Mycobacteria
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:
■ Mycobacterium tuberculosis, the agent of tuberculosis; one of the top three infectious diseases affecting humans globally
■ Mycobacterium bovis causes tuberculosis in humans as well as in cattle
■ Mycobacterium africanum, which also causes human tuberculosis
■ Mycobacterium leprae, the agent of leprosy, a disease affecting millions in Asia and Africa
■ mycobacteria other than tuberculosis (MOTT), such as Mycobacterium avium-intracellulare complex and Mycobacterium kansasii, which cause frequent disease in human immunodeficiency virus (HIV)-infected patients.
Mycobacterium tuberculosis
Habitat and transmission
Found in infected humans, mainly in the lungs; in the body, it resides primarily in the cells of the reticuloendothelial system; transmission is by coughing (droplet spread).
Characteristics
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 (Fig. 19.1). However, fluorescent microscopy, with auramine stain, is now used commonly for this purpose.
Culture and identification
This species does not grow on ordinary media and requires Lowenstein-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. 19.2).
Identification of mycobacteria is classically based on their rate of growth, optimum temperature requirements and pigment production in the presence or absence of light, together with biochemical tests. However, these slow procedures are being supplanted by more efficient nucleic acid amplification (polymerase chain reaction (PCR)) tests. Combination of nucleic acid tests together with a smear of sputum or the lesion with acid- and alcohol-fast bacilli is confirmatory of the disease.
Pathogenicity
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 granulomas, which have a concentric structure with a necrotic centre of caseation surrounded by a zone of multinucleated giant cells, monocytes and histiocytic cells with an outer ring of fibrosis; a consequence of cell mediated immune reactivity. No exotoxins or endotoxins.
Fig. 19.1 A sputum sample from a patient with tuberculosis stained with Ziehl- Neelsen stain showing the pink staining, slender, acid- and alcohol-fast bacilli. (Courtesy Professor Dr Willie Van Heerden, University of Pretoria, South Africa.)
Fig. 19.2 Growth of Mycobacterium tuberculosis on Lowenstein-Jensen medium: the bottle on the left is uninoculated; the bottle on the right shows ‘rough, tough and buff’ colonies of the organism.
Antibiotic sensitivity and control
Long-term (6-9 months) combination therapy with antituberculous drugs (isoniazid, rifampicin, pyrazinamide, ethambutol and streptomycin) is the corner stone of treatment. Tubercle bacilli are resistant to a number of antituberculous drugs, and multidrug-resistant tuberculosis (MDR-TB) and extensively drug-resistant tuberculosis (XDR) are a major worldwide concern. Hence, regimentation of drug delivery is critical for managing the disease, and 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.
Mycobacterium bovis
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 Lowenstein-Jensen medium and ready infection of rabbits.
Table 19.1 Comparison of the different types of leprosy
|
Tuberculoid |
Lepromatous |
|
|
Cell-mediated immunity |
++ |
- or ± |
|
Antibody response |
- |
++ |
|
Widespread lesions |
- |
+ |
|
Numbers of Mycobacterium |
± |
++ |
|
leprae in lesions |
||
|
++, predominant; +, common; ±, uncommon; -, absent. |
||
Mycobacterium leprae
Habitat and transmission
M. leprae is the agent of leprosy, and some 10 million cases of leprosy exist, mainly in Asia. Humans are the only known hosts of M. leprae, which resides mainly in the skin and nerves. Prolonged contact is thought to be the mode of transmission.
Characteristics
Aerobic, acid-fast bacilli (they are not alcohol-fast, i.e., decolourized by alcohol); no known toxins.
Culture and identification
Cannot be cultured in vitro but grows on the footpads of mice or armadillos, yielding chronic granulomas at the inoculation site.
Pathogenicity
The leprosy bacillus causes a slow, progressive, chronic disease that mainly affects the skin and the nerves; the lesions are predominantly seen in the cooler parts of the body. Two forms of leprosy are recognized (Table 19.1).
Lepromatous leprosy
The cell-mediated immune response is depressed or absent; M. leprae bacilli are usually seen in large numbers in the lesions and in blood; commonly involves mucosae, especially the nose (Fig. 19.3); leads to much disfigurement.
Tuberculoid leprosy
Associated with an intense cell-mediated immune response to the organisms; principally involves the nerves, with resultant anaesthesia and paraesthesia. Hence, damage to extremities is caused, with resultant loss of fingers and toes (see Chapter 35 for oral manifestations).
Antibiotic sensitivity and control
Antileprotic drugs are dapsone, rifampicin and clofazimine; several years of therapy are essential. As drug resistance is a growing problem, combination therapy, as in tuberculosis, is always given. No vaccine is available. Family contacts may be given dapsone.
Fig. 19.3 A patient with lepromatous leprosy. Note the saddle nose and associated general disfigurement and blindness.
Mycobacteria other than tuberculosis
MOTT is a collective name given to a group of mycobacteria of low human pathogenicity. These species include M. avium, M. intracellulare, M. kansasii, Mycobacterium marinum, Mycobacterium fortuitum and others.
Habitat and transmission
Isolated from soil, water, birds and animals.
Culture and identification
Grow on Lowenstein-Jensen medium but differ from 'pathogenic' mycobacteria in the colour of pigment produced and temperature requirements. Some species produce pigments in the dark (scotochromogens), others after exposure to light (photochromogens), and still others are non-chromogenic.
Pathogenicity and antibiotic sensitivity
In the main, MOTT cause pulmonary infection, often with M. tuberculosis; infections are especially seen in compromised individuals (e.g., in HIV disease). These mycobacteria are thought to be passengers in the disease process. They are usually sensitive to the normal antituberculous drugs.
M. marinum, associated with the keeping of tropical fish, causes skin ulcers.
Legionella
There are currently some 39 recognized species belonging to the genus Legionella, but Legionella pneumophila, the species first described, is the most important human pathogen. They cause atypical pneumonia, in both community dwellers and hospitalized patients.
Legionella pneumophila
Habitat and transmission
Ubiquitous organism found in soil and water, including airconditioning units, domestic and hospital water supplies, and sometimes in dental unit water systems. More than 50 species are known. They survive well within biofilms and free-living amoebae. Spread is known to occur by contaminated aerosols.
Characteristics
Gram-negative slender rods, which stain faintly with the standard Gram stain.
Culture and identification
Does not grow on ordinary media; grows slowly (3 weeks) in a special medium (cysteine-charcoal-yeast extract agar) under 5% carbon dioxide. Identification is by direct immunofluorescence.
Pathogenicity
The portal of entry is the respiratory tract and infection results in legionnaires' disease, a severe form of pneumonia. Patients at risk for infection include the immunocompromised, older men who smoke in excess, and with chronic lung disease, and diabetics. The clinical picture is variable, ranging from mild influenza-like illness to severe pneumonia with mental confusion, diarrhoea, haematuria and proteinuria. A less severe form of pneumonia (Pontiac fever) may be produced by some legionellae.
As there has been some concern that Legionella forms biofilms in stagnant dental unit water line and because of the possibility of dental patients acquiring legionellosis through aerosols, dental unit water line hygiene must be maintained at all times (see Chapter 38).
Antibiotic sensitivity and control
Erythromycin is the drug of choice and may be combined with rifampicin or ciprofloxacin.
It is impossible to eradicate the organism from water supplies as it is ubiquitous, but protective measures include increasing chlorine concentration and the temperature of hospital water supplies; aerosolization of water should be minimized.
Key facts
• Mycobacteria are acid-fast, beaded bacilli and resist decolourization with strong acids. Hence, a special stain, the Ziehl-Neelsen stain, is used to visualize them.
• The aforementioned property is due to the high lipid content (40%-60%) of the cell wall (mycolic acid), which is also an effective defence mechanism resisting phagocytosis.
• Mycobacterial infections are chronic, granulomatous (leads to granuloma formation) and insidious.
• Mycobacterium tuberculosis, the agent of tuberculosis, is a long, slender, non-sporing, beaded bacillus.
• It grows slowly (up to 6 weeks) in Lowenstein-Jensen medium as ‘rough, tough and buff’ colonies.
• Humans acquire tuberculosis through inhalation of infested droplet nuclei
• The increasingly common multidrug-resistant tuberculosis (MDR-TB) and extensive drug-resistant tuberculosis (XTB) are a global concern
• Leprosy, a disfiguring, chronic illness, is caused by Mycobacterium leprae.
• Up to 39 species belonging to the genus Legionella are recognized; Legionella pneumophila is the most important human pathogen.
• Legionellae are Gram-negative slender rods, but stain faintly with the standard Gram stain.
• L. pneumophila causes legionnaires’ disease, a condition that may range from a mild influenza-like illness to severe pneumonia with mental confusion, especially in the elderly.
Review questions (answers on p. 365)
Please indicate which answers are true, and which are false.
19.1 Which of the following statements of tuberculosis are true?
A. Mycobacterium tuberculosis is the organism solely responsible for human disease
B. Pathogenesis is characterized by granuloma formation and multiorgan involvement
C. tuberculosis is commonly seen in human immunodeficiency virus (HIV) disease
D. tuberculosis of the oral cavity is often the primary lesion
E. tuberculosis needs multiple drugs for effective treatment
19.2 Tuberculosis can be diagnosed:
A. by culturing the organism in Lowenstein-Jensen medium
B. by the Mantoux test
C. by using polymerase chain reaction-based tests
D. by demonstrating acid- and alcohol-fast bacilli in a sputum sample
E. by isolating the organism from blood cultures
19.3 Leprosy:
A. may cause facial disfigurement
B. is caused by Mycobacterium marinum
C. is associated with HIV disease
D. may lead to deformed extremities
E. bacillus can be cultured in footpads of mice
19.4 Legionella pneumophila:
A. is a Gram-positive slender rod
B. causes a debilitating pneumonia in the elderly and alcoholics
C. is often associated with faulty air-conditioning systems
D. is easily isolated in routine culture media
E. is often susceptible to erythromycin
Further reading
Bagg, J. (1996). Tuberculosis: A re-emerging problem for health care workers. British Dental Journal, 180, 376-381.
Brooks, J. F., Carroll, K. C., Butel, J. S., et al. (Eds.), (2013). Legionella (Chapter 22) and Mycobacteria (Chapter 23). In Jawetz, Melnick & Adelberg's medical microbiology (26th ed., pp. 255-263). New York: McGraw Hill. [e-Book].
Greenwood, D., Slack, R., & Peutherer, J. (Eds.), (2003). Chapters 18 and 34. In Medical microbiology (16th ed.). Edinburgh: Churchill Livingstone.
Ricci, M. L., Fontana, S., Pinci, F., et al. (2012). Pneumonia associated with a dental unit waterline. Lancet, 379, 684.