Fitness Trade-Offs in Competence Differentiation of Bacillus subtilis

Abstract

In the stationary phase, Bacillus subtilis differentiates stochastically and transiently into the state of competence for transformation (K-state). The latter is associated with growth arrest, and it is unclear how the ability to develop competence is stably maintained, despite its cost. To quantify the effect differentiation has on the competitive fitness of B. subtilis, we characterized the competition dynamics between strains with different probabilities of entering the K-state. The relative fitness decreased with increasing differentiation probability both during the stationary phase and during outgrowth. When exposed to antibiotics inhibiting cell wall synthesis, transcription, and translation, cells that differentiated into the K-state showed a selective advantage compared to differentiation-deficient bacteria; this benefit did not require transformation. Although beneficial, the K-state was not induced by sub-MIC concentrations of antibiotics. Increasing the differentiation probability beyond the wt level did not significantly affect the competition dynamics with transient antibiotic exposure. We conclude that the competition dynamics are very sensitive to the fraction of competent cells under benign conditions but less sensitive during antibiotic exposure, supporting the picture of stochastic differentiation as a fitness trade-off.

A DegU-P and DegQ-Dependent Regulatory Pathway for the K-state in Bacillus subtilis

“…The K-state appears to be a persistent state in which a trade off between the costs of growth-arrest and the expression of the competence machinery are balanced by fitness benefits due to tolerance in the face of environmental insults, e.g., exposure to antibiotics (Nester and Stocker, 1963; Johnsen et al, 2009; Hahn et al, 2015; Yuksel et al, 2016), as well as the ability to acquire useful genetic information. In the face of this trade-off, bet-hedging due to the bistable expression of the K-state, presumably helps to maximize fitness (Dubnau and Losick, 2006; Veening et al, 2008).…”

“…Because these genes are not needed for transformation (J. Hahn, unpublished) the expression of ComK must result in phenotypes beyond competence that presumably enhance fitness. In fact, ComK also induces a period of growth arrest during which the cells that express ComK exhibit antibiotic tolerance (Nester and Stocker, 1963; Haijema et al, 2001; Johnsen et al, 2009; Briley et al, 2011; Hahn et al, 2015; Yuksel et al, 2016). This persistent state has been called the K-state, to emphasize that ComK regulates more than competence for transformation (Berka et al, 2002).…”

Experimental evolution of Bacillus subtilis

“…The K-state, for example, would be poorly adapted to a nutrient-rich environment. Yet these growth-arrested B. subtilis cells in laboratory tests show much greater resistance to antimicrobial compounds, which would likely be produced by neighbours themselves facing starvation, potentially increasing the likelihood that the cell and its genome will survive (Yuksel et al, 2016). The transformation competence associated with the K-state could further enhance the cell's survival chances, whether it imports homologous DNA and repairs its genome by recombination or imports heterologous DNA and acquires potentially beneficial genetic traits (Norman et al, 2015), or possibly simply uses DNA as a nutrient (Johnston et al, 2014).…”

Adaptive evolution of hybrid bacteria by horizontal gene transfer

“…Since competence development is not subject of this study, we circumvented this problem by setting the expression of the master regulator for competence, comK , under the control of an IPTG-inducible promoter. The recipient strain Bs166 ( his leu met amyE::P hs comK(spc) comK::kan ) was generated by transforming strain BD3836 (3) with genomic DNA from strain Bs075 (4). For competition experiments, we generated the reporter strain Bs175 ( his leu met amyE::P hs comK(spc) comK::kan lacA::PrrnE-gfp (erm) ) as follows.…”

“…For competition experiments, we generated the reporter strain Bs175 ( his leu met amyE::P hs comK(spc) comK::kan lacA::PrrnE-gfp (erm) ) as follows. PrrnE-gfpmut2 was amplified from genomic DNA of Bs139 (4) using primers MeY87 (5’-AAGGAATTCCGGATCCGAAGCAGGTTAT-3’) and MeY88 (5’-GGACTAGTTTATTTGTATAGTTCATCCATGC-3’). Both the PCR-fragment and the donor plasmid pBS2E (obtained from the Bacillus subtilis Stock Center) were double digested with EcoRI and SpeI.…”