Of 12 coagulase-negative staphylococcal species that have been recovered as normal commensals of human skin and anterior nares, the most abundant and important is S. epidermidis. For this reason some clinical laboratories designate all coagulase-negative staphylococci as S. epidermidis, a practice that is not encouraged. The second most important coagulase-negative staphylococcus is S. saprophyticus, which has a special medical niche. Coagulase-negative staphylococcal species are important agents of hospital-acquired infections associated with the use of implanted prosthetic devices and catheters.

A. Staphylococcus epidermidis

 S. epidermidis is present in large numbers as part of the normal flora of the skin . As such, it is frequently recovered from blood cultures, generally as a contaminant from skin. Despite its low virulence, it is a common cause of infection of implants such as heart valves and catheters (Figure 1). Acquired drug resistance by S. epidermidis is even more frequent than by S. aureus. Vancomycin sensitivity remains the rule, but vancomycin-resistant isolates have been reported. S. epidermidis produces an extracellular polysaccharide material called polysaccharide intercellular adhesin (sometimes called “slime”), that facilitates adherence to bioprosthetic material surfaces, such as intravenous catheters, and acts as a barrier to antimicrobial agents.

Fig1. Staphylococcus epidermidis attached by its biofilm and growing on the surface of a cathether.

 B. Staphylococcus saprophyticus

 This organism is a frequent cause of cystitis in women, probably related to its occurrence as part of normal vaginal flora . It tends to be sensitive to most antibiotics, even penicillin G. S. saprophyticus can be distinguished from S. epidermidis and most other coagulase-negative staphylococci by its natural resistance to novobiocin (Figure 2). [Note: Urinary coagulase-negative staphylococcus is often presumed to be S. saprophyticus; but novobiocin resistance can be used for confirmation.]. Figure 3 presents a summary of diseases caused by staphylococci.

Fig2. Summary of various species of staphylococci.

Fig3. Summary of staphylococcal disease. 1  Indicates first-line drugs; 2 indicates alternative drugs.