Two major clonal MRSP lineages have disseminated in Europe (ST71-J-t02-II-III) and North America (ST68-C-t06-V). Regardless of their geographical or clonal origin, the isolates displayed resistance to the major classes of antibiotics used in veterinary medicine and thus infections caused by MRSP isolates represent a serious therapeutic challenge.

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.

f Staphylococcus pseudintermedius is an opportunistic pathogen in dogs. Four housekeeping genes with allelic polymorphisms were identified and used to develop an expanded multilocus sequence typing (MLST) scheme. The new seven-locus technique shows S. pseudintermedius to have greater genetic diversity than previous methods and discriminates more isolates based upon host origin. Staphylococcus pseudintermedius, recently classified as a member of the Staphylococcus intermedius group (SIG), is the most common opportunistic pathogen in dogs. In this host it is frequently associated with pyoderma and other infections, such as those of the urinary tract, wounds, and otitis externa (1-3). It has also been isolated from infections in cats (4, 5), has zoonotic potential (6-8), and is an important nosocomial pathogen causing postsurgical infections in veterinary clinics (9, 10, 33). The incidence of methicillin-resistant S. pseudintermedius (MRSP) has increased significantly in the past few years (1,5,(11)(12)(13)(14)(15)(16)(17)(18)(19). MRSP has emerged as an important problem worldwide because of multidrug resistance and the limited number of drug choices remaining to treat infections caused by this organism. Multilocus sequence typing (MLST) has been used extensively to define the population genetic structure of Staphylococcus aureus and other bacterial species. This information has been used to predict founder strains as well as track the spread of methicillin resistance and identify epidemic clones (2,20). Likewise, the widespread application of this technique to S. pseudintermedius should help to identify methicillin-sensitive S. pseudintermedius (MSSP) progenitors of MRSP clones and provide a mechanism to track their spatial and temporal distribution (21). The identification of successful and/or virulent clonal populations of S. pseudintermedius may facilitate research into characteristics which provide a selective advantage and inform efforts to develop alternative methods of treatment and control, such as vaccines or phage therapy targeting the major clonal populations of S. pseudintermedius associated with disease.MLST is well established as a valuable method for genotyping bacteria based on the sequence variation of housekeeping genes (9,22). It provides accurate, portable data useful for global epidemiology studies and studies of evolution and population genetics (22)(23)(24)(25)(26). MLST techniques applied to diverse species of bacteria generally use at least seven loci (22,(24)(25)(26)(27). Sequence typing based on four loci (MLST-4) has provided insight into the overall genetic structure of the SIG (1). The development of an MLST method expanded to seven loci (MLST-7) for S. pseudintermedius was undertaken to increase its discriminating power with the same number of loci used for other species of staphylococci (24,32).DNA extracts from 125 previously characterized isolates of S. pseudintermedius from dogs (114 isolates), cats (5 isolates), and human beings (6 isolates) from diverse geographical regions (N...