these are species that normally exist as saprophytes of soil and water
four groups of mycobacteria had been classified based to their associate with human disease according to their production of yellow or orange pigment and their rate of growth :- Group 1 - photochromogens (pigmentation on exposure to light),Group 2 - scotochromogens (pigmentation formed in the dark),Group 3 - non-chromogens (no pigmentation) and Group 4 - rapid growers
all strains in Group 1,2, and 3 grow slowly
the photochromogens are colourless when incubated in the dark,but if young cultures are exposed to a light source for an hour or more and then re-incubated,they develop a bright yellow or orange coloration. The caps of the culture bottles must be loosened during exposure to light as oxygen is essential for pigment formation
a) Clinical Importance
i) Photochromogens
contain three species :- M.kansaii,M.simiae,M.marinum which the first two grow well at 37 degrees whereas the latter grow better at 33 degrees. M.kansaii and M.simiae had been isolated from cases of pulmonary disease,whereas M.marinum,previously termed the fish tubercle bacillus is the cause of a warty skin infection known as swimming pool granuloma
ii) Scotochromogens
the principal pathogen in this group is M.scrofulaceum and is associated with Scrofula or cervical lymphadenitis,although it will also causes pulmonary disease
other species includes :- M.szulgai and M.aquae,both are rare cause of pulmonary disease
iii) Non-chromogens
the most prevalent and important opportunistic pathogens of humans are avian tubercle bacillus,M.avium and the closely related M.intracellulare,formerly known as the battey bacillus
responsible for lymphadenitis,pulmonary lesions and disseminated disease,notably in patients with the acquired immune deficiency syndrome (AIDS)
other pathogens includes :- M.xenopi,isolated from Xenopus toad,is a thermophile and grows well at 45 degrees. It is principally responsible for pulmonary lesions and is of limited geographical distribution. M.malmoense is a cause of pulmonary disease and lymphadenitis. It grows very slowly,often taking as long as 10 weeks to appear on primary culture and is therefore likely to be missed if cultures are not maintained for this length of time. M.ulcerans,the cause of buruli ulcer,is a very slowly growing species which will only grow in vitro between 31 to 34 degrees. Colonies are non-pigmented or a pale lemon-yellow color. Unlike other mycobacterial pathogens,M.ulcerans produces a toxin which can cause tissue necrosis and is involved in the pathogenesis of the disease
iv) Rapid growers
only two of the rapidly growing species M.chelonae and M.fortuitum,are well-recognized pathogens of humans
these two species were originally identified as the turtle and frog tubercle bacilli
they occasionally cause pulmonary or disseminated disease but are principally responsible for post-injection abscesses and wound injections including corneal ulcers
b) Ecology
wet soil,marshland,streams,rivers and estuaries
able to colonize piped-water supplies,thus human beings are exposed to mycobacteria as a resulting of drinking,washing,showering and inhalation of natural aersols
c) Disease due to environmental mycobacteria
four types of opportunist mycobacterial disease of humans have been described : skin lesions,following traumatic inoculation of bacteria,localized lymphadenitis,tuberculosis-like pulmonary lesions and disseminated disease
lymphadenitis is caused by a number of different species and in most cases,a single node,usually a tonsillar node is involved and the majority of patients are children aged less than 5 years
lymphadenitis occasionally occurs as part of a more disseminated infection particularly in individuals with AIDS
three types of skin lesions have been described :- post-injection (and post-traumatic) abscesses,swimming pool granuloma,buruli ulcer
post-injection abscesses are usually caused by the rapidly growing pathogens M.chelonae and M.fortuitum. It is due to batches of injectable materials being contaminated by these bacteria
abscesses develop within a week or so or up to a year or more after the injection. They are painful and may become quite large
treatment is by drainage with curettage or total excision. Chemotherapy is not necessary unless there is a local spread of disease or multiple abscesses,as may occur in insulin-dependent diabetics
swimming pool granuloma is caused by M.marinum and most of those affected are users of swimming pools,keepers of tropical fish and other involved in aquatic hobbies
the bacilli enter scratches and abrasions and cause warty lesions and these lesions usually occur on the knees and elbows of swimmers and on the hands of aquarium keepers. This is termed sporotrichoid spread
buruli ulcer is caused by M.ulcerans and the geographical distribution is limited to certain localities,which characteristically,are low-lying marshy areas subject to periodic flooding
M.ulcerans is a free living species that is introduced into the human dermis by minor injuries,particularly by spiky grasses
the first manifestation of the disease is a hard cutaneous nodule which is often itchy. This enlarges and develops central softening and fluctuation due to necrosis of the underlying adipose subcutaneous tissues caused by a toxin. The overlying skin becomes anoxic and breaks down,the liquefied necrotic contents of the lesion are discharged and one or more ulcers with deeply underminded edges are therefore formed
during this anergic stage,the lesion may progressively extend to an enormous size,sometimes involving an entire limb or a major part of the trunk
for unknown reasons,the anergic phase eventually gives way to an immunoreactive phase when a granulomatous response develops in the lesion,the acid-fast bacilli disappear and the patient reacts to burulin (a skin test reagent prepared from M.ulcerans). Healing then occurs but the patient is often left with considerable disfigurement and disability due to extensive scarring and contractures
pulmonary disease is most frequently seen in middle-aged or elderly men with lung damage due to smoking or exposure to industrial dusts. It also occurs in individuals with cogenital or acquired immune deficiencies,malignant disease and cystic fibrosis but a substantial minority of cases occur in persons with no apparent underlying localized or generalized disorder
disseminated disease is caused by M.avium complex and it occurs in individuals with cogenital or acquired immune deficiencies and other such as renal transplantation
d) Treatment
most environmental mycobacteria are resistant to many antituberculosis drugs in vitro
regimens containing five or six antituberculosis drugs have been used for pulmonary disease due to the M.avium complex,M.kansaii,M.xenopi,M.malmoense,but equal success has been obtained with standard triple therapy - rifampicin,isoniazid and ethambutol
