Classify mycobacteria, describe the morphology and clinical manifestations of M. tuberculosis, add a note on laboratory diagnosis of pulmonary tuberculosis.
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1) Classification of mycobacteria
A practical medical microbiology classification is:
- Mycobacterium tuberculosis complex (MTBC)
- M. tuberculosis, M. bovis, M. africanum, M. caprae, M. microti, M. pinnipedii (plus proposed species in some texts).
- Nontuberculous mycobacteria (NTM)
- Slow growers (>7 days on subculture), e.g. MAC, M. kansasii, M. marinum, M. xenopi.
- Rapid growers (<7 days), e.g. M. fortuitum, M. chelonae, M. abscessus.
- Some older systems further classify NTM by Runyon groups (photochromogens, scotochromogens, nonchromogens, rapid growers).
(From Tietz and Medical Microbiology textbook sections.)
2) Morphology of Mycobacterium tuberculosis
- Slender, straight or slightly curved rod-shaped bacillus.
- Strongly acid-fast (due to lipid-rich, mycolic acid-rich cell wall).
- Weakly Gram-positive appearance but not reliably seen on routine Gram stain.
- Obligate aerobe, nonmotile, non-spore-forming, non-capsulated.
- Slow grower: colonies on LJ/Middlebrook usually visible in weeks (commonly 2-8 weeks on solid media; earlier in liquid systems).
- Intracellular survival in macrophages; inhibits phagolysosome fusion.
3) Clinical manifestations of tuberculosis (especially pulmonary TB)
Typical pulmonary features:
- Persistent cough (may be productive)
- Fever (often low-grade), night sweats
- Weight loss, malaise, fatigue, anorexia
- Hemoptysis (usually in more advanced disease)
- Chest pain, dyspnea in some patients
Patterns:
- Primary TB: often middle/lower zone involvement with hilar lymphadenopathy; may have pleural effusion.
- Reactivation TB: classically upper lobe/apical-posterior disease, often cavitary.
- Disseminated (miliary) TB: hematogenous spread with severe systemic illness.
- Extrapulmonary disease can involve lymph nodes, meninges, bone/joint, genitourinary tract, pericardium, etc., especially in immunocompromised patients.
4) Note on laboratory diagnosis of pulmonary tuberculosis
A concise exam-style outline:
A. Specimen
- Sputum (spot + early morning; many guidelines still use 2-3 specimens).
- Induced sputum/BAL if non-productive cough.
B. Microscopy
- AFB smear:
- Ziehl-Neelsen (hot), Kinyoun (cold), or fluorochrome (auramine-rhodamine).
- Rapid and inexpensive, but limited sensitivity; cannot distinguish MTBC from all NTM by smear alone.
C. Nucleic acid amplification tests (NAAT)
- Recommended on at least one initial respiratory specimen.
- Provides rapid detection of MTBC DNA and often rifampicin resistance markers (e.g., Xpert platforms).
- Positive NAAT with compatible clinical picture supports early diagnosis.
- Negative NAAT does not fully exclude TB in high-suspicion cases.
D. Culture (gold standard for confirmation)
- Solid media (LJ, Middlebrook) and/or liquid broth systems.
- Higher sensitivity than smear; enables species ID and full drug susceptibility testing.
- Liquid culture detects growth faster than solid media.
E. Identification and drug resistance testing
- Molecular probes/line probe assays, sequencing, MALDI-TOF (from cultured isolates depending on setup).
- Phenotypic or genotypic DST for INH, rifampicin, and other drugs, especially with MDR/XDR risk.
F. Supportive tests
- Chest radiography/CT supports diagnosis but is not definitive.
- TST/IGRA indicate infection/exposure, not reliable stand-alone tests for active pulmonary TB diagnosis.
Source used: Medical Microbiology 9e, Tietz Textbook of Laboratory Medicine, and Murray & Nadel's Textbook of Respiratory Medicine.
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