Veterinarians should be aware of the potential for empiric drug treatment failures in instances where Staphylococcus spp infections are common (eg, pyoderma). Judicious use of bacterial culture and susceptibility testing is recommended.

Abstract. Clinical and Laboratory Standards Institute interpretive breakpoints for in vitro susceptibility tests that predict mecA-mediated oxacillin resistance in Staphylococcus pseudintermedius isolates from animals have been changed twice in the past decade. Moreover, there are no counterpart recommendations for human isolates of S. pseudintermedius. Individual medical and veterinary laboratories variably use interpretive breakpoints identical to those recommended for use with Staphylococcus aureus or identical to those recommended for use with coagulase-negative staphylococci. The purpose of the current study was to examine correlations between oxacillin disk diffusion, oxacillin gradient diffusion, oxacillin microbroth dilution, and cefoxitin disk diffusion tests used to predict mecA-mediated resistance in S. pseudintermedius and to retrospectively estimate, from disk diffusion zone diameter measurements, the prevalence and rate of increase of oxacillin resistance among canine S. pseudintermedius isolates submitted to a veterinary teaching hospital laboratory. Oxacillin disk diffusion zone diameters of #17 mm and oxacillin minimum inhibitory concentrations of $0.5 mg/ml were highly correlated with detection of mecA in canine S. pseudintermedius isolates by polymerase chain reaction. MecA-mediated resistance among S. pseudintermedius isolates from dogs increased from less than 5% in 2001 to near 30% in 2007. More than 90% of the methicillin-resistant S. pseudintermedius isolates in 2006 and 2007 were also resistant to representatives of $4 additional antimicrobial drug classes. Cefoxitin disk diffusion with the resistance breakpoint set at #24 mm significantly underestimated the presence of mecA in S. pseudintermedius.

The latex agglutination assay for the detection of PBP2a expression coupled with the PCR assay for the mecA gene may provide new information about emerging antimicrobial resistance among staphylococcal isolates.

Multiple tests were compared to the reference standard PBP2a latex agglutination test for detection of mecA-mediated oxacillin resistance in canine staphylococci. Cefoxitin disk diffusion, using breakpoints for human isolates of coagulase-negative Staphylococcus spp., had low sensitivity for detection of oxacillin resistance in members of the Staphylococcus intermedius group.

S schleiferi subsp schleiferi was detected in healthy dogs and dogs with otitis and pyoderma. Methicillin-resistant and -susceptible S schleiferi subsp schleiferi and S schleiferi subsp coagulans were detected as the predominant organisms in dogs with otitis.

Cefoxitin disk diffusion susceptibility testing is a widely used screening method to detect methicillin resistance in Staphylococcus aureus and coagulase-negative staphylococci (CNS) isolated from human beings.9 "Gold standard" confirmatory tests for detection of methicillin resistance include methicillin gene (mecA) detection by polymerase chain reaction (PCR) and penicillin-binding protein 2a (PBP2a) detection with specific antibody.9 In studies that included over 200 isolates each, cefoxitin disk diffusion tests for detection of methicillin resistance in S. aureus had sensitivities and specificities ranging from 96.5% to 100% and 99% to 100%, respectively. 5,6,16 Unlike similar testing with oxacillin, cefoxitin disk diffusion testing does not require additional supplementation of media or altered incubation conditions. 16 Zones of growth inhibition may also be more clearly demarcated and easier to interpret. 9,16 Previous studies have demonstrated that cefoxitin disk diffusion tests using interpretive guidelines recommended for human isolates of S. aureus and CNS were unreliable indicators of methicillin resistance in Staphylococcus pseudintermedius isolates from dogs. 3,4,12,15,18 Such tests resulted in unacceptably high levels of very major errors (resistant isolates called susceptible) and low (34-57%) agreement with mecA gene detection by PCR.3,4,15 However, cefoxitin disk diffusion test specificity was high (100%) and correlation with oxacillin disk diffusion was good (R = 0.85). 4Receiver operating characteristic analysis of one data set suggested that an optimum cefoxitin disk diffusion breakpoint for predicting methicillin resistance in S. pseudintermedius would be ≤33 mm. 4 The current study examined the feasibility of establishing cefoxitin disk diffusion interpretive criteria specific for S. pseudintermedius isolates from dogs. Isolates were from canine clinical samples received at the University of Tennessee Clinical Bacteriology and Mycology Laboratory (Knoxville, Tennessee) between 2006 and 2011. Isolates from each patient were unique with respect to body site of isolation, antimicrobial susceptibility profile, and year of isolation. In some cases, more than 1 isolate came from a patient but each isolate was unique in all of these factors. Results of cefoxitin disk diffusion susceptibility tests and mecA PCR were analyzed retrospectively for those isolates (total of 1,146) that had been subjected to both tests. This was a convenience sample that was not representative of the true distribution of methicillin-susceptible S. pseudintermedius (MSSP) and methicillin-resistant S. pseudintermedius (MRSP) in the local population of clinical isolates. The examined isolates did, however, contain nearly all MRSP from all years of the study, all MSSP from 2006, and approximately 25 MSSP from each quarter in years 2010 and 2011. The population was not entirely restricted in geographic origin to the immediate area of the laboratory, as a small proportion (not determined) of isolates were obtained from samples...

Use of the CCSP led to accurate exclusion of UTI in dogs without a UTI but was less reliable for diagnosis of UTI, particularly infections caused by gram-positive cocci. Standard aerobic microbiological culture remains the gold standard for detection of UTI in dogs.

Background Multidrug- and methicillin-resistant staphylococci are both veterinary and public health concerns due to their zoonotic potential. Therefore, the objective of this study was to investigate patterns of antimicrobial, multidrug, and methicillin resistance among four Staphylococcus spp. commonly isolated from canine clinical specimens submitted to the Clinical Bacteriology Laboratory at the University of Tennessee College of Veterinary Medicine (UTCVM). Methods Results of antimicrobial susceptibility testing and mecA polymerase chain reaction (PCR) for isolates of four common Staphylococcus spp. isolates were obtained from the Bacteriology Laboratory at the UTCVM between 01/01/2006 and 12/31/2017. Cochran-Armitage trend test was used to assess temporal trends of antimicrobial resistance (AMR), multidrug resistance (MDR), and methicillin resistance. Kappa test of agreement was used to assess agreement between the results of PCR and disk diffusion tests. Results Most of the 7805 isolates were S. pseudintermedius (6453 isolates), followed by S. coagulans (860), S. aureus (330), and S. schleiferi (162). Among S. pseudintermedius isolates, 45.5% were MDR, and 30.8% were methicillin-resistant (MRSP). There was a significant temporal increase in MRSP (p = 0.017). Chloramphenicol resistance increased among both MRSP and methicillin-susceptible (MSSP) isolates (p <  0.0001). Among S. aureus isolates, 40.9% were MDR, 37.4% were methicillin-resistant (MRSA), and the proportion of MRSA isolates increased significantly (p = 0.0480) over time. There was an increasing temporal trend in the proportion of MDR isolates among MSSP (p = 0.0022), but a decrease among MRSP (p <  0.0001) and MRSA (p = 0.0298). S. schleiferi had the highest percentage (56.9%) of methicillin-resistant isolates. Oxacillin disk diffusion was superior to cefoxitin for the detection of mecA-mediated resistance and had almost perfect agreement with mecA PCR assay for S. pseudintermedius (95.4% agreement, kappa (κ) = 0.904; p <  0.0001), S. coagulans (95.6%, κ = 0.913; p <  0.0001) and S. schleiferi (97.7%, κ = 0.945; p <  0.0001). However, cefoxitin disk diffusion was superior to oxacillin disk diffusion and had almost perfect agreement with mecA PCR assay for S. aureus (95.3%, κ = 0.834; p <  0.0001). Conclusions The levels of resistance and increasing temporal trends are concerning. These findings have implications for treatment decisions and public health due to the zoonotic potential of staphylococci. Continued surveillance and use of antibiograms to guide clinical decisions will be critical.