Photochromogens

Photochromogens

These strains do not produce any pigments in the colonies that are incubated in dark, but form pigments when the young culture is exposed to light for 1 hour in the presence of air and is reincubated for 24-48 hours. These bacteria produce yellow-orange pigments during such conditions. These mycobacte-ria are slow-growing ones, but grow faster (after 7 days) than the tubercle bacilli. The most common species are M. kansasii,

Mycobacterium marinum, and Mycobacterium simiae. Some ofthese mycobacteria produce disease similar to pulmonary tuberculosis. Most of these mycobacteria have been isolated from water and soil in the environment.

    kansasii reduces nitrates to nitrites (Color Photo 43) andshows a positive Tween 80 hydrolysis (Color Photo 44) test within 3–6 hours, while other mycobacteria species produce positive test within 3–10 days. M. kansasii causes a disease identical to pulmonary tuberculosis, which is associated with formation of cavity and scarring, usually in the upper lobe of the lungs. This species is the second most common NTM, next to Mycobacterium avium complex (MAC), as causative agent of lungdiseases. These strains have been frequently isolated from tap water, and infected tap water is believed to be the major source of infection. M. kansasii grows rapidly on LJ medium, producing visible colonies within 2 weeks. It is sensitive to rifampicin and other antitubercular drugs.

      marinum closely resemblesM. kansasiibut is differenti-ated from it by its (a) poor growth at 37°C, (b) failure to reduce nitrate to nitrite, and (c) failure to produce the enzyme catalase. M. marinum—originally isolated from fish—is the causativeagent of swimming pool or fish tank granuloma. This condi-tion is associated with development of superficial granuloma-tous lesions in the skin.

      simiae associated with pulmonary disease in humans wasoriginally isolated from monkeys. This species grows well at 37°C and is niacin positive, like M. tuberculosis.