coagulase-negative

**Definition:** Coagulase is an enzyme that binds prothrombin and converts fibrinogen to fibrin, which clots plasma. It is the classic way to separate Staphylococcus aureus (coagulase-positive) from the coagulase-negative staphylococci (CoNS). Both groups are Gram-positive cocci in clusters and catalase-positive.

**Key CoNS species:** - **S. epidermidis** - Part of normal skin flora. - Novobiocin-sensitive. - Produces a polysaccharide biofilm (slime, polysaccharide intercellular adhesin) that lets it adhere to plastic and foreign material. - Causes infections of prosthetic joints, prosthetic heart valves (early prosthetic valve endocarditis), intravascular catheters, CSF shunts, and other indwelling devices. - It is the most common cause of bacteremia from contaminated blood cultures and of central line infections. - Often methicillin-resistant (altered PBP, mecA), so vancomycin is the usual treatment. - Management usually involves removing the device. - **S. saprophyticus** - Novobiocin-resistant. - Second most common cause of uncomplicated UTI in sexually active young women, after E. coli. - Urease-positive, so it can produce alkaline urine and struvite stones. - Treat with TMP-SMX or a fluoroquinolone for uncomplicated cystitis (nitrofurantoin is also an option). - **Others:** S. lugdunensis behaves more like S. aureus (aggressive native-valve endocarditis). It is coagulase-negative but can give a false-positive slide clumping test because of clumping factor-like activity.

**Lab differentiation:** - Coagulase-positive: S. aureus, which is also mannitol fermenting, with protein A and golden pigment. - Coagulase-negative: epidermidis and saprophyticus. - Novobiocin test: S = epidermidis; R = saprophyticus ("Saprophyticus is Resistant").

**Clinical pearls:** - A single positive blood culture with CoNS is usually a contaminant. Multiple positive sets, especially in a patient with a prosthesis or line, suggest true infection. - IV drug users with endocarditis: think S. aureus. Prosthetic valve within 60 days: think S. epidermidis. - Virulence is driven mainly by biofilm formation, not toxins. This differs from S. aureus, which has many toxins (TSST-1, enterotoxins, exfoliative toxin) and protein A. - Staphylococci are catalase-positive, which separates them from streptococci (catalase-negative).