Step 1 · Microbiology

Gram-Positive Cocci

Name: Gram-Positive Cocci
Creator: Veli Bakalov, MD

Catalase, coagulase, hemolysis - the branching algorithm

Microbiology·Bacteriology·5 min read·8 high-yield points·Reviewed by Veli Bakalov, MD @HemeOncBuddy

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Catalase splits Staphylococcus (+) from Streptococcus/Enterococcus (−); coagulase splits S. aureus (+) from coagulase-negative staph; hemolysis (alpha, beta, gamma) sorts the streptococci.
S. aureus: protein A binds IgG Fc (blocks opsonization); TSST-1 superantigen → toxic shock; exfoliative toxin → scalded skin; PVL → necrotizing MRSA pneumonia; MRSA = mecA/PBP2a.
S. aureus food poisoning starts 2–6 hours after eating because its heat-stable enterotoxin is preformed in the food; vomiting predominates.
NO StRESs: novobiocin-Sensitive S. epidermidis forms biofilm on prosthetics/catheters; novobiocin-Resistant S. saprophyticus is the second most common cause of UTI in young women.
Optochin/bile: S. pneumoniae is sensitive and bile-soluble (capsule, quellung+, MOPS); viridans strep are resistant and insoluble (S. mutans caries, S. sanguinis dextran → endocarditis after dental work).
Bacitracin: group A S. pyogenes is sensitive and PYR+; group B S. agalactiae is resistant, CAMP+, hippurate+ and causes neonatal meningitis/sepsis (intrapartum penicillin).
Rheumatic fever is type II (anti-M protein vs myosin), follows pharyngitis only, and is prevented by antibiotics; PSGN is type III, follows pharyngitis or impetigo, and is not.
Enterococcus grows in bile and 6.5% NaCl (UTI, endocarditis; VRE → linezolid/daptomycin); S. gallolyticus grows in bile only and its bacteremia warrants colonoscopy for colorectal cancer.

The identification algorithm

Three bench tests. Catalase, coagulase, and hemolysis turn a smear of gram-positive cocci into a named organism. Catalase breaks down hydrogen peroxide: staphylococci (usually seen in clusters) make it, while streptococci and enterococci (chains or pairs) do not. Each branch then has its own discriminating test.

CatalasePositive → Staphylococcus; negative → Streptococcus/Enterococcus
Staph: coagulasePositive → S. aureus; negative → novobiocin splits S. epidermidis (sensitive) from S. saprophyticus (resistant)
Strep: hemolysisAlpha (partial, green), beta (complete, clear), or gamma (none/variable)
Alpha: optochin and bileS. pneumoniae sensitive and bile-soluble; viridans resistant and insoluble
Beta: bacitracinGroup A sensitive; group B resistant
Group D: 6.5% NaClEnterococcus grows; S. gallolyticus does not (both grow in bile)

Gram-positive cocci, step by step

Streptococci and enterococci

Organism	Hemolysis	Key test	Classic disease
S. pneumoniae	Alpha	Optochin-sensitive, bile-soluble, quellung-positive	Meningitis, otitis media, pneumonia, sinusitis
Viridans (S. mutans, S. sanguinis)	Alpha	Optochin-resistant, bile-insoluble	Dental caries, subacute endocarditis
Group A (S. pyogenes)	Beta	Bacitracin-sensitive, PYR-positive	Pharyngitis, skin infection, scarlet fever
Group B (S. agalactiae)	Beta	Bacitracin-resistant, CAMP- and hippurate-positive	Neonatal meningitis, pneumonia, sepsis
Enterococcus	Gamma/variable	Grows in bile and 6.5% NaCl	UTI, biliary infection, endocarditis
S. gallolyticus	Gamma/variable	Grows in bile, not 6.5% NaCl	Bacteremia/endocarditis with colorectal cancer

Staphylococcus aureus

Identity and virulence. S. aureus is catalase-positive, coagulase-positive, and beta-hemolytic, with golden colonies; it commonly colonizes the anterior nares. Coagulase converts fibrinogen to fibrin, helping wall off abscesses. Protein A binds the Fc portion of IgG, blocking opsonization and complement activation.

Invasive disease. Skin abscesses, osteomyelitis, septic arthritis, acute endocarditis (tricuspid valve in IV drug users), and post-viral pneumonia, classically after influenza.

S. aureus toxins

Toxin	Mechanism	Disease
TSST-1	Superantigen: cross-links MHC II to the TCR, flooding the body with IL-2 and IFN-γ	Toxic shock: fever, rash, desquamation, hypotension, even without bacteremia (tampons, nasal packing)
Exfoliative toxin	Cleaves desmoglein-1 in the superficial epidermis	Scalded skin syndrome, mainly in young children
Panton-Valentine leukocidin	Pore-forming toxin that kills leukocytes	Necrotizing MRSA pneumonia
Enterotoxin	Preformed in food, heat-stable	Food poisoning 2–6 hours after eating

Food poisoning. Symptoms begin 2–6 hours after eating contaminated mayonnaise, meats, or dairy because the enterotoxin is already preformed in the food; reheating does not destroy it, and no bacterial growth in the host is needed. Vomiting is prominent, and the illness is short and self-limited.

MRSA. Resistance comes from mecA, which encodes an altered penicillin-binding protein (PBP2a) with low affinity for β-lactams. Serious infections are treated with vancomycin; linezolid and daptomycin are alternatives (daptomycin is inactivated by surfactant, so it is not used for pneumonia).

Coagulase-negative staphylococci

S. epidermidis. A novobiocin-sensitive skin commensal that forms biofilm on prosthetic joints, heart valves, and catheters. It is a common contaminant of blood cultures, so a single positive bottle is interpreted with caution; true device infections are often methicillin-resistant and treated with vancomycin.

S. saprophyticus. Novobiocin-resistant; the second most common cause of UTI in young women, after E. coli.

NO StRESs: Novobiocin – Saprophyticus Resistant, Epidermidis Sensitive.

Beta-hemolytic streptococci: groups A and B

Group A (S. pyogenes). Beta-hemolytic, bacitracin-sensitive, and PYR-positive. M protein is antiphagocytic and drives molecular mimicry. Streptolysin O is the antigen behind the ASO titer, and pyrogenic (erythrogenic) exotoxin produces the sandpaper rash and strawberry tongue of scarlet fever. Diseases: pharyngitis, impetigo, cellulitis, erysipelas, necrotizing fasciitis, and scarlet fever; penicillin remains the treatment of choice.

Post-streptococcal sequelae

Feature	Rheumatic fever	PSGN
Mechanism	Type II: anti-M protein antibodies cross-react with myosin	Type III: immune complexes
Preceding infection	Pharyngitis only	Pharyngitis or skin (impetigo)
Antibiotics	Treating the strep prevents it	NOT prevented by antibiotics
Presentation	Jones criteria: carditis, polyarthritis, chorea, erythema marginatum, subcutaneous nodules	Cola-colored urine, periorbital edema, hypertension, low C3

Group B (S. agalactiae). Beta-hemolytic, bacitracin-resistant, CAMP-test positive, and hippurate positive. It colonizes the vagina and causes neonatal meningitis, pneumonia, and sepsis. Pregnant women are screened with a vaginal-rectal culture at 36 0/7–37 6/7 weeks (ACOG 2019; older CDC guidance used 35–37 weeks), and colonized women receive intrapartum penicillin prophylaxis.

Alpha-hemolytic and group D organisms

S. pneumoniae. An alpha-hemolytic, optochin-sensitive, bile-soluble, lancet-shaped diplococcus. Its polysaccharide capsule (positive quellung reaction) is the main virulence factor, aided by IgA protease. MOPS: leading cause of Meningitis, Otitis media, Pneumonia (lobar, rusty sputum), and Sinusitis. As an encapsulated organism it is especially dangerous in asplenia and sickle cell disease; conjugate and polysaccharide vaccines target the capsule.

Viridans streptococci. S. mutans and S. sanguinis are alpha-hemolytic but optochin-resistant and bile-insoluble, which separates them from pneumococcus. These oral commensals cause dental caries (S. mutans) and subacute endocarditis after dental work: S. sanguinis makes dextran that lets it adhere to fibrin on damaged valves.

Enterococcus. E. faecalis and E. faecium are hardy group D organisms that grow in bile and 6.5% NaCl. As gut flora they cause UTI, biliary infection, and subacute endocarditis after GI/GU procedures. They are intrinsically resistant to cephalosporins. Vancomycin-resistant enterococci (VRE), which replace D-Ala-D-Ala with D-Ala-D-Lac, are nosocomial and treated with linezolid or daptomycin.

S. gallolyticus (formerly S. bovis). A non-enterococcal group D streptococcus that grows in bile but not in 6.5% NaCl. Bacteremia or endocarditis with it is associated with colorectal cancer and should prompt colonoscopy.

Bovis in the blood, cancer in the colon.