Alternative ribosome expression alone is insufficient to drive antibiotic tolerance in Mycobacterium bovis BCG

Mycobacterium tuberculosis and other members of the M. tuberculosis-complex are obligate pathogens of mammals. Their unique life and transmission cycle finds them in micro-environments with extremes in concentration of zinc. In the macrophage, zinc is pumped into the phagosome by the host cell causing zinc poisoning. However once M. tuberculosis exits from the phagosome and macrophage, the extracellular milieu and caseum of the tubercle lesion are zinc restricted due to zinc sequestration by the neutrophil protein calprotectin. To counter low zinc levels, M. tuberculosis expresses genes that are members of the "Zur" regulon. These include zinc uptake transporters, and five "alternative" ribosome paralogues that are zinc-independent. Under zinc replete conditions, the Zur protein is bound to zinc and acts as a transcriptional repressor, binding to conserved DNA sites in promoter regions of the Zur regulon. During zinc limiting conditions, the apo form of Zur dissociates from promoter regions allowing transcription of the Zur regulon including alternative ribosome proteins. The expression of alternative ribosomes by mycobacteria is of considerable interest due to the association of expression with antibiotic tolerance.

In this study, we have utilised a fluorescent reporter construct and flow cytometry to evaluate alternative ribosome expression in M. bovis BCG under a range of culture conditions and laboratory media.

Our data show two general scenarios under which alternative ribosome expression is induced - under zinc restriction and in extended stationary phase under certain culture conditions. Zinc-restricted bacilli were not found to be more tolerant to antibiotic treatment than those grown in zinc-replete media. For bacilli in extended stationary phase, alternative ribosome expression did not consistently correlate with tolerance to streptomycin or rifampicin.

Our data suggest that rather than causing antibiotic tolerance, expression of alternative ribosomes can occur concurrently with other physiological changes that result in antibiotic tolerance.