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Bacterial type VI secretion systems (T6SSs) are puncturing molecular machines that transport effector proteins to kill microbes, manipulate eukaryotic cells, or facilitate nutrient uptake. How and why T6SS machines and effectors differ within a species is not fully understood. Here, we applied molecular population genetics to the T6SSs in a global population of the opportunistic pathogen Pseudomonas aeruginosa . We reveal varying occurrence of up to four distinct T6SS machines. Moreover, we define conserved core T6SS effectors, likely critical for the biology of P. aeruginosa , and accessory effectors that can exhibit mutual exclusivity between strains. By ancestral reconstruction, we observed dynamic changes in the gain and loss of effector genes in the species’ evolutionary history. Our work highlights the potential importance of T6SS intraspecific diversity in bacterial ecology and evolution. Populations of a single bacterial species can possess a great diversity of type VI secretion systems (T6SSs) and secreted effectors. Here, Habich et al. apply molecular population genetics to the T6SSs of Pseudomonas aeruginosa , highlighting the potential importance of T6SS intraspecific diversity in bacterial ecology and evolution.

The causative agent of cholera, Vibrio cholerae, regulates its diverse virulence factors to thrive in the human small intestine and environmental reservoirs. Among this pathogen's arsenal of virulence factors is the tightly regulated type VI secretion system (T6SS). This system acts as an inverted bacteriophage to inject toxins into competing bacteria and eukaryotic phagocytes. V. cholerae strains responsible for the current 7th pandemic activate their T6SS within the host. We established that T6SS-mediated competition occurs upon T6SS activation in the infant mouse, and that this system is functional under anaerobic conditions. When investigating the intestinal host factors mucins (a glycoprotein component of mucus) and bile for potential regulatory roles in controlling the T6SS, we discovered that once mucins activate the T6SS, bile acids can further modulate T6SS activity. Microbiota modify bile acids to inhibit T6SS-mediated killing of commensal bacteria. This interplay is a novel interaction between commensal bacteria, host factors, and the V. cholerae T6SS, showing an active host role in infection.

Vibrio cholerae is an aquatic microbe that can be divided into three subtypes: harmless environmental strains, localised pathogenic strains, and pandemic strains causing global cholera outbreaks. Each type has a contact-dependent type VI secretion system (T6SS) that kills neighbouring competitors by translocating unique toxic effector proteins. Pandemic isolates possess identical effectors, indicating that T6SS effectors may affect pandemicity. Here, we show that one of the T6SS gene clusters (Aux3) exists in two states: a mobile, prophage-like element in a small subset of environmental strains, and a truncated Aux3 unique to and conserved in pandemic isolates. Environmental Aux3 can be readily excised from and integrated into the genome via site-specific recombination, whereas pandemic Aux3 recombination is reduced. Our data suggest that environmental Aux3 acquisition conferred increased competitive fitness to pre-pandemic V. cholerae , leading to grounding of the element in the chromosome and propagation throughout the pandemic clade. Vibrio cholerae uses a type VI secretion system (T6SS) to kill neighbouring competitors. Here, Santoriello et al. show that a T6SS gene cluster (Aux3) exists as a mobile, prophage-like element in some environmental strains, and as a stable truncated form in pandemic isolates. They propose that Aux3 acquisition increased competitive fitness of pre-pandemic V. cholerae .

Tit-for-tat is a familiar principle from animal behavior: individuals respond in kind to being helped or harmed by others. Remarkably some bacteria appear to display tit-for-tat behavior, but how this evolved is not understood. Here we combine evolutionary game theory with agent-based modelling of bacterial tit-for-tat, whereby cells stab rivals with poisoned needles (the type VI secretion system) after being stabbed themselves. Our modelling shows tit-for-tat retaliation is a surprisingly poor evolutionary strategy, because tit-for-tat cells lack the first-strike advantage of preemptive attackers. However, if cells retaliate strongly and fire back multiple times, we find that reciprocation is highly effective. We test our predictions by competing Pseudomonas aeruginosa (a tit-for-tat species) with Vibrio cholerae (random-firing), revealing that P. aeruginosa does indeed fire multiple times per incoming attack. Our work suggests bacterial competition has led to a particular form of reciprocation, where the principle is that of strong retaliation, or ‘tits-for-tat’. Game theory has contributed much to the understanding of social evolution. In an elegant combination of experimental tests and modelling, this study suggests that when bacteria face intense competition, repeated retaliation outcompetes a single tit-for-tat response to attack.

The Vibrio cholerae type VI secretion system (T6SS) assembles as a molecular syringe that injects toxic protein effectors into both eukaryotic and prokaryotic cells. We previously reported that the V. cholerae O37 serogroup strain V52 maintains a constitutively active T6SS to kill other Gram-negative bacteria while being immune to attack by kin bacteria. The pandemic O1 El Tor V. cholerae strain C6706 is T6SS-silent under laboratory conditions as it does not produce T6SS structural components and effectors, and fails to kill Escherichia coli prey. Yet, C6706 exhibits full resistance when approached by T6SS-active V52. These findings suggested that an active T6SS is not required for immunity against T6SS-mediated virulence. Here, we describe a dual expression profile of the T6SS immunity protein-encoding genes tsiV1, tsiV2, and tsiV3 that provides pandemic V. cholerae strains with T6SS immunity and allows T6SS-silent strains to maintain immunity against attacks by T6SS-active bacterial neighbors. The dual expression profile allows transcription of the three genes encoding immunity proteins independently of other T6SS proteins encoded within the same operon. One of these immunity proteins, TsiV2, protects against the T6SS effector VasX which is encoded immediately upstream of tsiV2. VasX is a secreted, lipid-binding protein that we previously characterized with respect to T6SS-mediated virulence towards the social amoeba Dictyostelium discoideum. Our data suggest the presence of an internal promoter in the open reading frame of vasX that drives expression of the downstream gene tsiV2. Furthermore, VasX is shown to act in conjunction with VasW, an accessory protein to VasX, to compromise the inner membrane of prokaryotic target cells. The dual regulatory profile of the T6SS immunity protein-encoding genes tsiV1, tsiV2, and tsiV3 permits V. cholerae to tightly control T6SS gene expression while maintaining immunity to T6SS activity.

The rise of antimicrobial resistance (AMR) in bacterial pathogens is acknowledged by the WHO as a major global health crisis. It is estimated that in 2050 annually up to 10 million people will die from infections with drug resistant pathogens if no efficient countermeasures are implemented. Evolution of pathogens lies at the core of this crisis, which enables rapid adaptation to the selective pressures imposed by antimicrobial usage in both medical treatment and agriculture, consequently promoting the spread of resistance genes or alleles in bacterial populations. Approaches developed in the field of Evolutionary Medicine attempt to exploit evolutionary insight into these adaptive processes, with the aim to improve diagnostics and the sustainability of antimicrobial therapy. Here, we review the concept of evolutionary trade-offs in the development of AMR as well as new therapeutic approaches and their impact on host-microbiome-pathogen interactions. We further discuss the possible translation of evolution-informed treatments into clinical practice, considering both the rapid cure of the individual patients and the prevention of AMR.

The lifestyle of Vibrio cholerae is primarily environmental, yet a chance encounter with a human host can lead to cholera, a potentially lethal form of diarrhea. Strains belonging to O1 and O139 serogroups have pandemic potential, but the contribution of non-O1/non-O139 serovars towards the genesis of cholera remains unclear. Endemic V. cholerae lineages were investigated given several historical accounts describing cholera epidemics and sporadic, contemporary cholera-like outbreaks along the lower Rio Grande Delta (LRGD). Seven isolates were recovered from an urban segment of the Rio Grande and six from a rural segment where the river empties into the Gulf of Mexico. Urban isolates all encode ß -lactamase, and with one exception are phylogenetically closely related, rough (do not express O-antigen), harbor identical plasmids, exhibit a disabled Type VI Secretion System (T6SS), and decreased protease activity. In contrast, rural strains belong to distinct serogroups, are sensitive to ß -lactams, express proteases, and kill Escherichia coli in T6SS competition assays. Genome-scale phylogenetics and multilocus sequence typing indicate that urban and rural isolates belong to distinct and novel phylogroups. These results suggest that an urban niche heavily impacted by anthropogenic pressures and a downstream protected rural niche are inhabited by distinct V. cholerae phylotypes.

The gram-negative bacterium Vibrio cholerae is the causative agent of the diarrhoeal disease cholera and is responsible for seven recorded pandemics. Several factors are postulated to have led to the decline of 6th pandemic classical strains and the rise of El Tor biotype V. cholerae , establishing the current 7th pandemic. We investigated the ability of classical V. cholerae of the 2nd and 6th pandemics to engage their type six secretion system (T6SS) in microbial competition against non-pandemic and 7th pandemic strains. We report that classical V. cholerae underwent sequential mutations in T6SS genetic determinants that initially exposed 2nd pandemic strains to microbial attack by non-pandemic strains and subsequently caused 6th pandemic strains to become vulnerable to El Tor biotype V. cholerae intraspecific competition. The chronology of these T6SS-debilitating mutations agrees with the decline of 6th pandemic classical strains and the emergence of 7th pandemic El Tor V. cholerae . The bacterium Vibrio cholerae has caused seven recorded cholera pandemics. The factors responsible for the decline of 6th pandemic classical biotype strains are not well understood. Here, Kostiuk et al. propose that classical strains underwent sequential mutations in type-six secretion system genes that disadvantaged them when confronted with 7th pandemic El Tor biotype strains.

How do coalitional dynamics matter for the capacity of states to maintain social inclusion in coordinated models of capitalism? Taking its departure in scholarship emphasizing the influence of employers on the extent of state intervention in post-industrial economies, this paper argues that employer influence depends on which actors they team up with – unions or parties. If unions depend on employers for their organizational influence in a policy field, unions become a strong coalitional partner for employers in weakening demands for inclusiveness from the parliamentary arena. Conversely, if unions have influence independent of any coalition with employers, both unions and employers are likely to team up with political parties aligned with their preferences. This makes the level of inclusion resulting from increased state intervention more fluctuating, depending on who holds government power. A comparative study of reforms of Danish and Austrian vocational education institutions corroborates the empirical purchase of the argument.

Pseudomonas aeruginosa , a versatile Gram-negative opportunistic pathogen, relies on multiple virulence mechanisms, including a Type III Secretion System (T3SS) and several Type VI Secretion Systems (T6SS), to establish infections. The bacterial universal second messenger cyclic di-guanylate (c-di-GMP) orchestrates the lifestyle transitions of Pseudomonas aeruginosa between motile and biofilm-associated states and influences the expression of virulence traits. While it is clear that these systems are interconnected, their precise interaction on the single-cell level has remained unclear. In this study, we use single-cell analysis to dissect the role of c-di-GMP in the heterogeneity of virulence factors in P. aeruginosa populations. Our results confirm earlier findings that on the population level, high c-di-GMP levels lead to increased formation and activity of the H1-T6SS, while negatively influencing formation and activity of the T3SS. On the single-cell level, we further characterize the virulence crosstalk within P. aeruginosa populations by presenting a cooperative relationship among T3SS and flagellum and antagonistic relationships between presence of the H1-T6SS and the T3SS as well as the flagellum. Overall, this c-di-GMP-orchestrated heterogeneity and crosstalk of virulence systems suggest a strategy to optimize survival and pathogenicity under varying environmental conditions in the framework of the motile-sessile lifestyle transition. Pseudomonas aeruginosa coordinates several virulence factors – the type 3 and H1-6 secretion systems and the flagellum – on a single cell level using the second messenger cyclic di-GMP, promoting subpopulations with distinct virulence states.