Staphylococcus Aureus Pneumonia: Emergence of MRSA in the Community

Suzanne F. Bradley, MD


Semin Respir Crit Care Med. 2005;26(6):643-649. 

In This Article

Pitfalls in Antimicrobial Susceptibility Testing for CA-MRSA

CA-MRSA may harbor inducible resistance genes for macrolides, lincosamides, streptogramins, and tetracycylines; resistance may emerge on therapy.[9,60] Similarly, MRSA frequently contains clones with varying degrees of resistance to ciprofloxacin (heteroresistance). This drug should be used with caution because rapid selection of resistance is likely to occur.[9]

In the past, resistance to macrolides automatically implied that lincosamides such as clindamycin could not be used due to cross resistance. However, some erythromycin resistance is mediated by an efflux gene mrs(A) that has no impact on clindamycin susceptibility. To determine if inducible lincosamide resistance is present in macrolide-resistant strains, a D test can be performed. In a truly lincosamide-resistant strain, the placement of an erythromycin disk in close proximity will blunt the normally circular zone of inhibition caused by clindamycin forming a D shape, or positive, test. A positive D test means that inducible resistance to clindamycin is present and the drug should not be used.[60]


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