Factors influencing numbers of Mycobacterium avium, Mycobacterium intracellulare, and other mycobacteria in drinking water distribution systems

Eight water distribution systems were sampled over an 18-month period (528 water and 55 biofilm samples) to measure the frequency of recovery and numb er of mycobacteria, particularly Mycobacterium avium and Mycobacterium intr acellulare, in raw source waters before and after treatment and within the distribution system. The systems were chosen to assess the influence of sou rce water, treatment, and assimilable organic carbon levels on mycobacteria l numbers. Overall, mycobacterial recovery from the systems was low (15% of samples). Numbers of mycobacteria ranged from 10 to 700,000 CFU liter(-1), The number of M. avium in raw waters was correlated with turbidity. Water treatment substantially reduced the number of mycobacteria in raw waters by 2 to 4 log units. Mycobacterial numbers were substantially higher in the d istribution system samples (average, 25,000-fold) than in those collected i mmediately downstream from the treatment facilities, indicating that mycoba cteria grow in the distribution system. The increase in mycobacterial numbe rs was correlated with assimilable organic carbon and biodegradable organic carbon levels (r(2) = 0.65, P = 0.03), Although M. intracellulare was seld om recovered from water samples, it was frequently recovered (six of eight systems) in high numbers from biofilms (average, 600 CFU/cm(2)), Evidently, the ecological niches of M. avium and M. intracellulare are distinct.