S. epidermidis

Laboratory identification is by catalase positivity, which separates staphylococci from streptococci, coagulase negativity, which separates it from S. aureus, and novobiocin sensitivity, which separates it from S. saprophyticus, the second commonest cause of uncomplicated cystitis in young sexually active women. Its pathogenicity depends almost entirely on adherence: a polysaccharide glycocalyx allows colonization of catheters, prosthetic valves, ventriculoperitoneal shunts, pacemaker leads, and joint prostheses, followed by biofilm formation that shields the organism from antibiotics and phagocytes, so cure usually requires device removal. Prosthetic valve endocarditis within the first year of surgery is most often caused by S. epidermidis. Because it is ubiquitous on skin, it is also the commonest blood culture contaminant, and distinguishing infection from contamination depends on multiple positive cultures from separate sites, the time to positivity, and clinical context. Methicillin resistance is common among coagulase-negative staphylococci, so empirical therapy is vancomycin, often with rifampin added for prosthetic material.