Microbiology Cheat Sheet
Lower Airway: Smear Before Species
Lower airway infections for NEET-PG: sputum quality, pneumococcal pneumonia, Klebsiella, atypicals, Legionella, and TB AFB smear and GeneXpert.
MedNext Academy | 3 min read
Lower Airway: Smear Before Species
Lower airway infections for NEET-PG: sputum quality, pneumococcal pneumonia, Klebsiella, atypicals, Legionella, and TB AFB smear and GeneXpert.
This topic covers the etiopathogenesis and laboratory diagnosis of lower respiratory tract infections, emphasising sputum quality and identification by Gram stain and acid-fast staining.
High-yield lines
- Specimen quality is the key pre-analytical factor: acceptable sputum has fewer than 10 squamous epithelial cells and more than 25 neutrophils per low-power field.
- Streptococcus pneumoniae is the most common cause of community-acquired bacterial pneumonia and appears as lancet-shaped Gram-positive diplococci with a capsule.
- S. pneumoniae is optochin-sensitive and bile-soluble, distinguishing it from optochin-resistant viridans streptococci.
- Klebsiella pneumoniae causes severe necrotising pneumonia in alcoholics and diabetics with currant-jelly sputum and a bulging fissure sign.
- Mycoplasma pneumoniae lacks a cell wall, does not stain on Gram stain, is resistant to beta-lactams, and causes walking pneumonia with cold agglutinins.
- Legionella pneumophila is an intracellular pathogen diagnosed by urinary antigen test, growing only on buffered charcoal yeast extract agar with cysteine and iron.
- Legionnaires disease features high fever, diarrhoea, confusion, hyponatraemia, and relative bradycardia, treated with macrolides or fluoroquinolones, not beta-lactams.
- Mycobacterium tuberculosis has a mycolic-acid-rich wall making it acid-fast, and appears as bright red bacilli on Ziehl-Neelsen stain.
- GeneXpert MTB/RIF is the first-line NTEP test, detecting M. tuberculosis and rifampicin resistance in about 2 hours from a single sputum sample.
- AFB smear requires 5,000 to 10,000 bacilli per millilitre and a negative smear does not exclude TB, as up to half of pulmonary TB is smear-negative.
- Culture on Lowenstein-Jensen medium is the gold standard for TB (3 to 8 weeks) and enables full drug-susceptibility testing.
- Pseudomonas aeruginosa causes ventilator-associated pneumonia, is oxidase-positive, non-lactose-fermenting, and produces blue-green pyocyanin.
- Report AFB seen on smear rather than TB positive, since Nocardia and other mycobacteria are also acid-fast.
- The tubercle granuloma shows central caseous necrosis with epithelioid macrophages and Langhans giant cells.
Mapped competency codes
- MI6.1
- MI6.3
Continue into the full chapter
This summary maps to MI6-respiratory-tract-infections.
Frequently Asked Questions
What defines an acceptable sputum specimen for culture?
Fewer than 10 squamous epithelial cells and more than 25 neutrophils per low-power field, indicating lower airway rather than salivary origin.
Why does Mycoplasma pneumoniae fail Gram staining and beta-lactam therapy?
It lacks a cell wall entirely, so it does not retain Gram stain and is intrinsically resistant to cell-wall-active beta-lactams.
What is the first-line TB diagnostic test under NTEP?
GeneXpert MTB/RIF (CBNAAT), which detects M. tuberculosis and rifampicin resistance simultaneously within about 2 hours.
How should acid-fast smear results be reported?
As AFB seen on smear rather than TB positive, because Nocardia and non-tuberculous mycobacteria are also acid-fast.
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