The purpose of this study was to determine the relative health risks of pigmented bacteria found in drinking water samples. These pigmented bacteria include opportunistic pathogens such as Flavobacterium, Pseudomonas, Corynebacterium, Nocardia, Mycobacterium, Erwinia, Enterobacter, Serratia, and Micrococcus. Flavobacterium meningosepticum is the most clinically important of the flavobacteria but causes <0.08% of meningitis cases in the US. Pseudomonas aeruginosa is an important opportunistic pathogen causing >10% of nosocomial infections. Human feeding tests show that oral doses of 106 to 108CFU results in colonisation of the gastrointestinal (GI) tract but do not produce any morbidity in healthy volunteers. Corynebacteria rarely cause disease except for toxigenic C. diphtheriae which is non-pigmented on R2A agar. The most important member of Nocardia is N. asteroides which is an opportunistic pathogen for certain immunocompromised population segments. National surveys show that only 9.2–19.2% of mycobacterial clinical isolates are pigmented with M. kansasii being the most frequent (4–10%). This bacterium is associated with disease but would not be detected as part of the HPC bacteria using a 7d incubation period. Pigmented strains of Erwinia, Enterobacter, Serratia, and Micrococcus can cause disease but they are relatively unimportant compared to other pathogenic bacteria as shown in ranking studies.

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