Tit-for-Tat: Type VI Secretion System Counterattack during Bacterial Cell-Cell Interactions

Abstract

The bacterial Type VI secretion system (T6SS) is dynamic organelle that bacteria use to target prey cells for inhibition via translocation of effector proteins. Time-lapse fluorescence microscopy has documented striking dynamics of opposed T6SS organelles in adjacent sister cells of Pseudomonas aeruginosa. Such cell-cell interactions have been termed "T6SS dueling" and likely reflect a biological process that is driven by T6SS antibacterial attack. Here we show that T6SS dueling behavior strongly influences the ability of P. aeruginosa to prey upon heterologous bacterial species. We show that in the case of P. aeruginosa, T6SS-dependent killing of either Vibrio cholerae or Acinetobacter baylyi is greatly stimulated by T6SS activity occurring in those prey species. Our data suggest that in P. aeruginosa, T6SS organelle assembly and lethal counterattack are regulated by a signal that corresponds to the point of attack of the T6SS apparatus elaborated by a second aggressive T6SS+ bacterial cell.

Bacterial killing via a type IV secretion system

“…By comparison, Xac cell lysis was very rarely observed in these experiments. The rapid and violent cell death that we observe contrasts with that which has been reported for T6SS-mediated killing by Pseudomonas aeruginosa, in which target cells change their morphology (cell rounding, blebbing and plasmolysis) over a period of many minutes before lysis 29,30 .…”

“…We have provided evidence that the Xac T4SS, and by extension its homologues in other Xanthomonadaceae species, target toxins (X-Tfes or XVIPs) to bacterial cells, thereby providing Xanthomonas an advantage to survive in an environment populated by a number of different bacterial species in a manner similar to that recently described for several T6SSs [29][30][31][32][33] . Using the presence of a XVIPCD as search criteria, we have identified 500 potential X-Tfes in the public databases, of which all but two are derived from members of the Xanthomonadaceae family (Table 1).…”

Cryo‐ EM reconstruction of Type VI secretion system baseplate and sheath distal end

“…Temperature was set to 30°C, and humidity was regulated to 95% by an Okolab T‐unit (Okolab). Fiji (Schindelin et al , 2012) was used for additional image processing as described previously (Basler et al , 2013). …”

Salmonella Typhimurium utilizes a T6SS-mediated antibacterial weapon to establish in the host gut

“…For example, the human pathogen V. cholerae employs the T6SS to kill E. coli or to disable the amoebae Dictyostelium discoideum and macrophage cell lines (14,48,49). Recent studies demonstrated that T6SS organelles in V. cholerae, P. aeruginosa, and entero-aggregative E. coli cells are very dynamic and likely expel their T6SS spike/tube VgrG/Hcp complex toward prokaryotic cells and cause target cell lysis by delivering antibacterial toxins (50,51). A broad diversity of antibacterial toxins have been described so far (10,42).…”