Mycobacterium

Generation time in vitro is 14–15 hours; optimum temperature 37°C (no growth below 25°C or above 40°C); optimum pH 6.4–7.0. M. tuberculosis is an obligate aerobe growing luxuriantly, while M. bovis is microaerophilic on primary isolation and grows sparsely. On solid media, M. tuberculosis forms dry, rough, raised, wrinkled, creamy-white colonies becoming yellowish on further incubation; M. bovis forms flat, smooth, moist, white colonies that break up easily.

Key differentiating tests include the niacin test (positive in human type M. tuberculosis, negative in bovine type — canary yellow colour with cyanogen bromide/aniline), aryl sulphatase test (positive only in atypical mycobacteria), neutral red test (binds neutral red in virulent strains), catalase-peroxidase test (tubercle bacilli weakly catalase-positive but peroxidase-positive, unlike most atypical mycobacteria), and nitrate reduction (positive in M. tuberculosis, negative in M. bovis).

Infection spreads by inhalation of aerosolised droplet nuclei from an open pulmonary case — one open case may infect 25 contacts, with coughing releasing up to 3000 infectious nuclei. Alveolar macrophages ingest bacilli; cell-mediated immunity via activated CD4+ Th-1 cells (interferon-γ, interleukins) contains infection, while Th-2 cytokines drive delayed-type hypersensitivity and tissue destruction. The characteristic lesion is the tubercle — an avascular granuloma with central giant cells (± caseation) and a peripheral zone of lymphocytes and fibroblasts. Primary tuberculosis forms the Ghon focus plus hilar lymphadenopathy (primary complex); post-primary (reactivation) disease affects the upper lobes with cavitation.

Sputum microscopy by Ziehl-Neelsen staining is the most reliable single method — at least 300 oil-immersion fields must be examined before a negative report, and ≥2 typical bacilli are needed for a positive report; results are graded 1+ to 4+ by bacilli count per field. Petroff's concentration method (4% NaOH digestion and centrifugation) increases yield. Culture on Löwenstein-Jensen (LJ) medium detects as few as 10–100 bacilli/mL but needs 8–12 weeks incubation before a negative report. The Mantoux test injects 0.1 mL PPD (5 TU) intradermally; induration ≥10 mm at 48–72 hrs is positive, indicating past or present infection or BCG vaccination.

M. leprae is a straight/curved rod, 1–8 × 0.2–0.5 µm, Gram-positive and more readily stained than the tubercle bacillus, but less strongly acid-fast (5% sulphuric acid used for decolourisation instead of 20%). Bacilli occur singly, in groups, and as lipid-bound agglomerates called 'globi'. Leprosy is a chronic granulomatous disease with four clinical types: lepromatous (low host resistance, multibacillary, lepromin-negative, most infective), tuberculoid (high resistance, paucibacillary, lepromin-positive), borderline/dimorphous, and indeterminate. Diagnosis relies on slit-skin smears (Ziehl-Neelsen with 5% acid) from nasal mucosa, skin lesions and ear lobules, with the Bacteriological Index (BI) calculated from graded smears, and the Lepromin (Mitsuda) test used for classification and prognosis, not diagnosis.

Because a minimum bacillary concentration (about 10,000 bacilli/mL of sputum) is needed for reliable direct smear detection, thorough examination across 300 fields is required to avoid missing low bacillary load, paucibacillary cases.

The Mantoux test uses PPD to detect delayed hypersensitivity to M. tuberculosis and can indicate infection or BCG vaccination, whereas the Lepromin test uses lepromin antigen and is used only to classify existing leprosy and assess prognosis, not to diagnose leprosy.

Mycobacteria are extremely slow-growing (generation time 14–15 hours compared to 20 minutes for many bacteria), so cultures must be observed for 8–12 weeks before a true negative can be confidently declared.