Search

Natural competence for transformation is a primary mode of horizontal gene transfer (HGT). Competent bacteria are able to absorb free DNA from their surroundings and exchange this DNA against pieces of their own genome when sufficiently homologous. And while it is known that transformation contributes to evolution and pathogen emergence in bacteria, there are still questions regarding the general prevalence of non-degraded DNA with sufficient coding capacity. In this context, we previously showed that the naturally competent bacteriumVibrio choleraeuses its type VI secretion system (T6SS) to actively acquire DNA from non-kin neighbors under chitin-colonizing conditions. We therefore sought to further explore the role of the T6SS in acquiring DNA, the condition of the DNA released through T6SS-mediated killing versus passive cell lysis, and the extent of the transfers that occur due to these conditions. To do this, we herein measured the frequency and the extent of genetic exchanges in bacterial co-cultures on competence-inducing chitin under various DNA-acquisition conditions. We show that competentV. choleraestrains acquire DNA fragments with an average and maximum length exceeding 50 kbp and 150 kbp, respectively, and that the T6SS is of prime importance for such HGT events. Collectively, our data support the notion that the environmental lifestyle ofV. choleraefosters HGT and that the coding capacity of the exchanged genetic material is sufficient to significantly accelerate bacterial evolution.Significance StatementDNA shuffled from one organism to another in an inheritable manner is a common feature of prokaryotes. It is a significant mechanism by which bacteria acquire new phenotypes, for example by first absorbing foreign DNA and then recombining it into their genome. In this study, we show the remarkable extent of the exchanged genetic material, frequently exceeding 150 genes in a seemingly single transfer event, inVibrio cholerae. We also show that to best preserve its length and quality, bacteria mainly acquire this DNA by killing adjacent, healthy neighbors then immediately absorbing the released DNA before it can be degraded. These new insights into this prey-killing DNA acquisition process shed light on how bacterial species evolve in the wild.

SummaryBacteria of the genusVibrioare common members of aquatic environments where they compete with other prokaryotes and defend themselves against grazing predators. A macromolecular protein complex called the type VI secretion system (T6SS) is used for both purposes. Previous research showed that the sole T6SS of the human pathogenV. choleraeis induced by extracellular (chitin) or intracellular (low c-di-GMP levels) cues and that these cues lead to distinctive signalling pathways for which the proteins TfoX and TfoY serve as master regulators. In this study, we tested whether the TfoX- and TfoY-mediated regulation of T6SS was conserved in non-cholera species, and if so, how these regulators affected the production of individual T6SSs in double-armed vibrios. We show that, alongside representative competence genes, TfoX regulates at least one T6SS in all testedVibriospecies. TfoY, on the other hand, fostered motility in all vibrios but had a more versatile T6SS response in that it did not foster T6SS-mediated killing inV. fischeriwhile it induced both systems inV. alginolyticus. Collectively, our data provide evidence that the TfoX- and TfoY-mediated signalling pathways are mostly conserved in diverseVibriospecies and important for signal-specific T6SS induction.Originality-Significance StatementThis work provides new insight into the regulatory circuits involved in type VI secretion in diverseVibriospecies. Specifically, it is the first study to compare the effects of the two regulatory proteins TfoX and TfoY on the primary or secondary type VI secretion systems of non-cholera vibrios. Importantly, this work also shows that decreased c-di-GMP levels inV. parahaemolyticuslead to TfoY production without changingtfoYtranscript levels, thereby indirectly linking TfoY production to surface sensing.

Natural competence for transformation is a widely used and key mode of horizontal gene transfer that can foster rapid bacterial evolution. Competent bacteria take-up DNA from their environment using Type IV pili, a widespread and multi-purpose class of cell surface polymers. However, how pili facilitate DNA-uptake has remained unclear. Here, using direct labelling, we show that in the Gram-negative pathogenVibrio choleraeDNA-uptake pili are highly dynamic and that they retract prior to DNA-uptake. Unexpectedly, these pili can self-interact to mediate auto-aggregation of cells into macroscopic structures. This phenotype is conserved in disease causing pandemic strains. However, extensive strain-to-strain variability in the major pilin subunit PilA, present in environmental isolates, controls the ability of pili to interact without affecting transformation. We go on to show that interactions between pili are highly specific, enabling cells producing pili composed of different PilA subunits to discriminate between one another. On chitin surfaces, a natural habitat ofV. cholerae, pili connect cells within dense networks, suggesting a model whereby DNA-uptake pili function to promote inter-bacterial interactions during surface colonisation. Moreover, our results provide evidence that type IV pili could provide a simple and potentially widespread mechanism for bacterial kin recognition.

Vibrio cholerae, which causes the diarrheal disease cholera, is a species of bacteria commonly found in aquatic habitats. Within such environments, the bacterium must defend itself against predatory protozoan grazers. Amoebae are prominent grazers, withAcanthamoeba castellaniibeing one of the best-studied aquatic amoebae. We previously showed thatV. choleraeresists digestion byA. castellaniiand establishes a replication niche within the host’s osmoregulatory organelle. In this study, we deciphered the molecular mechanisms involved in the maintenance ofV. cholerae’s intra-amoebal replication niche and its ultimate escape from the succumbed host. We demonstrated that minor virulence features important for disease in mammals, such as extracellular enzymes and flagellum-based motility, play a key role role in the replication and transmission ofV. choleraein its aqueous environment. This work, therefore, describes new mechanisms that provide the pathogen with a fitness advantage in its primary habitat, which may have contributed to the emergence of these minor virulence factors in the speciesV. cholerae.