Affiliation:
1. Department of Immunology and Microbiology, University of Colorado Anschutz Medical Campus, Aurora, Colorado, USA
2. Division of Infectious Diseases, Department of Medicine, University of Colorado Anschutz Medical Campus, Aurora, Colorado, USA
Abstract
Bacteria use population heterogeneity, the presence of more than one phenotypic variant in a clonal population, to endure diverse environmental challenges – a ‘bet-hedging’ strategy. Phenotypic variants have been described in many bacteria, but the phenomenon is not well-understood in mycobacteria, including the environmental factors that influence heterogeneity. Here, we describe three reproducible morphological variants in
M. smegmatis
– smooth, rough, and an intermediate morphotype that predominated under typical laboratory conditions.
M. abscessus
has two recognized morphotypes, smooth and rough. Interestingly,
M. tuberculosis
exists in only a rough form. The shift from smooth to rough in both
M. smegmatis
and
M. abscessus
was observed over time in extended static culture, however the frequency of the rough morphotype was high in pellicle preparations compared to planktonic culture, suggesting a role for an aggregated microenvironment in the shift to the rough form. Differences in growth rate, biofilm formation, cell wall composition, and drug tolerance were noted among
M. smegmatis
and
M. abscessus
variants. Deletion of the global regulator lsr2 shifted the
M. smegmatis
intermediate morphotype to a smooth form but did not fully phenocopy the naturally generated smooth morphotype, indicating Lsr2 is likely downstream of the initiating regulatory cascade that controls these morphotypes. Rough forms typically correlate with higher invasiveness and worse outcomes during infection and our findings indicate the shift to this rough form is promoted by aggregation. Our findings suggest that mycobacterial population heterogeneity, reflected in colony morphotypes, is a reproducible, programmed phenomenon that plays a role in adaptation to unique environments and this heterogeneity may influence infection progression and response to treatment.
Funder
National Institute of Allergy and Infectious Diseases