Arabinose. V52 and the isogenic vasK mutant were used as positive and negative controls, respectively. Pellets and culture supernatants were separated by centrifugation. The supernatant portions were concentrated by TCA precipitation and both fractions were subjected to SDS-PAGE followed by western blotting using the antibodies indicated. (B) Survival of 25033180 E. coli MG1655 after mixing with V. cholerae. V. cholerae and E. coli were mixed in a 10:1 ratio and incubated for 4 hours at 37uC before the resulting spots were resuspended, serially diluted, and plated on E. coli-selective media. Data represent the averages of three independent experiments. Standard deviations are included. (C) Survival of D. discoideum after mixing with V. cholerae. D. discoideum was plated with V. cholerae and the number of plaques formed by surviving D. discoideum were counted after a 3-day incubation at 22uC. Data are representative of three independent experiments. Standard deviations are shown. 
doi:10.1371/journal.pone.0048320.gDNA manipulations39-Myc-tagged vasH was PCR-amplified from V. cholerae V52 chromosomal DNA with primers 59vasH and 39vasH::myc (Table 1). The resulting PCR product was restricted with 59EcoRI and 39-XbaI, cloned into pGEM T-easy (Promega), and subcloned into pBAD18. In-frame deletion of vasK was performed as described by Metcalf et al. [23] using the pWM91-based vasK knockout construct [9]. During sucrose selection, sucrose concentration was increased from 6 to 20 for all RGVC gene deletions because these SIS 3 isolates exhibited increased tolerance to sucrose compared to V52. For complementation, vasK was amplified from V52 chromosomal DNA using primers 59-vasK-pBAD24 and 39-vasKpBAD24 (Table 1). The resulting PCR product was purified using the Qiagen PCR cleanup kit, digested with EcoRI and XbaI, and cloned into pBAD24.Results RGVC Isolates Exhibit T6SS-Mediated Antimicrobial PropertiesWe previously demonstrated that clinical V. cholerae O37 serogroup strain V52 uses its T6SS to kill E. coli and Salmonella Typhimurium [6]. To determine the role of the T6SS in environmental strains, we employed two different types of V. cholerae isolated from the Rio Grande: smooth isolates with distinct O-antigens as part of their lipopolysaccharides (LPS), and rough isolates that lack O-antigen (Table 3). Due to concerns that rough bacteria are ML-264 web genetically unstable because the lack of O-antigen allows the uptake of chromosomal DNA [24], we assessed the virulence potential of two separately isolated but genetically identical rough isolates DL2111 and DL2112 (as determined by deep sequencing (Illumina platform) of a polymorphic 22-kb fragment [Genbank accession numbers JX669612 and JX669613]) to minimize the chance of phenotypic variation due to genetic exchange.Competition Mechanisms of V. choleraeFigure 5. Alignment of VasH polypeptide sequences of RGVC isolates. VasH of V52, N16961, and four RGVC isolates were aligned. In the rough isolates, a guanine was inserted at position 157 of vasH to restore the open reading frame. Colored 
bars indicate substitutions compared to VasH from V52. doi:10.1371/journal.pone.0048320.gTo determine whether environmental RGVC V. cholerae are capable of killing bacteria, we performed an E. coli killing assay (Figure 1). RGVC isolates and E. coli strain MG1655 were spotted on LB nutrient agar plates, and the number of surviving MG1655 cells was determined after a 4-hour incubation at 37uC. V52 and V52DvasK were used as virule.Arabinose. V52 and the isogenic vasK mutant were used as positive and negative controls, respectively. Pellets and culture supernatants were separated by centrifugation. The supernatant portions were concentrated by TCA precipitation and both fractions were subjected to SDS-PAGE followed by western blotting using the antibodies indicated. (B) Survival of 25033180 E. coli MG1655 after mixing with V. cholerae. V. cholerae and E. coli were mixed in a 10:1 ratio and incubated for 4 hours at 37uC before the resulting spots were resuspended, serially diluted, and plated on E. coli-selective media. Data represent the averages of three independent experiments. Standard deviations are included. (C) Survival of D. discoideum after mixing with V. cholerae. D. discoideum was plated with V. cholerae and the number of plaques formed by surviving D. discoideum were counted after a 3-day incubation at 22uC. Data are representative of three independent experiments. Standard deviations are shown. doi:10.1371/journal.pone.0048320.gDNA manipulations39-Myc-tagged vasH was PCR-amplified from V. cholerae V52 chromosomal DNA with primers 59vasH and 39vasH::myc (Table 1). The resulting PCR product was restricted with 59EcoRI and 39-XbaI, cloned into pGEM T-easy (Promega), and subcloned into pBAD18. In-frame deletion of vasK was performed as described by Metcalf et al. [23] using the pWM91-based vasK knockout construct [9]. During sucrose selection, sucrose concentration was increased from 6 to 20 for all RGVC gene deletions because these isolates exhibited increased tolerance to sucrose compared to V52. For complementation, vasK was amplified from V52 chromosomal DNA using primers 59-vasK-pBAD24 and 39-vasKpBAD24 (Table 1). The resulting PCR product was purified using the Qiagen PCR cleanup kit, digested with EcoRI and XbaI, and cloned into pBAD24.Results RGVC Isolates Exhibit T6SS-Mediated Antimicrobial PropertiesWe previously demonstrated that clinical V. cholerae O37 serogroup strain V52 uses its T6SS to kill E. coli and Salmonella Typhimurium [6]. To determine the role of the T6SS in environmental strains, we employed two different types of V. cholerae isolated from the Rio Grande: smooth isolates with distinct O-antigens as part of their lipopolysaccharides (LPS), and rough isolates that lack O-antigen (Table 3). Due to concerns that rough bacteria are genetically unstable because the lack of O-antigen allows the uptake of chromosomal DNA [24], we assessed the virulence potential of two separately isolated but genetically identical rough isolates DL2111 and DL2112 (as determined by deep sequencing (Illumina platform) of a polymorphic 22-kb fragment [Genbank accession numbers JX669612 and JX669613]) to minimize the chance of phenotypic variation due to genetic exchange.Competition Mechanisms of V. choleraeFigure 5. Alignment of VasH polypeptide sequences of RGVC isolates. VasH of V52, N16961, and four RGVC isolates were aligned. In the rough isolates, a guanine was inserted at position 157 of vasH to restore the open reading frame. Colored bars indicate substitutions compared to VasH from V52. doi:10.1371/journal.pone.0048320.gTo determine whether environmental RGVC V. cholerae are capable of killing bacteria, we performed an E. coli killing assay (Figure 1). RGVC isolates and E. coli strain MG1655 were spotted on LB nutrient agar plates, and the number of surviving MG1655 cells was determined after a 4-hour incubation at 37uC. V52 and V52DvasK were used as virule.