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?
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?
Which pairing of medium and expected use is correct?