Ga. Cangelosi et al., Colony morphotypes on Congo red agar segregate along species and drug susceptibility lines in the Mycobacterium avium-intracellulare complex, MICROBIO-UK, 145, 1999, pp. 1317-1324
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.