Which nonpathogenic Mycobacterium species is often isolated from clinical specimens and known as the 'tapwater bacillus'?

The species referred to as the 'tapwater bacillus' is M. gordonae. This mycobacterium is commonly found in environmental sources such as water, soil, and dust. It is considered nonpathogenic and is often recovered from clinical specimens, leading to its characterization as a contaminant in laboratory settings.

M. gordonae has garnered attention not only for its environmental prevalence but also because its presence in clinical samples can sometimes lead to confusion in diagnosis. It lacks significant virulence factors and does not typically cause disease in humans, which solidifies its status as a nonpathogenic species in contrast to other mycobacteria that are known to cause disease.

In contrast, other species like M. kansasii and the M. avium-intracellulare complex have been implicated in clinical disease, particularly in immunocompromised individuals. M. leprae, responsible for leprosy, is another pathogenic species known for its specific disease association and is not considered a contaminant from environmental sources. This context illustrates why M. gordonae is distinctly labeled as the 'tapwater bacillus.'