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Laboratory Testing for MRSA

What are the susceptibility patterns of clinical S. aureus isolates?


Approximately 10% of S. aureus isolates in the United States are susceptible to penicillin. However, many S. aureus strains, while resistant to penicillin, remain susceptible to penicillinase-stable penicillins, such as oxacillin and methicillin. Strains that are oxacillin and methicillin resistant, historically termed methicillin-resistant S. aureus (MRSA), are resistant to all ß-lactam agents, including cephalosporins and carbapenems, although they may be susceptible to the newest class of MRSA-active cephalosporins (e.g, ceftaroline). Strains of MRSA causing healthcare-associated infections often are multiply resistant to other commonly used antimicrobial agents, including erythromycin, clindamycin, fluoroquinolones and tetracycline, while strains causing community-associated infections are often resistant only to ß-lactam agents and erythromycin, may be resistant to fluoroquinolones. Since 1996, MRSA strains with decreased susceptibility to vancomycin (minimum inhibitory concentration [MIC], 4 – 8 μg/ml) and strains fully resistant to vancomycin (MIC ≥ 32 μg/ml) have been reported.

How should clinical laboratories test for MRSA?

In addition to broth microdultion testing, the Clinical and Laboratory Standards Institute (CLSI), recommends the cefoxitin disk screen test, the latex agglutination test for PBP2a, or a plate containing 6 μg/ml of oxacillin in Mueller-Hinton agar supplemented with 4% NaCl as alternative methods of testing for MRSA. For methods of inoculation, see CLSI Approved Standard M100-S23 1. In addition, there are now several FDA-approved selective chromogenic agars that can be used for MRSA detection.

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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 of staphylococci 2. All cells in a culture may carry the genetic information for resistance, but only a small number may express the resistance in vitro. This phenomenon is termed heteroresistance and occurs in staphylococci resistant to penicillinase-stable penicillins, such as oxacillin.

Cells expressing heteroresistance grow more slowly than the oxacillin-susceptible population and may be missed at temperatures above 35°C. This is why CLSI recommends incubating isolates being tested against oxacillin or cefoxitin at 33-35° C (maximum of 35°C) for a full 24 hours before reading 1.

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Can all susceptibility tests detect MRSA?

When used correctly, broth-based and agar-based tests usually can detect MRSA. The cefoxitin disk diffusion method can be used in addition to routine susceptibility test methods or as a back-up method.

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Are there additional tests to detect oxacillin/methicillin resistance?

Nucleic acid amplification tests, such as the polymerase chain reaction (PCR), can be used to detect the mecA gene, is the most common gene that mediates oxacillin resistance in staphylococci. However, mecA PCR tests will not detect novel resistance mechanisms such as mecC or uncommon phenotypes such as borderline-resistant oxacillin resistance.

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How is the mecA gene involved in the mechanism of resistance?

Staphylococcal resistance to oxacillin/methicillin occurs when an isolate produces an altered penicillin-binding protein, PBP2a, which is encoded by the mecA gene. The variant penicillin-binding protein binds beta-lactams with lower avidity, which results in resistance to this class of antimicrobial agents.

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What are the breakpoints for testing the susceptibility of staphylococci to oxacillin?

The CLSI breakpoints for S. aureus are different than those for coagulase-negative staphylococci (CoNS) 1.

Interpretive Criteria (in μg/ml) for Oxacillin MIC Tests

Susceptible Intermediate Resistant
S. aureus ≤ 2 μg/ml N/A ≥ 4 μg/ml
 CoNS ≤ 0.25 μg/ml N/A ≤ 0.5 μg/ml

Interpretive Criteria (in μg/ml) for Cefoxitin MIC Tests

Susceptible Intermediate Resistant
S. aureus ≤ 4 μg/ml N/A ≥ 8 μg/ml
  CoNS N/A N/A N/A

Interpretive Criteria (in mm) for Cefoxitin Disk Diffusion Test

Susceptible Intermediate Resistant
S. aureus ≥ 22 mm N/A ≤ 21 mm
  CoNS ≥ 25 mm N/A ≤ 24 mm

* Oxacillin disk diffusion testing is not reliable for detecting oxacillin/ methicillin resistance. Cefoxitin should be used as a surrogate for disk diffusion testing.

Why are oxacillin and cefoxitin tested instead of methicillin?

First, methicillin is no longer commercially available in the United States. Second, oxacillin maintains its activity during storage better than methicillin and is more likely to detect heteroresistant strains. However, cefoxitin is an even better inducer of the mecA gene, and tests using cefoxitin give more reproducible and accurate results than tests with oxacillin.

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If oxacillin and cefoxitin are tested, why are the isolates called “MRSA” instead of “ORSA”?

When resistance was first described in 1961, methicillin was used to test and treat infections caused by S. aureus. However, oxacillin, which is in the same class of drugs as methicillin, was chosen as the agent of choice for testing staphylococci in the early 1990s, and this was modified to include cefoxitin later. The acronym MRSA is still used by many to describe these isolates because of its historic role.

  1. CLSI. 2013. Performance standards for antimicrobial susceptibility testing. CLSI approved standard M100-S23. Clinical and Laboratory Standards Institute, Wayne, PA.
  2. Bannerman, TL. 2003. Staphylococcus, Micrococcus and other catalase-positive cocci that grow aerobically. In P.R. Murray, E.J. Baron, J.H. Jorgensen, M.A. Pfaller, R.H. Yolken [eds.], Manual of Clinical Microbiology 8 th ed. ASM Press, Washington, D.C.


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