Tested whether heterologous expression of vasH in the T6SS-silent RGVC GW610742 web isolates DL2111 and DL2112 restored T6SS-dependent protein synthesis/secretion. Myc-tagged vasH from V52 was cloned into pBAD18 to episomally express vasH. V52DvasH/pBAD18-vasH::myc was used as a control for the arabinose-dependent expression of vasH. As shown in Figure 6, episomal vasH::myc expression in V52DvasH induced Hcp production and subsequent secretion, while only synthesis but not secretion was restored 1655472 in the rough RGVC isolates.Competition Mechanisms of V. choleraeand are thus T6SS-negative. Following a 4-hour coincubation, we determined the number of surviving prey. T6SS-negative prey bacteria were not killed by their isogenic T6SS+ parent strain, but were killed by other T6SS+ isolates (Figure 8A ). Exposure to a predator with a disabled T6SS resulted in about 108 surviving prey bacteria. Similar numbers of surviving prey were obtained when the prey was mixed with an isogenic strain that was marked with a different antibiotic resistance cassette (data not shown). Thus, killing of T6SS-negative prey required a functional T6SS. Surprisingly, the vasK mutant of DL4215 displayed virulence towards V52DvasK, but not against DL4211DvasK or a differentlymarked DL4215DvasK sister strain (Figure 8C). Since DL4215DvasK does not kill V. communis, V. harveyi, or P. phenolica (Figure 7), we hypothesize that DL4215 exhibits some degree of selective T6SS-independent antimicrobial activity against V52DvasK. In conclusion, V. cholerae uses its T6SS not solely for competition with bacterial neighbors (Figure 7), but also for competition within its own species (Figure 8D).DiscussionWe examined environmental smooth and rough V. cholerae isolates (RGVCs) collected at two locations along the Rio Grande to study T6SS regulation in V. cholerae exposed to microbial competitors and predators. Our study showed that smooth RGVC isolates use their T6SS to kill other Gram-negative bacteria isolated from the Rio Grande delta. Deletion of the T6SS gene vasK resulted in a loss of bacterial killing. Importantly, the killing phenotype was restored by vasK complementation in trans. The requirement of VasK for killing implies that a constitutively active T6SS provides smooth RGVC isolates with a competitive advantage compared to their bacterial neighbors. By killing other bacteria, RGVC isolates might enhance their own survival in their environmental niche. In addition, we found that V. cholerae isolates use their T6SS to compete against each other. In our experiments, Hcp synthesis and secretion correlated with eukaryotic and prokaryotic host cell killing (Table 4). For example, smooth Hcp-secreting RGVC isolates DL4211 and DL4215 (Figure 3) displayed full virulence towards E. coli (Figure 1) and D. discoideum (Figure 2). Rough RGVC isolates with their frameshift mutations in the T6SS transcriptional GSK2816126A web activator gene vasH did not produce or secrete Hcp, and their virulence was attenuated. Sequencing and gene alignments of the T6SS transcriptional activator vasH in rough strains indicated a missing guanine at position 157 in rough isolates, resulting in a frameshift mutation. Because VasH was recently implicated in regulating both the large and auxiliary T6SS gene clusters in V. cholerae O395 [20], we speculated that the vasH frameshift mutation in the rough isolates silences T6SS expression. However, trans-complementation of the vasH mutation by episomal expression of V529s vasH restored syn.Tested whether heterologous expression of vasH in the T6SS-silent RGVC isolates DL2111 and DL2112 restored T6SS-dependent protein synthesis/secretion. Myc-tagged vasH from V52 was cloned into pBAD18 to episomally express vasH. V52DvasH/pBAD18-vasH::myc was used as a control for the arabinose-dependent expression of vasH. As shown in Figure 6, episomal vasH::myc expression in V52DvasH induced Hcp production and subsequent secretion, while only synthesis but not secretion was restored 1655472 in the rough RGVC isolates.Competition Mechanisms of V. choleraeand are thus T6SS-negative. Following a 4-hour coincubation, we determined the number of surviving prey. T6SS-negative prey bacteria were not killed by their isogenic T6SS+ parent strain, but were killed by other T6SS+ isolates (Figure 8A ). Exposure to a predator with a disabled T6SS resulted in about 108 surviving prey bacteria. Similar numbers of surviving prey were obtained when the prey was mixed with an isogenic strain that was marked with a different antibiotic resistance cassette (data not shown). Thus, killing of T6SS-negative prey required a functional T6SS. Surprisingly, the vasK mutant of DL4215 displayed virulence towards V52DvasK, but not against DL4211DvasK or a differentlymarked DL4215DvasK sister strain (Figure 8C). Since DL4215DvasK does not kill V. communis, V. harveyi, or P. phenolica (Figure 7), we hypothesize that DL4215 exhibits some degree of selective T6SS-independent antimicrobial activity against V52DvasK. In conclusion, V. cholerae uses its T6SS not solely for competition with bacterial neighbors (Figure 7), but also for competition within its own species (Figure 8D).DiscussionWe examined environmental smooth and rough V. cholerae isolates (RGVCs) collected at two locations along the Rio Grande to study T6SS regulation in V. cholerae exposed to microbial competitors and predators. Our study showed that smooth RGVC isolates use their T6SS to kill other Gram-negative bacteria isolated from the Rio Grande delta. Deletion of the T6SS gene vasK resulted in a loss of bacterial killing. Importantly, the killing phenotype was restored by vasK complementation in trans. The requirement of VasK for killing implies that a constitutively active T6SS provides smooth RGVC isolates with a competitive advantage compared to their bacterial neighbors. By killing other bacteria, RGVC isolates might enhance their own survival in their environmental niche. In addition, we found that V. cholerae isolates use their T6SS to compete against each other. In our experiments, Hcp synthesis and secretion correlated with eukaryotic and prokaryotic host cell killing (Table 4). For example, smooth Hcp-secreting RGVC isolates DL4211 and DL4215 (Figure 3) displayed full virulence towards E. coli (Figure 1) and D. discoideum (Figure 2). Rough RGVC isolates with their frameshift mutations in the T6SS transcriptional activator gene vasH did not produce or secrete Hcp, and their virulence was attenuated. Sequencing and gene alignments of the T6SS transcriptional activator vasH in rough strains indicated a missing guanine at position 157 in rough isolates, resulting in a frameshift mutation. Because VasH was recently implicated in regulating both the large and auxiliary T6SS gene clusters in V. cholerae O395 [20], we speculated that the vasH frameshift mutation in the rough isolates silences T6SS expression. However, trans-complementation of the vasH mutation by episomal expression of V529s vasH restored syn.
