Glucose protects E. coli from death by the Vibrio cholerae type VI secretion system

Abstract

Vibrio cholerae, the causative agent of the intestinal disease cholera, interacts with other bacteria in dense multispecies communities within both host and environmental settings. Using the harpoon-like type VI secretion system (T6SS), V. cholerae delivers toxic effector proteins into neighboring cells, causing cell lysis and death. The T6SS is frequently studied in V. cholerae using a qstR* mutant which constitutively expresses the T6SS. A qstR* V. cholerae strain can effectively kill target species Escherichia coli, Aeromonas veronii., and T6SS-sensistive V. cholerae cells in a standardized lab killing assay, causing a drop in viable cell counts of five orders of magnitude. This study finds that addition of glucose to a standardized killing assay against qstR* V. cholerae restores E. coli survival by three to four orders of magnitude, though the same effect is not found for Aeromonas or T6SS-sensitive V. cholerae. A growth assay revealed that E. coli doubling time does not affect killing by V. cholerae. Additional evidence shows that E. coli does not produce a diffusible molecule that represses the T6SS of V. cholerae. Investigation by fluorescence microscopy revealed that E. coli cells when entirely surrounded by V. cholerae cells survive in the presence but not the absence of glucose, which suggests that glucose causes a relevant physiological change in individual E. coli cells. We propose that further study should focus on the E. coli capsule as a potential mechanism for surviving T6SS attack. This study makes an unprecedented case that attack via the T6SS can be thwarted by sugar metabolism in target cells.