ABSTRACT
The UV resistance of bacterial endospores is an important quality supporting their survival in inhospitable environments and therefore constitutes an essential driver of the ecological success of spore-forming bacteria. Nevertheless, the variability and evolvability of this trait are poorly understood. In this study, directed evolution and genetics approaches revealed that the Bacillus cereus pdaA gene (encoding the endospore-specific peptidoglycan-N-acetylmuramic acid deacetylase) serves as a contingency locus in which the expansion and contraction of short tandem repeats can readily compromise (PdaAOFF) or restore (PdaAON) the pdaA open reading frame. Compared with B. cereus populations in the PdaAON state, populations in the PdaAOFF state produced a lower yield of viable endospores but endowed them with vastly increased UV resistance. Moreover, selection pressures based on either quantity (i.e., yield of viable endospores) or quality (i.e., UV resistance of viable endospores) aspects could readily shift populations between PdaAON and PdaAOFF states, respectively. Bioinformatic analysis also revealed that pdaA homologs within the Bacillus and Clostridium genera are often equipped with several short tandem repeat regions, suggesting a wider implementation of the pdaA-mediated phase variability in other sporeformers as well. These results for the first time reveal (1) pdaA as a phase-variable contingency locus in the adaptive evolution of endospore properties and (2) bet-hedging between what appears to be a quantity versus quality trade-off in endospore crops.
ABSTRACT
The specificity of phages and their ability to evolve and overcome bacterial resistance make them potentially useful as adjuncts in the treatment of antibiotic-resistant bacterial infections. The goal of this study was to mimic a natural grouping of phages of interest and to evaluate the nature of their proliferation dynamics with bacteria. We have, for the first time, transferred naturally occurring phage groups directly from their sources of isolation to in vitro and identified 13 P. aeruginosa and 11 K. pneumoniae phages of 18 different genera, whose host range was grouped as 1.2-17%, 28-48% and 60-87%, using a large collection of P. aeruginosa (n = 102) and K. pneumoniae (n = 155) strains carrying different virulence factors and phage binding receptors. We introduced the interpretation model curve for phage liquid culturing, which allows easy and quick analysis of bacterial and phage co-proliferation and growth of phage-resistant mutants (PRM) based on qualitative and partially quantitative evaluations. We assayed phage lytic activities both individually and in 14 different cocktails on planktonic bacterial cultures, including three resistotypes of P. aeruginosa (PAO1, PA14 and PA7) and seven K. pneumoniae strains of different capsular serotypes. Based on the results, the natural phage cocktails designed and tested in this study largely performed well and inhibited PRM growth either synergistically or in proto-cooperation. This study contributes to the knowledge of phage behavior in cocktails and the formulation of therapeutic phage preparations. The paper also provides a detailed description of the methods of working with phages.
