Coagulase-negative staphylococci within the hospital environment

The occurrence of several species of coagulase-negative staphylococci on the human skin surface is now well recognised; some are more closely associated with certain body sites such as the scalp and face, whereas others are more widespread in their habitat. Until about 20 years ago coagulase-negative staphylococci (CNS) isolated from specimens from humans were deemed as commensals or contaminants but now it is recognised that these bacteria can be pathogens in their own right. For example, Staphylococcus epidermidis is a not infrequent cause of infective bacterial endocarditis and its intermittent appearance in blood cultures has sometimes been attributed to environmental contamination. On the other hand, in patients with a prosthetic heart valve in situ it causes endocarditis frequently. In addition, the organism can cause infection of vascular prostheses and cerebrospinal fluid shunts for the relief of hydrocephalus and is a significant cause of urinary tract infection in children and young women. Of particular concern to the hospital bacteriologist is the determination of the significance of an isolate of CNS and especially in cases of suspected endocarditis, other parameters such as C-reactive protein and immune complexes have been used to assist diagnosis. Alternatively serum antibody to CNS may be used retrospectively to distinguish those patients with infection and from those only colonised. Various reasons have been given to explain the virulence of CNS, including production of toxins and aggressins, extracellular slime substance and an ability to adhere to natural and artificial surfaces. Each of these virulence characteristics may allow CNS to survive long enough in vivo to cause infection, especially of the immunocompromised host.