Studies on the Smooth and Rough colony morphotypes of Mycobacterium abscessus and their relevance to infection transmission

Mycobacterium abscessus (Mab) is an opportunistic pathogen causing severe infections in patients with pre-existing lung disease, notably cystic fibrosis; it was first described in 1953 after isolation from a knee abscess (type strain Mab ATCC 19977). Despite its presumed environmental reservoir, recent evidence indicates patient to patient transmission.

Mab is divided into two colony morphotypes, Smooth (S) and Rough (R); the former are associated with high production of glycopeptidolipid (GPL) while the latter have less or no GPL production. S to R transition is based on mutations that eliminate GPL expression. It was hypothesized that Smooth and Rough (R) colony morphotypes reflect differences in cellular hydrophobicity, which in turn affect capacity for aerosol transmission.

The properties of type and clinical S and R variants were compared. R strains grew differently in broth and biofilm assays, were consistently more hydrophobic and showed no demonstrable GPLs. However, no differences between S and R were observed in Goldberg drum aerosol or desiccation survival experiments and the ATCC S strain was clearly more resistant to UV exposure. Preparation of ffluc (luciferase)-expressing S and R strains facilitated the desiccation and UV assessments and enabled analysis of hydrophobicity attributable to the variants in mixed cultures. In the latter case, the S strain appeared to be as hydrophobic as the R when the two strains we mixed. It was speculated that redistribution of GPLs might be responsible for this and that this might explain the lack of difference between S and R observed in the aerosol survival experiments. Finally, genome and transcriptome analyses revealed several new and previously recognised polymorphisms associated with R transition.

Overall, little support for R strains being adapted to airborne transmission was found and a wider range of genomic targets than previously recognised appeared to underpin S to R transition.