Coagulase-Negative Staphylococci

Coagulase-Negative Staphylococci

S. epidermidis and a number of other species of coagulase-negative staphylococci are normal commensals of the skin, anterior nares, and ear canals of humans. Their large numbers and ubiquitous distribution result in frequent contamination of specimens col- lected from or through the skin, making these organisms among the most frequently iso- lated in the clinical laboratory. In the past, they were rarely the cause of significant infec- tions, but with the increasing use of implanted catheters and prosthetic devices, they have emerged as important agents of hospital-acquired infections. Immunosuppressed or neu- tropenic patients and premature infants have been particularly affected.

     Organisms may contaminate prosthetic devices during implantation, seed the device during a subsequent bacteremia, or gain access to the lumina of shunts and catheters when they are temporarily disconnected or manipulated. The outcome of the bacterial contamination is determined by the ability of the microbe to attach to the surface of the foreign body and to multiply there. Initial adherence is facilitated by the hydrophobic na- ture of the synthetic polymers used in medical devices and the natural hydrophobic nature of many coagulase-negative staphylococci. Following attachment, some strains produce a viscous extracellular polysaccharide slime or biofilm. This biofilm provides additional adhesion, completely covers the bacteria, and serves as a mechanical barrier to antimicro- bial agents and host defense mechanisms; it is also believed to enhance nutrition of the microbes by functioning as an ion-exchange resin. Strains able to produce the polysac- charide biofilm are more likely to colonize intravenous catheters but have no known advantage in adherence to human tissues such as heart valves. The resistance of many coagulase-negative staphylococci to multiple antimicrobic agents contributes further to their persistence in the body. Infections are generally low grade, but unless controlled, they can proceed to serious tissue damage or a fatal outcome.

     The interpretation of cultures that grow coagulase-negative staphylococci is fraught with difficulty. In most cases, the finding is attributable to skin contamination, although it can indicate infection when a patient has implanted devices, or has defenses that are otherwise compromised. The presence of at least moderate numbers of organisms or the repeated isolation of a strain with the same antibiogram argues for infection over skin contamination. There is no phage-typing system for coagulase-negative staphylococci but a number of molecular procedures  have been used to compare isolates for epidemiologic purposes.

    Most coagulase-negative staphylococci now encountered are resistant to penicillin, and many are also methicillin resistant. Resistance to multiple antimicrobics usually active against Gram-positive cocci, including vancomycin, is more common than with S. aureus. Eradication of coagulase-negative staphylococci from prosthetic devices and associated tissues with chemotherapy alone is very difficult unless the device is also removed.