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Laboratory Detection of Coagulase-negative Staphylococcus species with Decreased Susceptibility to the Glycopeptides Vancomycin and Teicoplanin

Are coagulase-negative Staphylococcus (CoNS) species 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 CoNS infection rate.

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What is the value of identifying CoNS to species level?

Although there are about 20 CoNS species, they are often 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.

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Why is the term "glycopeptide" used, instead of vancomycin, to describe the decreased susceptibility of these CoNS isolates?

The term glycopeptide refers to a group of antimicrobial agents that includes vancomycin, teicoplanin, and several other antimicrobial agents currently in U. S. clinical trials. Because of its broader scope, the term glycopeptide is used instead of vancomycin to describe these isolates. CoNS isolates with decreased susceptibility to glycopeptides have intermediate or resistant minimum inhibitory concentration (MIC) to vancomycin (>8 µg/ml) and teicoplanin (>16 µg/ml), according to National Committee for Clinical Laboratory Standards (NCCLS) breakpoints (1).

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Why are these isolates important?
  1. Limited treatment options. All clinical isolates with decreased susceptibility to glycopeptides have been oxacillin resistant and resistant to many other commonly used therapeutic agents. Because vancomycin is an important agent to treat clinically significant, oxacillin-resistant isolates, the decreased susceptibility of CoNS isolates to vancomycin, in particular, limits therapeutic options.
  2. Detection can be difficult. Like oxacillin-resistant CoNS, isolates with decreased susceptibility to glycopeptides can show heteroresistance. Slower growing cells within a culture that express resistance to vancomycin can be difficult to detect in commonly used susceptibility test methods. For this reason, CoNS isolates with vancomycin MICs of 8-16 µg/ml (intermediate) are important to detect.

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What CoNS species with decreased susceptibility to glycopeptides have been reported?

Isolates of S. haemolyticus and S. epidermidis with decreased susceptibility to glycopeptides have been reported.

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Can routine susceptibility tests detect these isolates?

Not all routine susceptibility test methods can detect CoNS isolates with decreased susceptibility to glycopeptides. Disk diffusion (Kirby-Bauer) testing and MicroScan rapid panels cannot detect these isolates. In addition, the current version of Vitek software may not report vancomycin MICs above 4 microg/ml for staphylococci other than S. haemolyticus. In general, MicroScan conventional panels and Etest can detect staphylococci with decreased susceptibility to vancomycin when the isolates being tested are incubated for a full 24 hours before reading.

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Should CoNS with decreased susceptibility to glycopeptides be reported to infection control?

Yes. Staphylococci with decreased susceptibility to vancomycin and teicoplanin have limited treatment options and require infection control precautions to decrease transmission and reduce infections. See Interim Guidelines for Prevention and Control of Staphylococcal Infection Associated with Reduced Susceptibility to Vancomycin. MMWR 1997;46:626-8,635.

(1) 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.

For information about previously isolated CoNS isolates with decreased susceptibility to glycopeptides:

Garrett, D.O., E. Jochimsen, K. Murfitt, B. Hill, S. McAllister, P. Nelson, R.V. Spera, R.K. Sall, F.C. Tenover, J. Johnston, B. Zimmer, W.R. Jarvis. 1999. The emergence of decreased susceptibility to vancomycin in Staphylococcus epidermidis. Infection Control and Hospital Epidemiology 20:167-170.

Schwalbe, R.S., J.T. Stapleton, P.H. Gilligan. 1987. Emergence of vancomycin resistance in coagulase-negative staphylococci. New England Journal of Medicine 316:927-931.

Tenover, F.C., M.V. Lancaster, B.C. Hill, C.D. Steward, S.A. Stocker, G.A. Hancock, C.M. O'Hara, McAllister, S.K., N.C. Clark, K. Hiramatsu. 1998. Characterization of staphylococci with reduced susceptibilities to vancomycin and other glycopeptides. Journal of Clinical Microbiology 36(4):1020-1027.

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