Colony morphotypes on Congo red agar segregate along species and drug susceptibility lines in the Mycobacterium avium-intracellulare complex

Isolates of the Mycobacterium avium-intracellulare complex (MAC) have long been known to segregate into transparent opaque and rough colony morphotype s that differ from each other in clinically important parameters including drug susceptibility and virulence. Here the authors report additional morph otypic variation that occurs on two levels: interspecific (between M. avium and M. intracellulare) and intraspecific (within individual M. avium isola tes). Clinical isolates of M. avium grown on Congo red (CR) plates formed r ed, pink or mixed (red and white) opaque colonies, while M. intracellulare isolates formed purely white opaque colonies. A quantitative CR binding ass ay showed that this interspecific differential applies to transparent as we ll as opaque colony variants; however, it was less pronounced among laborat ory reference strains than among recent clinical isolates. Opaque colonies of M. avium isolates with 'mixed' phenotypes segregated into stable opaque red and white variants with shared IS1245 banding patterns (intraspecific s egregation). White segregants of M. avium were more flocculent and signific antly more resistant to ciprofloxacin and rifamycin drugs than were red seg regants. Thus, cultivation on CR agar revealed a previously unknown multidr ug resistant colony morphotype of M. avium.