coagulase-positive

**Mechanism:** Coagulase binds prothrombin to form staphylothrombin. This activates the conversion of fibrinogen to fibrin, which forms a clot around the bacteria. The fibrin coat shields the organism from phagocytosis and helps wall off abscesses. There are two forms. Bound coagulase ("clumping factor") is detected by the slide test, and free coagulase is detected by the tube test.

**Lab identification:** - Staphylococci are gram-positive cocci in clusters and are catalase-positive. - Coagulase testing then splits them into two groups. Coagulase-positive is S. aureus. Coagulase-negative includes S. epidermidis and S. saprophyticus. - S. aureus also ferments mannitol (yellow colonies on mannitol salt agar), is often beta-hemolytic, and is golden-pigmented. - Other coagulase-positive staphylococci are uncommon. S. intermedius and S. pseudintermedius are animal-associated (dog bites), and S. schleiferi is a rare cause of human infection.

**Clinical significance of S. aureus:** - Skin and soft tissue: impetigo, cellulitis, folliculitis, furuncles, carbuncles, abscesses, and scalded skin syndrome (exfoliative toxin). - Invasive disease: bacteremia, acute infective endocarditis (especially in IV drug users, tricuspid valve), osteomyelitis (most common cause overall), septic arthritis, and pneumonia (post-influenza). - Toxin-mediated disease: toxic shock syndrome (TSST-1, a superantigen) and food poisoning (preformed enterotoxin, with onset in 1–6 hours). - Virulence factors: protein A (binds the Fc portion of IgG, which inhibits opsonization and phagocytosis), coagulase, hemolysins, and Panton-Valentine leukocidin (PVL, associated with CA-MRSA and necrotizing pneumonia). - MRSA: resistance comes from the mecA gene, which encodes an altered penicillin-binding protein (PBP2a). Treat with vancomycin, linezolid, or daptomycin (not for pneumonia, because surfactant inactivates it). For outpatient CA-MRSA, TMP-SMX or doxycycline can be used.

**Coagulase-negative staphylococci (contrast):** - S. epidermidis is novobiocin-sensitive. It forms biofilms on prosthetic devices, catheters, and shunts, and it is a common blood culture contaminant. - S. saprophyticus is novobiocin-resistant. It causes UTIs in sexually active young women, and is the second most common cause after E. coli.

**Test pearls:** - Do not confuse this with the coagulase test's role in identification vs. the coagulation cascade. Coagulase is a bacterial enzyme and is not a human clotting factor. - Catalase-positive organisms in clusters should prompt a coagulase test. Positive means S. aureus. Negative, then check novobiocin sensitivity. - Chronic granulomatous disease (NADPH oxidase deficiency) causes recurrent infections with catalase-positive organisms, including S. aureus.