Isolates of the complex (MAC) have long been known to segregate into transparent, opaque and rough colony morphotypes that differ from each other in clinically important parameters including drug susceptibility and virulence. Here the authors report additional morphotypic variation that occurs on two levels: interspecific (between and ) and intraspecific (within individual isolates). Clinical isolates of grown on Congo red (CR) plates formed red, pink or mixed (red and white) opaque colonies, while isolates formed purely white opaque colonies. A quantitative CR binding assay showed that this interspecific differential applies to transparent as well as opaque colony variants; however, it was less pronounced among laboratory reference strains than among recent clinical isolates. Opaque colonies of isolates with ‘mixed’ phenotypes segregated into stable opaque red and white variants with shared IS banding patterns (intraspecific segregation). White segregants of were more flocculent and significantly more resistant to ciprofloxacin and rifamycin drugs than were red segregants. Thus, cultivation on CR agar revealed a previously unknown multi-drug resistant colony morphotype of


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