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In the past 15 years, treatment of canine pyoderma and other infections in small animals has been made more difficult by the
emergence of antibiotic resistance in staphylococcal bacteria in the form of methicillin-resistant Staphylococcus aureus (MRSA), methicillin-resistant Staphylococcus pseudintermedius (MRSP) and methicillin-resistant Staphylococcus schleiferi (MRSS). MRSA infections have gained much public attention, and as veterinarians we need to understand methicillin-resistant
infections and how they relate to our animal patients as well as to pet owners.
Part 1 of this two-part article will give you the information you need to understand methicillin resistance in veterinary
practice. Next month in part 2, you'll learn how to diagnose and treat methicillin-resistant skin infections as well as implement
measures to reduce the spread of infection.
WHAT IS METHICILLIN RESISTANCE?
Methicillin is a beta-lactam antimicrobial introduced in the 1950s. It is relatively resistant to beta-lactamase, so it was
used to treat penicillin-resistant staphylococci. However, bacterial resistance to methicillin emerged soon after its introduction.
Methicillin resistance is mediated by bacterial production of an altered penicillin-binding protein (PBP2a), which does not
allow microbial binding of beta-lactam antibiotics. Therefore, methicillin-resistant isolates are resistant to all beta-lactam
antibiotics (penicillins, cephalosporins, and carbapenems) and are also frequently resistant to other classes of antibiotics.1,2 The protein PBP2a is encoded by the mecA gene that resides on a mobile genetic element called the staphylococcal cassette chromosome mec (SCCmec).
A laboratory diagnosis of MRSP is actually done by testing for bacterial resistance against oxacillin, a similar antibiotic
that is more stable for testing purposes.1,2 For S. aureus, cefoxitin has supplanted oxacillin as the marker for methicillin resistance.
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