Pathobiological significance of colony morphology in Mycobacterium avium complex

Mycobacterium avium complex (MAC) strains are known to exhibit variation in colony morphology. In addition to the smooth transparent (ST), smooth opaque (SO) and rough opaque (RO), which are the most common morphological forms, intermediate (IM) and pin point (PP) forms were also occasionally observed. In order to understand the pathobiological significance of these different colony forms, we investigated their virulence in beige mice, ability to bind to plastic and epithelial cells, differences in the lipids, and modulation of macrophage functions by the bacillary extracts. ST variants, the most common form seen in AIDS patients, were more virulent with increased multiplication in lungs, livers and spleens of beige mice and showed increased adherence to plastic and epithelial cells. SO, RO, PP colonial forms did not show increase in growth in any of the organs over a period of 4 weeks. IM colonial variants showed increased growth in lungs and spleens but not in livers. Thin layer chromatographic (TLC) analysis of lipid extracts showed one specific component in the high polar lipids of the SO variant, while ST variant did not show any specific component in any of the three families of lipids (high, intermediate and low polarity). The RO variant either expressed low levels or lost many of the components of lipids of high and intermediate polarity, however produced increased levels of lipids of low polarity. One of the components of low polar lipids was specific for RO variant and was produced in large quantity. The isogenic variants differed in the total lipid and sugar contents and also differed in their ability to modulate macrophage functions.