Gram-Positive Bacteria: Step 1 Algorithm & Toxins

Creator: Progress Note
Published: 2026-08-26
Keywords: gram positive bacteria, gram positive bacteria, gram positive cocci algorithm, catalase coagulase, staphylococcus aureus, streptococcus pyogenes, group b strep, streptococcus pneumoniae, enterococcus, novobiocin optochin bacitracin, clostridium toxins, listeria, mrsa treatment, rheumatic fever

bolt The 60-second version

Gram-positive bacteria have a thick peptidoglycan cell wall that retains crystal violet and stains purple. Step 1 expects you to sort them with a short lab algorithm — shape, catalase, coagulase, hemolysis — and then attach each organism to its virulence factors, classic infections, and treatment.

check_circleThick peptidoglycan with teichoic acids; no outer membrane, so no endotoxin (lipoteichoic acid triggers cytokines instead).
check_circleCocci: catalase-positive = Staphylococcus; catalase-negative = Streptococcus and Enterococcus.
check_circleS. aureus is coagulase-positive; S. epidermidis (novobiocin-sensitive) and S. saprophyticus (novobiocin-resistant) are coagulase-negative.
check_circleHemolysis splits streptococci: α (S. pneumoniae, viridans), β (group A, group B), γ (Enterococcus).
check_circleRods: spore-formers are Bacillus (aerobic) and Clostridium (anaerobic); non-spore-formers include Corynebacterium and Listeria.
check_circleMost are treated with β-lactams; MRSA needs vancomycin, and VRE needs linezolid or daptomycin.

target What you'll be able to do · aligned to USMLE Step 1 · Microbiology — Bacteriology

MICROApply the catalase, coagulase, hemolysis, and sensitivity tests to identify gram-positive cocci.
MICROMatch major gram-positive organisms to their virulence factors and toxins.
PATHExplain the immunologic basis of rheumatic fever and poststreptococcal glomerulonephritis.
PHARMSelect first-line therapy for MSSA, MRSA, streptococci, enterococci, and C. difficile.

How gram-positive bacteria are tested on Step 1

Vignettes give a lab clue (catalase, coagulase, hemolysis, optochin or bacitracin sensitivity) plus a clinical picture and ask for the organism, its toxin, or the drug of choice.

workspace_premium Why the stain works

A thick peptidoglycan layer traps the crystal violet–iodine complex during alcohol decolorization. Gram-negative cells have a thin wall and lose the dye, picking up the pink safranin counterstain.

The identification algorithm

Start with shape, then catalase for cocci. Catalase breaks down hydrogen peroxide; catalase-positive organisms are a particular threat in chronic granulomatous disease.

Gram-positive cocci. Staphylococci grow in clusters and are catalase-positive; streptococci and enterococci grow in chains and are catalase-negative.

Test	Distinguishes	Result
Coagulase	S. aureus vs other staphylococci	S. aureus positive
Novobiocin	S. epidermidis vs S. saprophyticus	Epidermidis sensitive; saprophyticus resistant
Optochin; bile solubility	S. pneumoniae vs viridans streptococci	Pneumoniae sensitive and soluble; viridans resistant
Bacitracin	Group A vs group B streptococcus	Group A sensitive; group B resistant
CAMP test, hippurate	Group B streptococcus	Positive
Growth in 6.5% NaCl and bile	Enterococcus vs nonenterococcal group D	Enterococcus grows

Mnemonic pairings: NO StRESs (Novobiocin — Saprophyticus Resistant, Epidermidis Sensitive); OVRPS (Optochin — Viridans Resistant, Pneumoniae Sensitive); B-BRAS (Bacitracin — group B Resistant, group A Sensitive).

Staphylococci

S. aureus causes disease both by invasion (abscesses, endocarditis, osteomyelitis) and by toxins that act at a distance.

Organism	Key virulence	Classic disease
S. aureus	Protein A (binds Fc of IgG), coagulase, TSST-1, enterotoxins, exfoliative toxin, PVL	Abscess, acute endocarditis (IV drug use, tricuspid), osteomyelitis, toxic shock, scalded skin, rapid-onset food poisoning
S. epidermidis	Biofilm on plastic	Prosthetic joint, catheter, and shunt infections; common blood-culture contaminant
S. saprophyticus	Adhesins for uroepithelium	Second most common cause of uncomplicated UTI in young women

Preformed S. aureus enterotoxin causes vomiting within 2–6 hours of eating; reheating does not destroy it.

Streptococci and enterococci

Streptococci are distinguished by hemolysis and Lancefield group. Group A streptococcus is unique for its nonsuppurative sequelae.

Organism	Key features	Classic disease
S. pyogenes (group A)	M protein, streptolysin O (ASO titer), pyrogenic exotoxins	Pharyngitis, impetigo, cellulitis, necrotizing fasciitis, scarlet fever; rheumatic fever, poststreptococcal glomerulonephritis
S. agalactiae (group B)	Polysaccharide capsule; colonizes vagina	Neonatal sepsis, meningitis, pneumonia — screen at 36–37 weeks
S. pneumoniae	Polysaccharide capsule, IgA protease; lancet-shaped diplococci	Most common cause of community-acquired pneumonia, meningitis, otitis media, sinusitis
Viridans group	Dextrans bind fibrin-platelet aggregates	Dental caries (S. mutans), subacute endocarditis on damaged valves (S. sanguinis)
S. gallolyticus (bovis)	Group D, nonenterococcal	Endocarditis — look for colon cancer
Enterococcus faecalis, faecium	Hardy; intrinsically resistant to cephalosporins	UTI, biliary infection, endocarditis after GI or GU procedures

Rheumatic fever follows pharyngitis only; glomerulonephritis can follow either pharyngitis or skin infection.

Rheumatic fever is a type II hypersensitivity. Prompt penicillin treatment of strep pharyngitis prevents it; Aschoff bodies and Anitschkow cells are the histologic hallmarks.

Gram-positive rods

Rods split into spore-formers and non-spore-formers. Spores survive boiling and alcohol; killing them requires autoclaving.

Gram-positive rods. Listeria escapes the phagosome with listeriolysin O and spreads cell to cell on actin tails, so clearing it depends on cell-mediated immunity — hence its risk in neonates, pregnancy, older adults, and the immunosuppressed.

functions Worked example — follow the algorithm

A 24-year-old woman has dysuria and frequency. Urine culture grows gram-positive cocci in clusters that are catalase-positive, coagulase-negative, and resistant to novobiocin. What is the organism?

Clusters and catalase-positive mean Staphylococcus.
Coagulase-negative excludes S. aureus.
Novobiocin resistance separates S. saprophyticus from S. epidermidis.
An uncomplicated UTI in a young, sexually active woman fits.

Answer. Staphylococcus saprophyticus — the second most common cause of uncomplicated cystitis after E. coli.

Treatment essentials

β-lactams remain first-line for most gram-positive infections; resistance determines the exceptions.

Organism	First-line therapy	Resistance note
MSSA	Nafcillin, oxacillin, dicloxacillin, or cefazolin	Penicillinase-resistant penicillins
MRSA	Vancomycin (daptomycin, linezolid alternatives)	Altered PBP2a from mecA gene
Group A streptococcus	Penicillin or amoxicillin	Universally penicillin-sensitive
S. pneumoniae	Ceftriaxone ± vancomycin for meningitis	Altered PBPs reduce penicillin binding
Enterococcus	Ampicillin (± gentamicin for endocarditis)	VRE: D-Ala-D-Lac; use linezolid or daptomycin
C. difficile	Oral vancomycin or fidaxomicin	Recurrence: fecal microbiota transplant

Daptomycin is inactivated by surfactant and should not be used for pneumonia.

Next in the roadmap

Gram-Positive Bacteria

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quiz Test yourself

Quick check — 3 questions

1. A neonate develops sepsis on day 1. Blood cultures grow β-hemolytic, bacitracin-resistant, CAMP-positive cocci in chains. What is the organism?

2. Which S. aureus virulence factor binds the Fc region of IgG to prevent opsonization?

3. Endocarditis caused by Streptococcus gallolyticus should prompt which investigation?

Frequently asked questions

How do you tell Staphylococcus from Streptococcus?expand_more

Staphylococci grow in grape-like clusters and are catalase-positive; streptococci grow in chains and are catalase-negative.

Which gram-positive bacteria form spores?expand_more

Bacillus (aerobic) and Clostridium (anaerobic) species. Spores resist heat and alcohol and are destroyed by autoclaving.

Why don't gram-positive bacteria have endotoxin?expand_more

Endotoxin is lipopolysaccharide in the outer membrane, which only gram-negative bacteria have. Gram-positive cell walls instead contain lipoteichoic acid, which also activates TLR2 and cytokine release.

What is the treatment for MRSA?expand_more

Vancomycin is standard for serious infections; alternatives include daptomycin (not for pneumonia), linezolid, and ceftaroline. Mild skin infections may respond to TMP-SMX, doxycycline, or clindamycin.

References & further reading

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Gram-Positive Bacteria: The USMLE Step 1 Guide

How gram-positive bacteria are tested on Step 1

The identification algorithm

Staphylococci

Streptococci and enterococci

Gram-positive rods

Treatment essentials

Quick check — 3 questions

Frequently asked questions

References & further reading

One high-yield concept + one question, every week

Turn this guide into questions you’ll actually remember

Practice & tools for USMLE

Keep going — it’s free