Classification and the diagnostic laboratory
Đã giới thiệu| English | Ý nghĩa |
|---|---|
| Gram stain | Gram stain |
| peptidoglycan/ˈpeptɪdɒɡlɪkən/ | A material made of sugar chains cross-linked by short peptides that gives most bacterial cell walls their strength. |
| PCR/ˌpiː siː ˈɑː/ | PCR |
The tree, walked
- Specimen → culture (24–48 h) → Gram stain (positive purple / negative pink) → shape → catalase, oxidase, biochemistry → a named organism ✓.
Report read: Gram-positive cocci in clusters, catalase positive, coagulase positive → clusters say staphylococci, catalase rules out streptococci, coagulase names Staphylococcus aureus ✓. Three lines are three forks.
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Gram-positive cocci in clusters, catalase positive, coagulase positive — the organism is —
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Clusters point to staphylococci; catalase separates them from streptococci; coagulase picks Staphylococcus aureus over the coagulase-negative skin flora.
Why the stain separates
- The Gram stain is wall chemistry: crystal violet–iodine complexes are trapped by the thick peptidoglycan of Gram-positive cells; Gram-negative walls lose them at the decolouriser and take the pink counterstain ✓.
- PCR names DNA in ~1–2 h — but only culture (24–48 h) proves viability and drug susceptibility.
{exercise:uni-med0801-gram} {exercise:uni-med0801-pcr}
Gram-positive cells stain purple because their wall —
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Decolouriser cannot wash the dye complex out of the thick peptidoglycan; the thin Gram-negative wall loses it and takes the pink counterstain.
PCR results in ~1-2 hours still need culture because PCR only shows —
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PCR amplifies nucleic acid of dead or live origin; viable organism and susceptibilities come only from culture (24-48 h).