimethoprim–sulfamethoxazole, and the majority of fluoroquinolones, with intermediate susceptibility to orbifloxacin in 57.1% of the samples, while 95.2% of the isolates were susceptible to macrolides, tetracyclines, and amphenicols. Resistance was observed only to aminoglycosides (100%), polymyxin B (intrinsic resistance), and fusidic acid (95.2%).

3.7.3. Enterobacteriaceae (n = 17)

Most isolates were resistant to penicillins (94.1%) and first‐generation cephalosporins (70.6%). However, 82.3% were susceptible to higher‐generation cephalosporins, 100% to aminoglycosides (except one resistant only to gentamicin), and 88.2% to fluoroquinolones. Less susceptibility was observed to tetracyclines (76.5%) and amphenicols (64.7%). 35.3% of the microorganisms were resistant to polymyxin B.

3.7.4. Pseudomonas spp. (n = 9)

Eight isolates belonged to the species P. aeruginosa, and one corresponded to P. citronellolis . P. aeruginosa isolates were uniformly resistant to tetracyclines, amphenicols, and trimethoprim–sulfamethoxazole, but susceptible to polymyxin B (75%) and aminoglycosides (100%). P. citronellolis was susceptible to aminoglycosides, fluoroquinolones, tetracyclines, amphenicols, and trimethoprim–sulfamethoxazole.

3.8. Multidrug Resistance (MDR) Strains

From the 100 isolates tested, 27% were classified as multidrug‐resistant strains (MDR), including 4% methicillin‐resistant S. pseudintermedius (MRSP) and 2% extensively drug‐resistant (XDR) isolates. The majority of MDR isolates corresponded to Staphylococcus spp. (66.7%), followed by Enterobacteriaceae (28.6%).

MDR prevalence was higher in samples collected during spring (44.4%) and summer (26.9%) when compared to autumn (11.1%) and winter (18.5%), although this difference did not reach statistical significance (p = 0.35).

Ulcer type was not significantly associated with multidrug resistance (p = 0.95). Among MDR isolates, 14/27 (51.9%) were recovered from stromal ulcers, 8/27 (29.6%) from superficial ulcers, and 5/27 (18.5%) from melting ulcers. One additional ulcer was classified as perforated and was excluded from comparative analysis because of insufficient sample size.

Brachycephalic conformation was not significantly associated with ulcer infection by multidrug‐resistant microorganisms (p = 0.43).

4. Discussion

The present study provides updated insight into the bacterial spectrum and antimicrobial resistance patterns associated with canine corneal ulcers in Germany. The predominance of Staphylococcus pseudintermedius and β‐hemolytic Streptococcus agrees with previous reports identifying Gram‐positive bacteria as the primary pathogens in canine bacterial keratitis [1, 14, 15, 16, 17, 18]. Some studies, however, show regional differences with a higher prevalence of Streptococcus in the Midwestern United States [8] and in the United Kingdom [19]. These findings underscore the persistent relevance of commensal skin and mucosal bacteria as opportunistic ocular pathogens [20, 21].

Differences have also been reported according to ulcer presentation, with prevalence of Staphylococcus spp. and β‐hemolytic Streptococcus in indolent or stromal ulcers [17, 22, 25]. In this topic, our data presented significant differences, with Staphylococcus spp. significantly more frequent in superficial ulcers, while β‐hemolytic Streptococcus was significantly associated with non‐melting stromal ulcers. The predominance of Gram‐positive organisms in superficial ulcers may support the empirical use of compounded cephalosporins or chloramphenicol as monotherapy for uncomplicated ulcers, whereas stromal or clinically infected ulcers may benefit from combination therapy including aminoglycosides.

For melting ulcers, our findings showed that Staphylococcus spp. and Enterobacteriaceae prevailed, which differs from previous reports with Pseudomonas aeruginosa and β‐hemolytic Streptococcus as the main agents [19, 23]. Despite these differences, melting ulcers were still predominantly associated with microorganisms showing greater susceptibility to aminoglycosides and fluoroquinolones, and these drugs, particularly aminoglycosides, should be considered as the first choice for empiric treatment.

The overall rate of positive bacterial culture (79.5%) was consistent with other studies that reported positivity ranging from 56% to 93% [14, 17, 24]. The relatively high isolation rate may reflect the referral nature of the ophthalmology clinic, where more severe or refractory cases are typically presented. Negative cultures may have been associated with low bacterial load, ulcer type, or the influence of prior antibiotic therapy; the latter being reported in nearly 80% of dogs.

The strong predominance of brachycephalic breeds, especially French Bulldogs, is in line with the increasing recognition of brachycephalic ocular syndrome as a major predisposing factor for corneal ulceration [3, 6, 14, 19, 22, 25]. Their ocular conformation, including shallow orbits, macroblepharon, and lagophthalmos, predisposes them to mechanical trauma, tear film instability, and reduced corneal sensitivity, which facilitates epithelial defects and secondary infection [3, 4, 5]. The high number of affected young to middle‐aged animals in this study agrees with reports from [14, 26].

Regarding the seasonality, some authors demonstrated that warmer seasons in China [17] and the summer in the Midwestern United States [8] increased the risk of yielding a positive bacterial culture from canine corneas, notably Pseudomonas species in the latter study. Although our series did not reach statistical significance, we observed a tendency towards a mild increase in Enterobacteriaceae during warmer months, especially in spring. This observation parallels findings in human ophthalmology, where Gram‐negative pathogens are more frequently isolated during summer months [27]. In addition, Bock et al. [28] demonstrated that higher outdoor temperatures may be associated with an increased prevalence of antibiotic‐resistant bacteria, particularly Enterobacteriaceae. It is also noteworthy that Pseudomonas, which is a common finding in corneal ulcers in dogs, was detected less frequently in our study, with no isolation in summer. Local environmental temperature and humidity, along with other factors, like cultural habits, may influence bacterial proliferation and ocular surface microbiota composition, although further studies are warranted to confirm this pattern in dogs.

The detection of MDR bacteria in 27% of isolates, including multidrug‐resistant Staphylococcus pseudintermedius (MRSP) and XDR strains, is clinically significant. In human medicine, there are several recent reports informing about the emergence of these multidrug‐resistant pathogens [29, 30], especially in hospital infections. This prevalence aligns with global trends showing an increase in MRSP‐associated infections in dogs [31, 32, 33]. Although the presence of MDR organisms did not significantly correlate with age or season, the overall frequency warrants continuous monitoring. Under the One Health framework, the detection of resistant staphylococci in companion animals is of particular concern due to their zoonotic potential [34, 35].

The classification of bacterial isolates as resistant, intermediate, or susceptible varies among authors, as antimicrobial susceptibility testing is based on minimum inhibitory concentration (MIC) breakpoints established for systemic drug concentrations. These breakpoints may not be directly applicable to ocular infections, which are primarily treated with topical agents (CLSI, 2023, cited in [36]). In addition, some antibiotics commonly used in ophthalmic practice, such as gatifloxacin and besifloxacin, are not used systemically, and standardized breakpoints for these agents are lacking [9, 36].

In vitro susceptibility testing also does not fully replicate the ocular environment. Although topical antibiotics may achieve high local concentrations, clinical response may occur even in isolates classified as resistant in vitro [37]. Factors such as tear film turnover, lacrimation, and limited drug residence time may reduce antimicrobial efficacy. Additionally, local conditions, including inflammatory exudates and oxygen tension, can further influence antimicrobial activity [23]. Although the correlation between in vitro susceptibility and clinical response in ocular infections remains inconsistent, these limitations should be taken into account when interpreting results and do not diminish the clinical value of culture and AST.

This study highlights the importance of early microbiological investigation and susceptibility‐guided therapy in canine corneal ulcers. Knowledge of the predominant regional pathogens may assist clinicians in selecting more appropriate empirical treatments while awaiting culture and AST results, thereby supporting more rational antimicrobial use.

This study has limitations inherent to its retrospective design, including variability in prior treatments, incomplete follow‐up in some cases, and the absence of data regarding other clinical conditions such as dermatological or systemic associated problems. In addition, antimicrobial susceptibility testing was based on minimum inhibitory concentration (MIC) breakpoints established for systemic administration rather than topical ophthalmic use, as mentioned, which may limit the clinical applicability of these results when selecting topical antibiotics. Nevertheless, the relatively large sample size and the use of standardized laboratory methods strengthen the robustness and reliability of our findings.

Therefore, routine bacterial culture and antimicrobial susceptibility testing should be considered in canine corneal ulcers, particularly in stromal, melting, or refractory cases. Regional surveillance data may further support empirical therapeutic decisions and help anticipate seasonal variations in bacterial profiles.

5. Conclusions

Staphylococcus pseudintermedius and β‐hemolytic Streptococcus remain the principal bacterial pathogens associated with canine corneal ulcers in Germany. Although most isolates were susceptible to commonly used ophthalmic antibiotics, the detection of multidrug‐resistant and methicillin‐resistant strains reinforces the need for ongoing microbiological surveillance, bacteriological cultures plus antimicrobial susceptibility tests before topical antibiotic prescription, and judicious antimicrobial use in veterinary ophthalmology.

Author Contributions

Ingrid Allgoewer: conceptualization, writing – review and editing, project administration. Adriana Morales: conceptualization, writing – original draft, data curation. Esmeralda Delgado: conceptualization, supervision, writing – review and editing. Hugo Vilhena: conceptualization, writing – review and editing.

Disclosure

The authors have not used AI to generate any part of the manuscript. AI tools were used to correct and improve grammar and to edit the manuscript to follow the journal guidelines.

Ethics Statement

This study complies with the Guidelines for Ethical Research in Veterinary Ophthalmology (GERVO) and is exempt from approval by an ethics committee.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.