Prevalence, Mechanism, Genetic Diversity, and Cross-Resistance Patterns of Methicillin-Resistant Staphylococcus Isolated from Companion Animal Clinical Samples Submitted to a Veterinary Diagnostic Laboratory in the Midwestern United States

Abstract

Methicillin-resistant Staphylococcus (MRS) is a leading cause of skin and soft tissue infections in companion animals, with limited treatment options available due to the frequent cross-resistance of MRS to other antibiotics. In this study, we report the prevalence, species distribution, genetic diversity, resistance mechanism and cross-resistance patterns of MRS isolated from companion animal (mostly dog and cat) clinical cases submitted to Iowa State University Veterinary Diagnostic Laboratory (ISU VDL) between 2012 and 2019. The majority of isolates were identified as Staphylococcus pseudintermedius (68.3%; 2379/3482) and coagulase-negative Staphylococcus (CoNS) (24.6%; 857/3482), of which 23.9% and 40.5% were phenotypically resistant to methicillin, respectively. Cross resistance to other β-lactams (and to a lesser extent to non-β-lactams) was common in both methicillin-resistant S. pseudintermedius (MRSP) and CoNS (MRCoNS), especially when oxacillin MIC was ≥4 μg/mL (vs. ≥0.5−<4 μg/mL). The PBP2a protein was detected by agglutination in 94.6% (521/551) MRSP and 64.3% (146/227) MRCoNS. A further analysis of 31 PBP2a-negative MRS isolates (all but one MRCoNS) indicated that 11 were mecA gene-positive while 20 were negative for mecA and other mec genes by PCR. The resistance to last-resort anti-staphylococcal human drugs (e.g., tigecycline, linezolid, vancomycin) among the MRS tested was none to very low. Even though genotyping indicated an overall high level of genetic diversity (87 unique PFGE patterns and 20 MLST types) among a subset of MRSP isolates tested (n = 106), certain genotypes were detected from epidemiologically connected cases at the same or different time points, suggesting persistence and/or nosocomial transmission. These results indicate a relatively high prevalence of MRS from companion animals in the Midwestern US; therefore, it is important to perform routine susceptibility testing of Staphylococcus in veterinary clinical settings for the selection of appropriate antimicrobial therapy.

Keywords: PBP2a; antimicrobial resistance; clinical cases; companion animals; genetic diversity; mec genes; methicillin-resistant Staphylococcus; veterinary diagnostic laboratory.

Figure 1

Prevalence of methicillin-resistant Staphylococcus spp. from companion animal clinical cases tested in this study. Total number of resistant and susceptible isolates are shown in the horizontal bars for each organism category. CoNS, coagulase-negative staphylococci excluding S. epidermidis; CoPS, coagulase-positive staphylococci excluding S. aureus and S. pseudintermedius.

Figure 2

Overall methicillin-resistant Staphylococcus (MRS) prevalence by year (2012–2019). Total numbers of isolates (both susceptible and resistant) for each year are shown in parenthesis on the x-axis.

Figure 3

PFGE dendrogram of representative (n = 106) methicillin-resistant S. pseudintermedius (MRSP) isolates from this study. MLST-based sequence types (STs) are also shown on the far-right column when available. Isolate prefixes (from left to right): numerals, isolate numbers; C, canine; F, feline; Ab, abscess; Bl, bladder; Dt, draining tract; Ea, ear; Es, esophagus; Ey, eye swab; Jf, joint fluid; Lu, lung; Ln, lymph node; Ma, mass; Ns, nasal swab; Pf, peritoneal fluid; Pw, prostatic wash; Sk, skin; Ts, tissue; Ur, urine; Ws, wound swab; numerals, the last two digits of isolation year (e.g., 18 would mean 2018).

Figure 4

MLST-based dendrogram of the S. pseudintermedius STs found in the current study and their distribution on a global scale, as available in the PubMLST database. The isolates from this study are shown in boldface. The dendrogram was generated without the branch lengths and contained six datasets (PubMLST ID number, ST number, MRSP-MRSS phenotype, host, source, country, and year) using iToL (https://itol.embl.de/, accessed date 15 January 2022). In order to provide a comprehensive view, all isolates of the matching STs available in the PubMLST database (as of January 2022) were included in the dendrogram, except for ST71. For this particular ST, only 13 representative isolates (selected based on host, source, and country) from the total 54 available in the database were included for simplicity. For the STs found in this study, only one isolate (the ones submitted to the PubMLST database) per ST was included in the dendrogram. MRSP: methicillin-resistant S. pseudintermedius; MSSP: methicillin-susceptible S. pseudintermedius.