Brian K. Hammer

The type six secretion system (T6SS) is a highly studied, versatile bacterial nanoweapon that is important in bacterial pathogenesis and evolution. Despite extensive study, its prevalence remains poorly quantified. Analyzing 35,467 representative genomes, we find that approximately 41% of genomes in the average well-sampled Gram-negative genus (more than 30 sampled genomes) encode a T6SS. Further, we show that the distribution of the T6SS is patchy at all taxonomic levels. The uneven distribution we found is consistent with context-dependent selection, repeated evolutionary loss, or greater evolutionary lability. Our dataset provides a foundation for studying the drivers of T6SS distribution, and our pipeline enables similar surveys of other secretion systems, gene clusters, and multi-protein complexes across bacteria.

Jeongeupia sacculi (HS-3) is a novel β -proteobacterium with a unique multicellular life cycle. The genetic tractability and mechanisms underlying differentiation are unknown. Here, we adapt a method of homologous recombination for genetic modification in HS-3 and generate a disruption mutant of rpoS , which encodes the alternative sigma factor σ ^S commonly associated with general stress response or major behavioral switches. We then characterize phenotypes associated with rpoS disruption. Under low-nutrient conditions, rpoS disruption reduced Congo red binding, consistent with rpoS acting as a regulator of low-nutrient response by upregulating production of biofilm-associated materials. Surprisingly, rpoS disruption did not significantly alter survival in response to other stressors in general stress response assays tested. Our work establishes a method for genetic manipulation in HS-3 and enables future investigation into the life cycle of HS-3, including whether rpoS has a role in differentiation triggered by low-nutrient response.

Bacteria often attach to surfaces and grow densely packed communities called biofilms. As biofilms grow, they expand across the surface, increasing their surface area and access to nutrients. Thus, the overall growth rate of a biofilm is directly dependent on its range expansion rate. A direct trade-off between horizontal and vertical growth impacts the range expansion rate and, crucially, the overall biofilm growth rate. The biophysical connection between horizontal and vertical growth remains poorly understood, in large part due to the difficulty in resolving the biofilm shape with sufficient spatial and temporal resolutions from small length scales to macroscopic sizes. Here we show that the horizontal expansion rate of bacterial colonies is strongly coupled to vertical expansion via the contact angle at the biofilm edge. Using white light interferometry, we measure the three-dimensional surface morphology of growing colonies, and find that small colonies are well described as spherical caps. At later times, nutrient diffusion and uptake prevent the tall colony centre from growing exponentially. We further show that a simple model connecting vertical and horizontal growth dynamics can reproduce the observed phenomena, suggesting that the spherical cap shape emerges due to the biophysical consequences of diffusion-limited growth.

The Type VI Secretion System (T6SS) is a widespread and highly effective mechanism of microbial warfare; it confers the ability to efficiently kill susceptible cells within close proximity. Due to its large physical size, complexity, and ballistic basis for intoxication, it has widely been assumed to incur significant growth costs in the absence of improved competitive outcomes. In this study, we precisely examine the fitness costs of constitutive T6SS firing in the bacterium Vibrio cholerae. We find that, contrary to expectations, constitutive expression of the T6SS has a negligible impact on growth, reducing growth fitness by 0.025 ± 0.5% (95% CI) relative to a T6SS− control. Mathematical modeling of microbial populations demonstrates that, due to clonal interference, constitutive expression of the T6SS will often be neutral, with little impact on evolutionary outcomes. Our findings underscore the importance of precisely measuring the fitness costs of microbial social behaviors and help explain the prevalence of the T6SS across Gram‐negative bacteria.

Vibrio cholerae, is the causative agent of cholera, that infects millions, annually. Chironomids are aquatic insects that hostV. cholerae. Toxigenic strains produce cholera toxin (CT) which is the main virulence factor that causes cholera symptoms. In contrast to other bacterial pathogens,V. choleraeproduces CT when at low cell densities while hemagglutinin/protease (HAP) is a high cell density‐controlled gene. WhenV. choleraebehavior was examined on chironomids, we showed that high cell densities of non‐toxigenic strains, increased HAP production in a toxigenic strain, conditions which could also potentially reduce CT production. Here we propose the value of studies that could support the potential ofV. choleraenon‐toxigenic strains to repress virulence gene expression in cholera‐infected humans. High cell densities of a non‐toxigenic strain present in an infected individual, may down‐regulate CT expression, reducing cholera symptoms. To further test the hypothesis supported by a chironomid model, additional experiments in animal models are first needed.

The Type VI Secretion System (T6SS) is a widespread and highly effective mechanism of microbial warfare; it confers the ability to efficiently kill susceptible cells within close proximity. Due to its large physical size, complexity, and ballistic basis for intoxication it has widely been assumed to incur significant growth costs in the absence of improved competitive outcomes. In this study, we precisely examine the fitness costs of constitutive T6SS firing in the bacterium Vibrio cholerae . We find that, contrary to expectations, constitutive use of T6SS has a negligible impact on growth, reducing growth fitness by 0.025 ± 0.5% (95% CI) relative to a T6SS-control. Mathematical modeling of microbial populations demonstrates that, due to clonal interference, constitutive expression of the T6SS will often be neutral, with little impact on evolutionary outcomes. Our findings underscore the importance of precisely measuring the fitness costs of microbial social behaviors, and help explain the prevalence of the T6SS across Gram negative bacteria.