Laboratory Detection of Oxacillin-resistant Coagulase-negative Staphylococcus spp.

Are coagulase-negative Staphylococcus (CoNS) spp. important?

Although the clinical significance of CoNS isolated in culture can be difficult to determine, members of this species have been associated with increasing numbers of hospital-acquired infections. The increased use of invasive devices, such as catheters, and the expanding number of patients with impaired host defenses have contributed to the occurrence of infections due to CoNS.

What is the value of identifying CoNS to species level?

Although there are about 20 CoNS species, they often are considered to be a single group. Some species are more resistant to commonly used antimicrobial agents than others. Identification to species level can aid in the recognition of outbreaks and in tracking resistance trends.

Which CoNS species are more frequently multidrug resistant?

S. epidermidis is the most common CoNS isolated in clinical laboratories. Usually, S. epidermidis, S. haemolyticus, and S. hominis are more likely to be multiply resistant to antimicrobial agents than are other CoNS species. However, resistance patterns of CoNS may differ between hospitals and wards.

Why is oxacillin resistance in CoNS important?

1. Limited treatment options. Oxacillin-resistant CoNS isolates are resistant to all b-lactam agents, including penicillins, cephalosporins, and carbapenems. In addition, oxacillin-resistant CoNS isolates are often resistant to other commonly used antimicrobial agents, so vancomycin is frequently the drug of choice for treatment of clinically significant infections.
2. Oxacillin-resistant strains are transmissible. An outbreak of oxacillin-resistant CoNS can occur when one strain is transmitted among patients.

Is it difficult to detect oxacillin resistance in CoNS?

Accurate detection of oxacillin resistance can be difficult. Colony sizes of CoNS are often smaller than those of S. aureus, making growth more difficult to read. In addition, like S. aureus, two subpopulations (one susceptible and the other resistant) may coexist within a culture (1). 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 at 35º C for a full 24 hours before reading (2).

What are the breakpoints for testing the susceptibility of CoNS to oxacillin and why are the oxacillin breakpoints for CoNS different from the breakpoints for S. aureus?

The 1999 National Committee for Clinical Laboratory Standards (NCCLS) breakpoints for CoNS are different from those for S. aureus (2).

When studies were performed to evaluate oxacillin breakpoints for CoNS, the current breakpoints for S. aureus failed to detect many CoNS that contained the mecA gene. In general, the new breakpoints for CoNS correlate better with mecA production for CoNS. Staphylococcal resistance to oxacillin/methicillin occurs when an isolate carries an altered penicillin-binding protein, PBP2a, which is encoded by the mecA gene. The alteration of the penicillin-binding protein does not allow the drug to bind well to the bacterial cell, causing resistance to β-lactam antimicrobial agents.

Why do the 1999 NCCLS guidelines not recommend testing CoNS using the oxacillin screen plate when previously published guidelines indicated the test could be used?

A recent study was performed that systematically evaluated the detection of oxacillin resistance in CoNS (3). Because many resistant strains were not detected in this study, the NCCLS decided that the oxacillin screen test should no longer be recommended. This view was supported by anecdotal reports with the same findings.

Why is oxacillin tested instead of methicillin?

Oxacillin is more resistant to degradation in storage and is more likely to detect most heteroresistant strains. Methicillin is not commercially available in the United States, and NCCLS does not provide breakpoints for methicillin or nafcillin for CoNS testing (2).

1. Kloos, W.E. and T. L. Bannerman. 1999. Staphylococcus and Micrococcus, p. 276. In P.R. Murray, E.J. Baron, M.A. Pfaller, F.C. Tenover, R.H. Yolken [ed.], Manual of Clinical Microbiology, 7^th ed. ASM Press, Washington, D.C.
2. National Committee for Clinical Laboratory Standards. 1999. Performance standards for antimicrobial susceptibility testing. NCCLS approved standard M100-S9. National Committee for Clinical Laboratory Standards, Wayne, PA.
3. Tenover, F.C., R.N. Jones, J.M. Swenson, B. Zimmer, S. McAllister, J. H. Jorgensen, and the NCCLS Staphylococcus Working Group. 1999. Methods for improved detection of oxacillin resistance in coagulase-negative staphylococci: results of a multicenter study. Journal of Clinical Microbiology 37(12):4051-4058.