STEPTRACKERMICROBIOLOGY | IDENTIFICATION | HIGH-YIELD REVIEW

Identification Algorithms

From Gram stain to genus in a few yes/no tests — the decision trees questions expect you to walk.

FOCUS ON

Gram-positive cocci: catalase + = Staph (clusters) → coagulase + = S aureus; − → novobiocin sensitive (S epidermidis) / resistant (S saprophyticus). Catalase − = Strep (chains) → hemolysis: α (optochin S = pneumococcus, R = viridans) · β (bacitracin S = group A, R = group B) · γ (Enterococcus grows in bile + 6.5% NaCl). Gram-negative rods: lactose fermenter? → fast (E coli, Klebsiella, Enterobacter) vs slow (Citrobacter, Serratia); non-fermenter → oxidase + (Pseudomonas) / − (Salmonella, Shigella, Proteus).

TEST

RESULT → ORGANISM

WHAT IT DETECTS

CLUE

Catalase

+ Staphylococcus · − Streptococcus, Enterococcus

Breaks H₂O₂ → bubbles

Catalase ⁺ organisms: CGD infections (S aureus, Nocardia, Aspergillus, Candida, Burkholderia, Serratia)

Coagulase

+ S aureus · − S epidermidis, S saprophyticus

Converts fibrinogen → fibrin

—

Novobiocin

Sensitive S epidermidis · resistant S saprophyticus

Antibiotic disc

Separates the two coagulase-negative staph

Hemolysis

α (partial, green): S pneumoniae, viridans · β (complete, clear): S aureus, GAS, GBS · γ: Enterococcus, S gallolyticus

Hemolysins on blood agar

—

Optochin & bile solubility

Sensitive + soluble: S pneumoniae · resistant + insoluble: viridans

—

Separates the two α-hemolytic strep

Bacitracin

Sensitive: group A (S pyogenes) · resistant: group B (S agalactiae)

—

Separates the two β-hemolytic strep

CAMP test · hippurate

+ Group B strep

Enhances S aureus hemolysis

—

Bile esculin + 6.5% NaCl

Enterococcus grows in both · S gallolyticus grows in bile only

—

S gallolyticus → colon cancer screening

Lactose fermentation

Fast: E coli, Klebsiella, Enterobacter · slow: Citrobacter, Serratia · none: Salmonella, Shigella, Proteus, Pseudomonas, Yersinia

MacConkey pink colonies

Pink = fermenter

Oxidase

+ Pseudomonas, Neisseria, Vibrio, Campylobacter, H pylori, Aeromonas · − Enterobacteriaceae

Cytochrome c oxidase

—

H₂S production

+ Salmonella, Proteus (on TSI) · − Shigella

Black precipitate

Separates Salmonella from Shigella

Urease

+ Proteus, Cryptococcus, H pylori, Ureaplasma, Nocardia, Klebsiella, S epidermidis, S saprophyticus

Ammonia raises pH

Struvite stones (Proteus) · urea breath test (H pylori)

Maltose / glucose (Neisseria)

N meningitidis: maltose + glucose · N gonorrhoeae: glucose only

Sugar fermentation

Maltose separates meningococcus

Picture it — sorting gram-positive cocci

Gram-positive cocciCatalase splits staph from strep. Staph: coagulase, then novobiocin for the coagulase-negative pair. Strep: hemolysis pattern, then optochin (α) or bacitracin (β); γ-hemolytic strep in this scheme is Enterococcus.

Schematic diagrams, not to scale.

Mechanism — gram-positive cocci tree

Gram ⁺ cocci

→

Catalase ⁺Staph

→

Coagulase ⁺S aureus

Coagulase −Novobiocin S: epidermidis · R: saprophyticus

Catalase −Strep

→

αOptochin S: pneumococcus · R: viridans

βBacitracin S: GAS · R: GBS

γNaCl ⁺: Enterococcus · − : S gallolyticus

Gram-negative rods tree

LACTOSE

NEXT TEST

ORGANISMS

Fast fermenter

—

E coli, Klebsiella, Enterobacter

Slow fermenter

—

Citrobacter, Serratia

Non-fermenter

Oxidase ⁺

Pseudomonas

Non-fermenter

Oxidase −, H₂S ⁺

Salmonella, Proteus

Non-fermenter

Oxidase −, H₂S −

Shigella, Yersinia

How it's tested

COMMON CONFUSIONBoth S pneumoniae and viridans are α-hemolytic — optochin sensitivity and bile solubility separate them; capsule is only in pneumococcus.

THE TRAPGroup D enterococci and nonenterococcal group D (S gallolyticus) both grow in bile; only Enterococcus tolerates 6.5% NaCl.

YOUR APPROACHGram → shape → catalase or lactose → one more discriminator test → organism.

If you see… think…

Gram ⁺ cocci in clusters, catalase ⁺, coagulase ⁺ → S aureus

Coagulase − staph resistant to novobiocin, UTI in young woman → S saprophyticus

α-hemolytic, optochin sensitive, lancet-shaped diplococci → S pneumoniae

β-hemolytic, bacitracin sensitive → group A strep

β-hemolytic, bacitracin resistant, CAMP ⁺ → group B strep

Grows in 6.5% NaCl and bile → Enterococcus

Lactose non-fermenter, oxidase ⁺, grape odor → Pseudomonas

Lactose non-fermenter, H₂S ⁺, motile → Salmonella

Lactose non-fermenter, H₂S −, nonmotile → Shigella

Ferments maltose and glucose → N meningitidis