Growth in Aging Colonies The Importance of Being Different

Sammanfattning: The accumulation of rifampicin resistant (RifR) mutants in aging bacterial colonies has previously been attributed to stress-induced mutagenesis. Mutations giving rise to RifR are located in the rpoB gene, coding for the ? subunit of RNA polymerase, RpoB. We showed that these mutants accumulate because they grow faster than the wild-type in the aging colonies. We found no evidence of increased mutagenesis in the RifR cells and their distribution, as localized clones in the aging colonies, indicated that they accumulated by selection and growth rather than by an increased rate of mutagenesis. Colony competition experiments with reconstructed strains showed that the RifR mutations were responsible for the growth advantage. We also found that deletion of rpoS, coding for the stationary phase sigma factor (RpoS), also gives a growth advantage on bacterial cells in aging colonies.We suggest that mutants lacking RpoS, having a different transcription pattern than the wild-type, may override the signals to enter stationary phase together with the rest of the population and instead keep growing for as long as possible. We found that the rpoB mutants mimicked the transcription pattern of the rpoS deletion mutant, thereby displaying a similar phenotype in the aging colonies. The pathways used in acetate metabolism (consisting of the enzymes Acs, AckA-Pta, PoxB and AceBAK) were shown to be important for the growth advantage mutants suggesting that acetate is one of the main carbon sources used to support their prolonged growth in the aging colonies.Rifampicin is a first-line drug used to treat M. tuberculosis infections. We used S. enterica as a model system for experimental evolution to ask whether compensatory mutations might be important in RifR mutants. In every lineage evolved compensatory mutations arose without any significant reduction in resistance. These mutations altered genes for the ?, ?, and ?’ subunits of RNA polymerase.