Laboratory Detection of Oxacillin/Methicillin-resistant Staphylococcus aureus (MRSA)
Why are MRSA important?
Pathogenicity.
MRSA are pathogenic and are common causes of hospital-acquired infections.
Limited treatment options.
Vancomycin often is the only drug of choice for treatment of severe MRSA infections, although some strains remain susceptible to fluoroquinolones, trimethoprim/sulfamethoxazole, gentamicin, or rifampin. Because of the rapid emergence of rifampin resistance, this drug should never be used as a single agent to treat MRSA infections.
MRSA are transmissible.
A MRSA outbreak can occur when one strain is transmitted to other patients. Often this occurs when a patient or health care worker is colonized with an MRSA strain (i.e., carries the organism but shows no clinical signs or symptoms of infection) and, through contact with others, spreads the strain. Handwashing and screening patients for MRSA should be performed to decrease transmission and reduce the number of patients infected with MRSA.
How should clinical laboratories screen for MRSA?
The National Committee for Clinical Laboratory Standards (NCCLS)-recommended "Screening Test for Oxacillin-resistant S. aureus" uses an agar plate containing 6 µg/ml of oxacillin and Mueller-Hinton agar supplemented with NaCl (4% w/v; 0.68 mol/L). For methods of inoculation, see NCCLS Approved Standard M100-S10.
Is it difficult to detect oxacillin/methicillin resistance?
Accurate detection of oxacillin/methicillin resistance can be difficult due to the presence of two subpopulations (one susceptible and the other resistant) that may coexist within a culture. All cells in a culture may carry the genetic information for resistance but only a small number can express the resistance in vitro. This phenomenon is termed heteroresistance and occurs in staphylococci resistant to penicillinase-stable penicillins, such as oxacillin. Heteroresistance is a problem for clinical laboratory personnel because cells expressing resistance may grow more slowly than the susceptible population. This is why NCCLS recommends incubating isolates being tested against oxacillin, methicillin, or nafcillin at 35? C for a full 24 hours before reading .
Can all susceptibility tests detect MRSA?
When used correctly, broth-based and agar-based tests usually can detect MRSA. Oxacillin screen plates can be used in addition to routine susceptibility test methods or as a back-up method.
How is the mecA gene involved in the mechanism of resistance?
Staphylococcal resistance to oxacillin/methicillin occurs when an isolate carries an altered penicillin-binding protein, PBP2a, which is encoded by the mecAgene. The alteration of the penicillin-binding protein does not allow the drug to bind well to the bacterial cell, causing resistance to ß-lactam antimicrobial agents.
What are the breakpoints for testing the susceptibility of staphylococci to oxacillin?
The 1999 NCCLS breakpoints for S. aureus are different than those for coagulase-negative staphylococci (CoNS).