Staphylococcus aureus vs Coagulase-negative staphylococci
How to tell Staphylococcus aureus and Coagulase-negative staphylococci apart when a board question makes them look alike.
Coagulase positive means aureus; coagulase negative splits by novobiocin and clinical setting.
Side by side
Coagulase-positive catalase-positive clusters causing abscesses, acute endocarditis, pneumonia, osteomyelitis, and toxin syndromes.
Coagulase-negative clusters: epidermidis is novobiocin-sensitive and device/biofilm-associated; saprophyticus is novobiocin-resistant and UTI-associated.
Coagulase and multiple virulence factors help S aureus form abscesses and invade tissue.
S epidermidis biofilm supports prosthetic infections; S saprophyticus adheres in the urinary tract.
Coagulase positive, beta hemolysis, abscess, or acute native-valve endocarditis points S aureus.
Coagulase negative plus prosthetic device points epidermidis; coagulase negative plus novobiocin resistance and cystitis points saprophyticus.
The trap
Catalase positivity confirms Staphylococcus but does not name the species, and the coagulase test is the next critical split. Coagulase-positive equals S aureus (skin and soft tissue infections, pneumonia, acute native-valve endocarditis, osteomyelitis, food poisoning from preformed enterotoxin, toxic shock syndrome from TSST-1, scalded skin syndrome from exfoliative toxin). Coagulase-negative splits two ways by novobiocin: novobiocin-sensitive equals S epidermidis (biofilm-producing, prosthetic device infections, catheter bacteremia; a single positive blood culture is more likely contamination than infection, but multiple sets positive plus a foreign body changes that call); novobiocin-resistant equals S saprophyticus (second-most-common cause of UTI in sexually active young women, after E coli).
The repair move
For Gram-positive cocci in clusters, walk the algorithm: (1) catalase confirms Staphylococcus (vs catalase-negative Streptococcus in chains); (2) coagulase positive equals S aureus, so start empiric MRSA coverage with vancomycin in any serious infection until susceptibilities return; (3) coagulase negative needs novobiocin (sensitive equals S epidermidis on prosthetic devices, treat only if multiple cultures positive or clinical sepsis; resistant equals S saprophyticus, treat as uncomplicated UTI with TMP-SMX or nitrofurantoin).
Try a free question
CoreA patient has an abscess with gram-positive cocci in clusters. The isolate is catalase positive and coagulase positive. Which organism is most likely?
Show answer and explanation
Answer: A. Staphylococcus aureus. S aureus is catalase-positive, coagulase-positive, and classically causes abscesses, toxin syndromes, pneumonia, and acute endocarditis.
Why the other choices are tempting
- B. S epidermidis is coagulase-negative and prosthetic-device associated.
- C. S saprophyticus is coagulase-negative and causes uncomplicated UTI in young sexually active women.
- D. S pyogenes is a beta-hemolytic streptococcus in chains, not clusters.
- E. Enterococcus is not coagulase-positive staphylococcus.
Flashcards
Tap a card to flip it.
What is the fastest separator for Staphylococcus aureus vs Coagulase-negative staphylococci?
Coagulase positive means aureus; coagulase negative splits by novobiocin and clinical setting.
What clue should make you choose Staphylococcus aureus?
Coagulase-positive catalase-positive clusters causing abscesses, acute endocarditis, pneumonia, osteomyelitis, and toxin syndromes.
Quick answers
What is the fastest way to tell Staphylococcus aureus from Coagulase-negative staphylococci?
Coagulase positive means aureus; coagulase negative splits by novobiocin and clinical setting.
What is the key clue for Staphylococcus aureus?
Coagulase-positive catalase-positive clusters causing abscesses, acute endocarditis, pneumonia, osteomyelitis, and toxin syndromes.
What is the key clue for Coagulase-negative staphylococci?
Coagulase-negative clusters: epidermidis is novobiocin-sensitive and device/biofilm-associated; saprophyticus is novobiocin-resistant and UTI-associated.
What is the common trap with Staphylococcus aureus vs Coagulase-negative staphylococci?
Catalase positivity confirms Staphylococcus but does not name the species, and the coagulase test is the next critical split. Coagulase-positive equals S aureus (skin and soft tissue infections, pneumonia, acute native-valve endocarditis, osteomyelitis, food poisoning from preformed enterotoxin, toxic shock syndrome from TSST-1, scalded skin syndrome from exfoliative toxin). Coagulase-negative splits two ways by novobiocin: novobiocin-sensitive equals S epidermidis (biofilm-producing, prosthetic device infections, catheter bacteremia; a single positive blood culture is more likely contamination than infection, but multiple sets positive plus a foreign body changes that call); novobiocin-resistant equals S saprophyticus (second-most-common cause of UTI in sexually active young women, after E coli).