Gram Stain, Media, and Biochemical Identification
Key Takeaways
The fastest identification path starts with Gram reaction, morphology, arrangement, colony features, and specimen source.
Catalase, coagulase, oxidase, indole, PYR, CAMP, bile esculin, 6.5% NaCl, urease, H2S, and optochin are high-yield bench tests for MLT-level bacteriology.
Media are not interchangeable: MacConkey, chocolate, Thayer-Martin, BCYE, Lowenstein-Jensen, Sabouraud, TCBS, and mannitol salt each answer different questions.
MacConkey lactose fermentation and oxidase testing quickly separate many Enterobacterales from nonfermenters and fastidious gram-negative organisms.
Identification should be reconciled with specimen context so normal flora, contaminants, and true pathogens are not treated the same way.
Build Identification From the First Clue
A good MLT microbiology answer rarely comes from one test in isolation. Start with the Gram stain: gram-positive or gram-negative, cocci or rods, clusters or chains, diplococci or curved rods. Then add colony morphology, hemolysis, media reaction, specimen source, and rapid biochemical tests.
Gram stain quality matters. Over-decolorization can make gram-positive organisms look gram-negative. Thick smears can trap stain. Mixed morphology may reflect a polymicrobial specimen, contamination, or poor specimen quality. Exam questions often give the stain and two or three tests because they want you to choose a workflow conclusion, not recite every organism trait.
Media Selection Table
| Medium | Main purpose | High-yield exam clue |
|---|---|---|
| Blood agar | General growth and hemolysis | Beta-hemolysis supports group A strep or group B strep workup depending on tests and source |
| Chocolate agar | Fastidious organisms | Haemophilus and Neisseria may require enriched media and CO2 |
| MacConkey agar | Selects gram-negative rods and differentiates lactose fermentation | Pink colonies suggest lactose fermenters such as E. coli or Klebsiella |
| CNA or PEA agar | Selects gram-positive organisms by inhibiting many gram-negative rods | Useful when mixed flora would obscure gram-positive pathogens |
| Mannitol salt agar | Selects staphylococci and differentiates mannitol fermentation | Staphylococcus aureus commonly ferments mannitol |
| Thayer-Martin or modified Thayer-Martin | Selective recovery of pathogenic Neisseria | Neisseria gonorrhoeae workup depends on correct transport and selective media |
| BCYE | Legionella isolation | Pneumonia isolate grows on BCYE with poor growth on routine blood agar |
| Lowenstein-Jensen | Mycobacterial culture | Slow-growing acid-fast organisms |
| Sabouraud dextrose agar | Fungal isolation | Yeasts and molds rather than routine urine bacteriology |
| TCBS | Vibrio selection and sucrose differentiation | Suspected Vibrio from stool or exposure history |
| Campylobacter selective media | Recovery under microaerophilic conditions, often at 42 C | Curved gram-negative rods from stool with oxidase positivity |
Gram-Positive Identification Flow
- Gram-positive cocci in clusters: perform catalase.
- Catalase positive: think Staphylococcus or related genera.
- Coagulase positive: Staphylococcus aureus is the classic answer.
- Coagulase negative: consider coagulase-negative staphylococci; use specimen context before calling a contaminant.
- Gram-positive cocci in chains or pairs: catalase negative points toward Streptococcus or Enterococcus.
- Beta-hemolytic, PYR positive, bacitracin susceptible: supports Streptococcus pyogenes group A.
- Beta-hemolytic, CAMP positive: supports Streptococcus agalactiae group B.
- Bile esculin positive and growth in 6.5% NaCl: supports Enterococcus.
- Alpha-hemolytic diplococci, optochin susceptible, bile soluble: supports Streptococcus pneumoniae.
- Alpha-hemolytic but optochin resistant: think viridans streptococci, especially in oral-flora contexts.
Gram-Negative Identification Flow
- Gram-negative rod: check MacConkey growth and lactose fermentation.
- Lactose fermenter plus indole positive: Escherichia coli is a classic pattern.
- Lactose fermenter plus mucoid colonies and indole negative: Klebsiella pneumoniae is a common pattern.
- Non-lactose fermenter, oxidase negative, nonmotile: Shigella is a common stool-culture answer.
- Non-lactose fermenter, H2S positive, motile: Salmonella is a common stool-culture answer.
- Swarming, urease positive, H2S positive: Proteus mirabilis is a classic pattern.
- Oxidase positive nonfermenter with grape-like odor or blue-green pigment: Pseudomonas aeruginosa.
- Curved gram-negative rod, oxidase positive, microaerophilic growth at 42 C: Campylobacter jejuni.
- Gram-negative diplococci, oxidase positive: think Neisseria and use carbohydrate utilization plus source.
- Small pleomorphic gram-negative coccobacillus requiring X and V factors: Haemophilus influenzae.
Rapid Test Memory Table
| Test | Positive result points toward | Negative or alternate clue |
|---|---|---|
| Catalase | Staphylococcus over Streptococcus | Streptococcus and Enterococcus are usually catalase negative |
| Coagulase | Staphylococcus aureus | Coagulase-negative staphylococci need source context |
| Oxidase | Pseudomonas, Neisseria, Vibrio, Campylobacter | Enterobacterales are usually oxidase negative |
| Indole | Escherichia coli, Proteus vulgaris | Klebsiella pneumoniae and Proteus mirabilis are classically indole negative |
| PYR | Group A strep and Enterococcus | Helps separate some beta-hemolytic streptococci |
| CAMP | Group B strep | Use with beta-hemolysis and source such as genital screening |
| Bile esculin plus 6.5% NaCl | Enterococcus | Bile esculin alone is not enough for every distinction |
| Optochin and bile solubility | Streptococcus pneumoniae | Viridans streptococci are optochin resistant and bile insoluble |
| Urease and H2S | Proteus patterns | Interpret with motility and lactose reaction |
| X and V factors | Haemophilus influenzae | Satellitism around Staphylococcus aureus is a classic supporting clue |
Avoid Identification Traps
Do not let one reaction overrule the whole case. A pink colony on MacConkey suggests lactose fermentation, but you still need organism-level identification. A positive oxidase test in a gram-negative rod points away from Enterobacterales, but it does not automatically mean Pseudomonas unless morphology, growth, and source fit.
Also separate organism identification from clinical significance. Viridans streptococci from a throat culture may be expected oral flora. The same group from multiple blood culture sets in an endocarditis scenario may be significant. Coagulase-negative staphylococci from a single blood culture bottle may be contamination; from several catheter-drawn and peripheral sets, they require more careful interpretation.
A wound culture grows gram-positive cocci in clusters. The isolate is catalase positive and coagulase positive. Which identification is most consistent with this pattern?
Staphylococcus aureus
Streptococcus pyogenes
Enterococcus faecalis
Streptococcus pneumoniae
A urine isolate is a gram-negative rod that forms pink colonies on MacConkey agar and is indole positive. Which organism is the best match?
Escherichia coli
Pseudomonas aeruginosa
Shigella species
Campylobacter jejuni
Which pairing of medium and expected use is correct?
BCYE agar for Legionella isolation
Sabouraud agar for routine disk diffusion AST
MacConkey agar for selective recovery of pathogenic Neisseria
Lowenstein-Jensen medium for routine throat beta-hemolytic streptococci
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