Atypical Mycobacterium spp. multiply in a variety of suitable water environments,
notably biofilms. One of the most commonly occurring species is M. gordonae. Other
species have also been isolated from water, including M. avium, M. intracellulare, M.
kansasii, M. fortuitum and M. chelonae. High numbers of atypical Mycobacterium spp.
may occur in distribution systems after events that dislodge biofilms, such as flushing
or flow reversals. They are relatively resistant to treatment and disinfection and have
been detected in well operated and maintained drinking-water supplies with HPC less
than 500/ml and total chlorine residuals of up to 2.8 mg/litre. The growth of these
organisms in biofilms reduces the effectiveness of disinfection. In one survey, the
organisms were detected in 54% of ice and 35% of public drinking-water samples.