Methicillin-Resistant Staphylococcus aureus (MRSA): Identification and Susceptibility Testing Techniques

Extract

Many traditional techniques are useful for identification of MRSA strains, including techniques for detection of penicillin-resistance, such as the nitrocefin disk. Techniques for assessing methicillin-resistance vary from growth on special media or at a lower temperature, to detection of the mecA gene by manual (latex agglutination) and automated (PCR) methods. Technique development is now geared toward making MRSA identification more rapid. Real-time PCR has sped MRSA detection, but can be costly. Resistance to other drugs is also an issue. Clindamycin resistance may need to be induced, so a special disk diffusion test can be performed. Vancomycin resistance is becoming an issue, so alternative drugs need to be identified. Drugs that are currently available for MRSA infections include: daptomycin, linezolid, quinupristin/dalfopristin, and tigecycline. Drugs that are in the development phase include: ceftobiprole, dalbavancin, oritavancin, and telavancin. These drugs provide a promising arsenal against MRSA.

The traditional techniques used to identify a culture isolate as Staphylococcus aureus are neither difficult nor time consuming. When a patient specimen is cultured and S. aureus is a potential pathogen, the laboratory scientist looks for β-hemolytic white to yellowish colonies on the blood agar plate. A gram stain (which may or may not have also been performed on the direct specimen) of a suspect colony shows gram-positive cocci in clusters. As protocol usually dictates, a catalase test is performed, which should be positive. This confirms that the isolate probably contains staphylococci and not streptococci. Then the isolate is tested for coagulase production; either by the traditional…