DESCRIPTION OF ATYPICAL MYCOBACTERIA
Atypical mycobacteria have been divided into four provisional groups according to the capacity of the cultures to become yellow on exposure to light (photochromogens, group I), or to produce an orange colour in the dark of the incubator (scotochromogens, group II), or to remain colourless (nonchromogens, group III), or to grow in three days (rapid growers, group IV). This grouping has proved valuable and is widely used.
Further help in identification may be obtained from the bacterial morphology, colonial appearance, the effect on growth of incubating at low, medium, and high temperatures (say, 25°C, 37°C, 45°C); absence of niacin (nicotinic acid); catalase and arylsulphatase activity; susceptibility or resistance to, or effect on, certain chemicals; the pattern of sensitivity to anti-tuberculosis drugs; their pathogenicity or lack of it for guinea-pigs, rabbits, chickens and mice; precipitation and agglutination reactions; bacteriophage typing; and lipid analysis.
Some of these tests may be carried out in the hospital laboratory but the last three procedures are practicable only in certain reference centres. The most important photochromogen is Mycobacterium kansasii. The growth at first resembles Mycobacterium tuberculosis, but when a young culture is exposed for an hour to light in the presence of air and then re-incubated it becomes canary yellow. Mycobacterium kansasii infect€ the lungs of middle-aged men especially if there has been pre-existing disease. Consequently it is found in cities and industrial areas where the incidence of chronic bronchitis and lung damage due to air pollution is high. Women are seldom affected.
There is little evidence that the organism is transmitted from man to man, and its source is unknown. It is the commonest pathogen among the opportunistic mycobacteria . The scotochromogens are widely distributed in the environment and they occasionally contaminate cultures of tubercle bacilli. Some rather ill-defined strains—the scrofula scotochromogens —give rise to a mild cervical adenitis in children which simulates infection with the tubercle bacillus. The nonchromogens, which produce no colour in the dark or after exposure to light, include Mycobacterium intracellulare, formerly known as the Battey bacillus.
It is so closely related to Mycobacterium avium that some investigators regard them both as members of the same species. Infection with Mycobacteria intracellulare occurs particularly in the South Eastern states of the U.S.A. and in Western Australia, and in many warm climate countries. It is responsible for widespread, latent infection and tissue hypersensitivity which probably reduces the efficacy of BCG vaccination. It occasionally causes serious, progressive, pulmonary disease especially in middle-aged men whose lungs have already been damaged by air pollutants, and is, therefore, as a pathogen associated with urban and industrial life. Pulmonary disease due to Mycobacterium intracellulare is refractory to chemotherapy.
The source of the organism is obscure, but mycobacteria resembling or apparently identical with it have been isolated from soil. Human latent infections and tissue hypersensitivity may occur through contaminated skin lesions. Mycobacterium avium, another nonchromogen and the cause of tuberculosis in birds, may be endemic in flocks of poultry. Pigs that are closely associated with affected poultry may acquire an infection of the neck glands. Cattle may also be infected. Infection in man is rural rather than urban. The organism is an occasional cause of cervical adenitis in children. It may attack the damaged lungs of middle-aged men, and be the cause of a progressive, generalized infection with fatal outcome.
Fortunately, these last two infections are rare as they respond poorly to chemotherapy. A third nonchromogen is Mycobacterium xenopi which was first isolated from a cutaneous lesion in the South African toad, Xenopus laevis. It occasionally causes chronic lung infection in man: Mycobacterium xenopi is a thermophile, that is, it grows at 45°C as does Mycobacteria avium ; and it is moderately pathogenic for chickens. Because human infections have a coastal and estuarine distribution the mycobacterium may be derived from sea birds. It is susceptible to some of the antituberculosis drugs. The rapid growers are usually saprophytes or commensals. They appear from time to time in pathological specimens and may be a source of confusion. Mycobacteria smegmatis which is found in smegma (the sebum beneath the prepuce or around the clitoris and on the labia minora) may contaminate specimens of urine and may be seen as acid-fast rods in films of the deposits stained with the Ziehl-Neelsen stain.
They usually do not resist decolourization with alcohol nor do they usually survive treatment with 4 per cent sodium hydroxide. One rapid grower, Mycobacterium fortuitum, is a pathogen of fish, amphibia and reptiles, and it may be recovered from water, soil and manure. In man it has been isolated from pustules and superficial abscesses, sometimes at the site of an injection. It has on rare occasions been the cause of lymphadenitis and pulmonary infections. It may harmlessly colonize the lungs of pneumoconiotics for a time without causing damage . When first isolated it will grow rapidly at 37°C. Rapid growth for a mycobacterium is 4 to 5 days.