Methicillinresistant staphylococci

Staph. aureus is a well-recognized pathogen capable of causing infections virtually anywhere in the body, and Staph. epidermidis is also emerging as a pathogen of critical importance (Bane 1993). Potential sites of nosocomial Gram-positive infections include the urinary tract, surgical wounds, intravascular loci (primary bacteremia, catheter-related infections, suppurative phlebitis), the lung and pleural space, the facial sinuses or parotid gland, and the peritoneal cavity. Infections of prosthetic devices are a particularly important subgroup of wound infections from staphylococci. Infections may be sporadic, or they may contribute to major nosocomial outbreaks.

Staphylococci possess many properties that enhance their virulence. Peptidoglycans in the bacterial cell wall activate complement and cause other immune responses, and the teichoic acid moiety promotes attachment to epithelial cells. Staphylococci also produce numerous cytotoxins, including a- and b-toxins and leukocidin. Capsular antigens, such as protein A, and a glycocalyx or 'slime' (particularly for Staph. epidermidis) decrease opsonization and antigen presentation.

Staph. aureus is well established as the second most prevalent nosocomial Gram-positive pathogen (Table d) (Ja.rYis.e.t.a.l: 1991). Coagulase-negative staphylococci, of which Staph. epidermidis is most commonly isolated, are less prevalent as nosocomial pathogens than Staph. aureus, but are recognized as formidable pathogens in surgical wounds, prosthetic devices such as vascular grafts, prosthetic joints, and bacteremia originating from intravascular catheters. Staphylococcal resistance to methicillin is an endemic problem in many hospitals; ongoing epidemiological surveillance is necessary to avert episodic epidemics. Approximately 25 per cent of Staph. aureus isolates are now resistant to methicillin (MRSA). Hope that newer antibiotics, such as the quinolones, would be effective against MRSA has largely dissipated; vancomycin remains the treatment of choice, and should be used even if in vitro susceptibility testing suggests an alternative. Of equal concern is the emergence of methicillin-resistant Staph. epidermidis (MRSE). Between 1980 and 1989, the incidence of MRSE in the United States increased from approximately 20 per cent to more than 60 per cent, reflecting the emergence and virulence of this pathogen. It is no longer true that a single positive blood culture for Staph. epidermidis can be dismissed as a contaminant.

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Table 4 Relative importance of Gram-positive bacteria as nosocomial pathogens Chapter References

Barie, P.S. (1993). Emerging problems in Gram-positive infections in the postoperative patient. Surgery, Gynecology, and Obstetrics, 177, S55-64. Bueno-Cavanillas, A., et al. (1994). Influence of nosocomial infection on mortality rate in an intensive care unit. Critical Care Medicine, 22, 55-60. Dean, D.A. and Burchard, K.W. (1996). Fungal infection in surgical patients. American Journal of Surgery, 171,374-82.

Jarvis, W.R., et al. (1991). Nosocomial infection rates in adult and pediatric intensive care units in the United States. American Journal of Medicine, 91 (Supplement 3B), 1855-1915.

Pittet, D., Tarara, D., and Wenzel, R.P. (1994). Nosocomial bloodstream infection in critically ill patients: excess length of stay, extra costs, and attributable mortality. Journal of the American Medical Association, 271, 1598-1601.

Vincent, J.-L., et al. (1995). The prevalence of nosocomial infection in intensive care units in Europe: results of the European Prevalence of Infection in Intensive Care (EPIC) Study. Journal of the American Medical Association, 274, 639-44.

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