Abstract
We investigated the distribution and antimicrobial resistance of 120 Staphylococcus felis isolates from feline patients in Japan, mainly from the urinary tract (28.3%), abscesses (23.3%), ears (22.5%), and nasal cavity (10.8%). The distribution of S. felis differed from those of previous studies in Japan and other countries. Antimicrobial susceptibility testing revealed a relatively high resistance to penicillin (PEN, 33.3%), followed by erythromycin (ERY, 15.8%), clindamycin (CLI, 13.3%), and levofloxacin (5.0%). However, oxacillin resistance was not detected. Notably, 11/120 (9.2%)S. felis isolates exhibited multidrug resistance, i.e., resistance to more than three classes of drugs, which mainly consisted of PEN-ERY-CLI resistance phenotypes. This is the first investigation on antimicrobial-resistant S. felis isolates from feline patients in Japan.
Keywords: antimicrobial susceptibility, cat, isolation site, Staphylococcus felis
Staphylococcus felis is a member of the coagulase-negative staphylococci family. It is phenotypically similar to S. simulans but can be differentiated based on novobiocin susceptibility [10]. This bacterium causes opportunistic infections in cats and is frequently isolated from various sites of diseased cats, including the urinary tract [11], skin, respiratory tract [13], ears [5], nasal cavity [15], and eyes [7]. In addition, S. felis can infect humans by zoonotic transmission and thus poses a public health concern [19]. Igimi et al. [9] had previously reported the distribution of S. felis in cats in Japan. However, this investigation was conducted in one facility over 30 years ago, and the current distribution of S. felis in cats in Japan is unknown.
The prevalence of antimicrobial resistance in pathogens must be understood to guide the appropriate selection of antimicrobial drugs for treatment [18]. Notably, methicillin-resistant staphylococci (MRS), one of the representative multidrug-resistant (MDR) bacteria, is widely prevalent in companion animals [4, 14]. Thus, antimicrobial resistance in staphylococci are significant concern in companion animal medicine. However, few reports have described the prevalence of antimicrobial resistance, including methicillin resistance, in S. felis worldwide [6, 21], and none in Japan.
In this study, we retrospectively investigated the distribution and antimicrobial resistance of S. felis isolates from feline patients in Japan to understand the current epidemiologic status of this organism and to analyze antimicrobial resistance in cats in Japan using the organism as a marker.
The relevant data on a total of 120 isolates of S. felis isolates from clinical specimens were extracted from the database of Sanritsu Zelkova Veterinary Laboratory between November 2023 and May 2024 (Supplementary Table 1). These specimens were obtained from different feline patients that visited veterinary hospitals (n=83) in Japan. All patients had been clinically diagnosed with bacterial infectious diseases by each veterinarian. No information was available regarding their antimicrobial treatment histories. This study focused on bacterial aspects, and thus, ethical approval was not required according to the guidelines for epidemiological research set by the Japanese government.
Bacterial identification was initially performed using a MicroScan WalkAway 96 Plus Microbiology System (Beckman Coulter, Inc., Brea, CA, USA). This automated system has lower reliability rates for the identification of less common species of coagulase-negative staphylococci, compared with conventional methods [16]. Thus, we finally identified S. felis by matrix-assisted laser desorption/ionization (MALDI)–time-of-flight mass spectrometry with a Bruker MALDI Biotyper System (Bruker Daltonik, Bremen, Germany), which is widely used for identification of Staphylococcus species, including S. felis, because of its high accuracy [1, 21, 22]. All confirmed S. felis isolates were stored at −80°C in 10% skim milk.
Susceptibilities to penicillin (PEN), oxacillin (OXA), gentamicin, minocycline, clindamycin (CLI), erythromycin (ERY), chloramphenicol, trimethoprim–sulfamethoxazole, levofloxacin, rifampicin, vancomycin, teicoplanin, and linezolid were evaluated using the broth microdilution method on the MicroScan WalkAway 96 Plus system. The breakpoints of minimum inhibitory concentration (MIC) for all drugs were defined with reference to the Clinical and Laboratory Standards Institute (CLSI) guidelines [3]. Methicillin resistance in S. felis was defined as an OXA MIC of ≥1 µg/mL, according to the CLSI guideline [3]. Staphylococcus aureus ATCC 29213 was used as the quality control strain. All of the CLI non-resistant and ERY-resistant strains were also subjected to broth microdilution method using a combination of 4 µg/mL ERY and 0.5 µg/mL CLI in a single well to detect inducible CLI resistance [20].
To perform statistical analysis, we used commercially available software (BellCurve for Excel; Social Survey Research Information Co., Ltd., Tokyo, Japan). A 95% confidence interval (CI) for each proportion was calculated by Clopper-Pearson Exact method. Fisher’s exact test was carried out to compare data between two groups, whereas Marascuilo procedure, following chi-square test, was used to compare data between multiple groups. Statistical significance was set as P<0.05.
In the present study, 120 S. felis isolates from patients at many hospitals were obtained from the urinary tract (n=34; 28.3% [95% CI, 20.4–37.3%]), abscess (n=28; 23.3% [16.0–31.9%]), ear (n=27; 22.5% [15.4–31.0%]), nasal cavity (n=13; 10.8% [5.8–17.9%]), eye (n=7; 5.8% [2.3–11.8%]), skin (n=6; 5.0% [1.9–10.6%]), and the others (n=5; 4.2% [1.4–9.5%]). Approximately 30 years ago, Igimi et al. [9] reported that this organism was significantly less frequently isolated from abscesses (n=3; 7.1% [1.2–20.0%]) of feline patients at the one university hospital in Japan. In Poland, S. felis was most frequently isolated from the nasal cavity (35.8% [21.2–54.5%]) and eyes (24.0% [12.1–39.4%]) of sick cats [1]. Such variations in the isolation sites of S. felis by age, method, and/or area for investigation should be considered to understand the distribution of this organism. Incidentally, 62 of 120 (51.7% [42.4–60.9%]) strains used in this study were isolated together with the other bacteria (data not shown), implying that S. felis is not always the main causative bacteria even if identified. Thus, a comprehensive interpretation would be required for the pathogenicity significance of S. felis in feline patients with mixed infection by this organism and the other pathogens.
Few reports have explored the antimicrobial susceptibilities of S. felis isolates. Worthing et al. [21] demonstrated that out of 37 isolates in Australia, only 3 exhibited resistances to PEN and/or tetracycline. This study is the first to investigate the antimicrobial susceptibilities of S. felis isolates in Japan. The highest resistance rate was observed in PEN (33.3% [24.9–42.5%]), followed by ERY (15.8% [9.7–23.7%]), CLI (13.3% [7.7–20.8%]), levofloxacin (5.0% [1.9–10.6%]), gentamicin (2.5% [0.5–7.1%]), chloramphenicol (2.5% [0.5–7.1%]), and rifampin (0.8% [0.0–5.0%]) (Table 1). Notably, PEN resistance rate was significantly higher in our isolates than those reported in Australia (8.1% [1.7–21.9%]) [21], possibly because of the heavy use of PENs for companion animals in Japan [12]. Multiple comparison test revealed that there were no significant differences in PEN resistance rate, as well as those to the other drugs, among isolation sites. Therefore, PENs are not recommended as first-choice drugs to treat cats with S. felis infection in Japan, even if the infection occurs in any body parts.
Table 1. Prevalence of antimicrobial resistance in 120 Staphylococcus felis isolates from feline patients in Japan in 2023–2024.
| Drugsa) | Breakpoints (µg/mL)b) |
No. of resistance (%) |
|||||||
|---|---|---|---|---|---|---|---|---|---|
| Urinary tract (n=34) |
Abscess (n=28) |
Ear (n=27) |
Nasal Cavity (n=13) |
Eye (n=7) |
Skin (n=6) |
Others (n=5) |
Total (n=120) |
||
| PEN | ≥0.25 | 13 (38.2) | 6 (21.4) | 8 (29.6) | 3 (23.1) | 3 (42.9) | 3 (50.0) | 4 (80.0) | 40 (33.3) |
| OXA | ≥1 | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) |
| GEN | ≥16 | 0 (0) | 1 (3.6) | 1 (3.7) | 0 (0) | 0 (0) | 0 (0) | 1 (20.0) | 3 (2.5) |
| MIN | ≥16 | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) |
| ERY | ≥8 | 5 (14.7) | 5 (17.9) | 3 (11.1) | 1 (7.7) | 1 (14.3) | 2 (33.3) | 2 (40.0) | 19 (15.8) |
| CLI | ≥4 | 2 (5.9) | 4 (14.3) | 3 (11.1) | 2 (15.4) | 1 (14.3) | 2 (33.3) | 2 (40.0) | 16 (13.3) |
| CHL | ≥32 | 0 (0) | 1 (3.6) | 1 (3.7) | 0 (0) | 0 (0) | 0 (0) | 1 (20.0) | 3 (2.5) |
| SXT | ≥4/76 | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) |
| LVX | ≥4 | 0 (0) | 2 (7.1) | 3 (11.1) | 0 (0) | 0 (0) | 0 (0) | 1 (20.0) | 6 (5.0) |
| RFP | ≥4 | 0 (0) | 1 (3.6) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 1 (0.8) |
| VAN | ≥32 | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) |
| TEC | ≥32 | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) |
| LZD | ≥8 | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) | 0 (0) |
a) PEN, penicillin; OXA, oxacillin; GEN, gentamicin; MIN, minocycline; ERY, erythromycin; CLI, clindamycin; CHL, chloramphenicol; SXT, sulfamethoxazole-trimethoprim; LVX, levofloxacin; RFP, rifampin; VAN, vancomycin; TEC, teicoplanin; LZD, linezolid. b)All breakpoints were set by the Clinical and Laboratory Standards Institute guidelines [3].
Furthermore, we found that 11 of the 120 (9.2% [4.7–15.8%]) S. felis isolates exhibited MDR, i.e., resistance to more than three classes of drugs, which was not detected in Australia [21] and Saudi Arabia [6]. To the best of our knowledge, this is the first report on MDR S. felis isolates from feline patients. These MDR isolates were obtained from various body parts (Table 2), which should be taken into account when treating feline patients infected with S. felis. Notably, all of the MDR isolates exhibited PEN resistance. In addition, 10 of the 11 MDR isolates exhibited resistance to both ERY and CLI, indicating that most of the drug resistance in this study is constitutively attributed to alterations in the ribosomal binding site (i.e., constitutive macrolide, lincosamide, and streptogramin B [MLSB] resistance) [8]. One of the five CLI non-resistant and ERY-resistant strains grew in ERY-CLI combination well, indicating that this strain exhibited inducible CLI resistance (i.e., inducible MLSB resistance) [20]. Thus, it is likely that in addition to PEN resistance, the distribution of constitutive and inducible MLSB resistances contributes to the occurrence of MDR S. felis among cats in Japan.
Table 2. Antimicrobial resistance phenotypes in Staphylococcus felis isolates from feline patients in Japan in 2023–2024.
| Number of resistance | Resistance patterna) | Isolation sites |
|---|---|---|
| 5 | PEN-GEN-ERY-CLI-CHL (1) | Abscess (1) |
| PEN-ERY-CLI-CHL-LVX (1) | Genitals (1) | |
| 4 | PEN-GEN-ERY-CLI (1) | Airway (1) |
| 3 | PEN-ERY-CLI (7) | Ear (2), Skin (2), Urinary tract (1), Abscess (1), Nasal cavity (1) |
| PEN-CHL-LVX (1) | Ear (1) | |
| 2 | ERY-CLI (4) | Urinary tract (1), Abscess (1), Ear (1), Eye (1) |
| PEN-ERY (2) | Urinary tract (1), Abscess (1) | |
| PEN-CLI (2) | Abscess (1), Nasal cavity (1) | |
| PEN-LVX (2) | Ear (2) | |
| PEN-GEN (1) | Ear (1) | |
| 1 | PEN (22) | Urinary tract (11), Eye (3), Abscess (2), Ear (2), Nasal cavity (1), Skin (1), Cerebrospinal fluid (1), Genitals (1) |
| ERY (3) | Urinary tract (2), Abscess (1) | |
| LVX (2) | Abscess (2) | |
| RFP (1) | Abscess (1) | |
a) PEN, penicillin; GEN, gentamicin; ERY, erythromycin; CLI, clindamycin; CHL, chloramphenicol; LVX, levofloxacin; RFP, rifampin. The numbers in parentheses indicate the number of isolates.
In this study, OXA MICs in all our collection were ≤0.25 µg/mL, and thereby less than the CLSI breakpoint for Staphylococcus except for S. aureus, S. lugdunensis, S. epidermidis, S. pseudintermedius, and S. schleiferi [3]. However, antimicrobial susceptibility testing cannot fully detect MRS because of the presence of low-level OXA resistance (i.e., OXA MIC of ≤1 µg/mL) in several species of MRS, including S. aureus [2] and S. pseudintermedius [17]; all these low-level MRS strains had OXA MIC of at least 0.5 µg/mL. Therefore, the remarkably low MICs of OXA in our collection strongly indicate that the risk of acquiring methicillin-resistant S. felis remains extremely low in Japan, as well as in the other countries investigated [6, 21]. On the other hand, OXA resistance rate in S. aureus isolates from feline patients during the same investigation period was 35.3% (25.2–46.4%) based on the database of Sanritsu Zelkova Veterinary Laboratory (unpublished data). This strongly indicates that species-specific factors, in addition to selective pressure, can contribute to the occurrence of methicillin resistance in feline staphylococci. Further studies would be needed to elucidate such species-specific factors.
In conclusion, this study found that the distribution of S. felis in cats has shifted in Japan with time. Although methicillin resistance was less prevalent in S. felis in Japan, we found a relatively higher rate of MDR, specifically PEN and MLSB resistance, than those reported in other countries. We believe that continued surveillance of S. felis isolates is required to ensure the appropriate use of antimicrobial treatment in cats with S. felis infections.
POTENTIAL CONFLICTS OF INTEREST
There are no conflicts of interest to declare.
Supplementary
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