Nonphotochromogens

Nonphotochromogens

Nonphotochromogens are the mycobacteria that do not produce pigment in dark or on exposure to light. These include M. aviumcomplex (MAC), Mycobacterium xenopi, and  Mycobacterium ulcerans. 

Group III nonphotochromogensalso includes Mycobacterium terrae, Mycobacterium triviale, and Mycobacterium nonchromogenicum, which rarely cause infectionsin humans.

M.aviumcomplex: MAC consists of M.avium and Mycobacterium intracellulare. They are found commonly in theenvironment such as in soil and in water (brackish, ocean, and drinking water). MAC organisms are strongly acid-fast mycobacteria with a cell wall rich in lipids. They are weakly positive aerobic bacteria. This complex possesses 28 agglutination types. Types 1, 2, and 3 are considered as M. avium and other types as M. intracellulare. Ability to grow inside the cells is the main virulence determinant of the bacteria. In infected humans, development of the disease depends primarily on response of host to infection. MAC produces a disease similar to pulmonary tuberculosis in patients with compromised pulmonary function, such as patients with chronic bronchitis or obstructive pulmonary disease, i.e., previous damage to lungs as a result of infection or other disease.

      In patients with human immunodeficiency virus (HIV) infection, MAC produces a typically disseminated disease affecting virtually every organ in infected humans. Intensity of infections caused by MAC in these patients is very high. In some patients, tissues are overloaded with mycobacteria, and blood contains nearly 100–1000 mycobacteria/mL of blood. The disseminated infection is commonly seen in persons at the terminal stage of the immunological disorder, with their CD4 count falling below 10 cells/cu mm.

      Patients with acquired immunodeficiency syndrome (AIDS) acquire MAC infection following inhalation of infectious aerosols. The infection is also acquired by ingestion of the water or food contaminated with mycobacteria. Person-to-person transmission does not occur.

      Diagnosis of infection is made by microscopy of clini-cal specimens and culture on LJ medium. Treatment with clarithromycin or azithromycin combined with ethambutol and rifabutin for a long period is effective for the treatment of infection. Chemoprophylaxis with clarithromycin or azithro-mycin in AIDS patients with low CD4 cell count has reduced the incidence of MAC drastically in these patients.

M. xenopi was originally isolated from a cutaneous lesionof a South African toad (Xenopus laevis). The organism is a thermophilic mycobacterium, which may occasionally cause chronic pulmonary lesions in humans. Most of these infec-tions have been documented from London. The bacteria have been isolated from mostly hot water taps and also from main water supplies in hospitals.

M. ulcerans is a skin pathogen, which was originally iso-lated from human ulcerative skin lesions in Australia (1958). Subsequently, the species causing similar cutaneous lesions have been documented from Uganda (Buruli ulcer), Nigeria, Congo, Malaysia, New Guinea, and Mexico. M. ulcerans grows slowly on LJ medium in 4–8 weeks when incubated at 31–34°C. However, this species fails to grow when incubated at 37°C in primary culture.

Ulcers of the skin are typically seen on the legs or on the arms. After an incubation period of few weeks, initially the infection begins by minor injuries on the skin through which mycobac-teria gain access. The infection begins with the appearance of an indurated nodule at the site of inoculation, which breaks down forming an indolent ulcer. Diagnosis of the condition is made by microscopy of the smears, collected from the edge of the ulcer. Large number of acid-fast and alcohol-fast bacilli are seen in the stained smear by microscopy. Finally, the ulcer heals with formation of scars.