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

Abstract

Isolates of the Mycobacterium avium-intracellulare 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 M. avium and M. intracellulare) and intraspecific (within individual M. avium isolates). Clinical isolates of M. avium grown on Congo red (CR) plates formed red, pink or mixed (red and white) opaque colonies, while M. intracellulare 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 M. avium isolates with 'mixed' phenotypes segregated into stable opaque red and white variants with shared IS1245 banding patterns (intraspecific segregation). White segregants of M. avium 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 multidrug resistant colony morphotype of M. avium.

Growth, Congo Red Agar Colony Morphotypes and Antibiotic Susceptibility Testing of Mycobacterium avium subspecies paratuberculosis

“…They are smooth opaque, smooth transparent, and rough. 34 Smooth variants have a dome-shaped appearance, whereas rough variants are characterized by flat, transparent colonies with irregular edges. M. avium subsp.…”

“…Transparent variants predominate in fresh clinical isolates, whereas opaque variants appear with repeated passage in vitro. 34 Conversion to the rough phenotype is irreversible and involves a chromosomal deletion. 34 This change has been observed in M. avium subsp.…”

“…34 Conversion to the rough phenotype is irreversible and involves a chromosomal deletion. 34 This change has been observed in M. avium subsp. avium following passage of the isolate through animals 43 in which these variants grew at a faster rate in vivo and were more virulent resulting in 60% to 70% mortality versus 10% for their smooth counterparts.…”

“…avium has been shown to undergo red-white morphotypic switching when grown on agar media containing Congo Red. 28,34 White opaque variants were found to be more resistant to multiple antibiotics than red opaque variants.…”

Pathophysiological Implications of Cell Envelope Structure in Mycobacterium tuberculosis and Related Taxa

“…Preliminary studies are under way to compare the hydrophobicity of "M. canettii" and M. tuberculosis. For cultures grown on solid media in the presence of Congo Red [93], the smooth colonies of M. kansasii and "M. canettii" resisted staining by the dye, but two different rough strains of M. tuberculosis absorbed so much dye that they almost merged into the background (Figure 12). In preliminary standard partitioning experiments between hexadecane and water [94], it was found that M. tuberculosis was approximately one fifth more hydrophobic than "M. canettii" (details not shown).…”