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peer revIewed METHICILLIN-RESISTANT STaphylococcal InfecTIonS Recent Developments T he increasing prevalence of staphylococcal antimicrobial resistance, particularly methicillin resistance, presents a challenge to veterinary practitioners treating clinical infections. Failure to recognize staphylococcal antimicrobial resistance frequently results in ineffective empiric therapeutic choices and protracted clinical disease. In addition, concern is developing regarding potential transmission of antimicrobialresistant strains from humans to animals and vice versa. METHICILLIN RESISTANCE Methicillin-resistant staphylococci possess the mecA gene, carried on the mobile genetic element staphylococcal chromosome cassette mec (SCCmec), which encodes for an altered penicillin-binding protein (PBP2a). Production of this low-affinity, penicillin-binding protein renders resistance to virtually all beta-lactam derivatives, including:1 • Carbapenems (eg, imipenem, meropenem) • Cephalosporins (eg, cephalexin, cefpodoxime, cefovecin) • Penicillins (eg, penicillin, amoxicillin) • Potentiated penicillins (eg, amoxicillin and clavulanic acid). Although references to methicillin resistance are commonplace in the medical literature, oxacillin is often used by veterinary microbiology laboratories as the correlate for testing antimicrobial resistance. Both methicillin and oxacillin are semisynthetic penicillinase-resistant penicillins, but oxacillin is more stable in vitro.2,3 Cefoxitin susceptibility testing is also used by human microbiol26 Today's Veterinary Practice May/June 2013 Christine L. Cain, DVM, Diplomate ACVD ogy laboratories to screen for methicillin resistance in Staphylococcus aureus isolates, but may not reliably detect methicillin resistance in S pseudintermedius.2 More recently, a mecA homologue—mecC—has been identified in some European staphylococcal isolates from humans and animals that exhibit methicillin resistance but lack the mecA gene.4-6 The prevalence of this gene in staphylococci is currently unknown. MULTIDRUG RESISTANCE Although methicillin-resistant staphylococci are not necessarily more virulent than methicillin-susceptible staphylococci,7 treatment options are often severely limited by multidrug resistance. This is particularly true for infections caused by methicillin-resistant S pseudintermedius (MRSP); MRSP isolates are increasingly multidrug resistant.2,8-11 Resistance to non–beta-lactam antimicrobials is common and conveyed by genetic mechanisms other than the mecA gene;2,8-11 these antimicrobials include: • Fluoroquinolones • Lincosamides • Macrolides • Potentiated sulfonamides • Tetracyclines. Due to the prevalence of multidrug resistance in methicillin-resistant strains, empirical switching of antimicrobial classes when treating staphylococcal infections that fail to respond to first-line antimicrobials (particularly beta-lactams) is NOT recommended. Treatment choices should be based on culture and susceptibility testing.