Chapter 19 Mycobacteria and legionellae
According to the World Health Organization (WHO), nearly 2 billion people, one-third of the world’s population, have disease caused by mycobacteria, particularly tuberculosis. Mycobacteria are widespread both in the environment and in animals and cause two major human diseases – tuberculosis and leprosy. They are aerobic, acid-fast bacilli (not stained by the Gram stain because of the high lipid component of the cell wall). The major medically important pathogens are:
Acid- and alcohol-fast, slender, beaded bacilli; non-sporing. As the organisms do not take up the Gram stain because of the long-chain fatty acids (mycolic acid) in the cell wall, a special stain (the Ziehl–Neelsen stain) is required to visualize them. However, fluorescent microscopy, with auramine stain, is now used commonly for this purpose.
This species does not grow on ordinary media and requires Löwenstein–Jensen medium for growth (constituents: whole egg, asparagine, glycerol, malachite green). Slow-growing (2–3 weeks; sometimes up to 6 weeks) at 37°C. They grow as ‘rough, tough and buff’ colonies – rough due to dry, irregular growth; tough due to difficulty in lifting the colony from the surface; and buff due to the pale yellow colour (Fig. 19.1).
In general, identification of mycobacteria is based on their rate of growth, optimum temperature requirements and pigment production in the presence or absence of light; biochemical tests are also helpful. These slow procedures are being supplanted by more efficient nucleic acid probe techniques.
This organism is the agent of tuberculosis, a chronic, granulomatous, slowly progressive infection, usually of the lungs; eventually, many other organs and tissues may be affected. A pandemic disease, tuberculosis is especially common in the developing world owing to HIV infection (15–20% of individuals with HIV disease may have tuberculosis). The oral cavity is affected secondary to primary disease elsewhere (see Chapter 35). The hallmark of the disease is granuloma formation and caseation mediated by cellular immunity. No exotoxins or endotoxins.
Long-term therapy (6–9 months) with antituberculous drugs (isoniazid, rifampicin, pyrazinamide, ethambutol). As drug resistance is growing and a persistent problem, combination therapy should always be given. Tubercle bacilli resistant to a number of antituberculous drugs (multidrug-resistant tuberculosis (MDR-TB)) is a growing problem. Hence, regimentation of drug delivery is a cornerstone of managing the disease, which is achieved by a global programme termed directly observed therapy (DOT).
Prevention is by bacille Calmette–Guérin (BCG) vaccination containing live attenuated organisms, in childhood. Pasteurization of milk and general improvement of living standards have played a valuable role in prevention.
This organism infects cattle. Humans become infected by ingesting M. bovis-contaminated milk. Infection is rarely seen in the West owing to eradication of the disease in cattle. The organism specifically causes the childhood disease scrofuloderma, characterized by enlarged, caseous cervical lymph nodes. M. bovis is similar in many respects to M. tuberculosis; in the laboratory, it can be distinguished from the latter by its poor growth on Löwenstein–Jensen medium and ready infection of rabbits.