Mild stress conditions during laboratory culture promote the proliferation of mutations that negatively affect sigma B activity in Listeria monocytogenes

Guerreiro DN, Wu J, Dessaux C, Oliveira AH, Tiensuu T, Gudynaite D, Marinho CM, Boyd A, García-del Portillo F, Johansson J, O’Byrne CP. 2020. Mild stress conditions during laboratory culture promote the proliferation of mutations that negatively affect sigma B activity in Listeria monocytogenes. J Bacteriol 202:e00751-19. https://doi.org/10.1128/jb.00751-19

Abstract

In Listeria monocytogenes, the full details of how stress signals are integrated into the σB regulatory pathway are not yet available. To help shed light on this question, we investigated a collection of transposon mutants that were predicted to have compromised activity of the alternative sigma factor B (σB). These mutants were tested for acid tolerance, a trait that is known to be under σB regulation, and they were found to display increased acid sensitivity, similar to a mutant lacking σB (ΔsigB). The transposon insertions were confirmed by whole-genome sequencing, but in each case, the strains were also found to carry a frameshift mutation in the sigB operon. The changes were predicted to result in premature stop codons, with negative consequences for σB activation, independently of the transposon location. Reduced σB activation in these mutants was confirmed. Growth measurements under conditions similar to those used during the construction of the transposon library revealed that the frameshifted sigB operon alleles conferred a growth advantage at higher temperatures, during late exponential phase. Mixed-culture experiments at 42°C demonstrated that the loss of σB activity allowed mutants to take over a population of parental bacteria. Together, our results suggest that mutations affecting σB activity can arise during laboratory culture because of the growth advantage conferred by these mutations under mild stress conditions. The data highlight the significant cost of stress protection in this foodborne pathogen and emphasize the need for whole-genome sequence analysis of newly constructed strains to confirm the expected genotype.

Funder

European Union’s Horizon 2020
Department of Agriculture, Food and the Marine

Publisher

American Society for Microbiology

Rights

CC BY-NC-ND

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Collections

Microbiology (Scholarly Articles)