Secretion systems play a critical role in resistance to predation by Bdellovibrio bacteriovorus

Bdellovibrio bacteriovorus, a Gram-negative predatory bacterium belonging to the Bdellovibrio and like organisms (BALOs), predates on Gram-negative bacteria. BALO strains differ in prey range but so far, the genetic basis of resistance against BALO predation is hardly understood. We developed a loss-of-function approach to screen for sensitive mutants in a library of strain M6, a predation-resistant strain of the plant pathogen Acidovorax citrulli. The screen is based on tracking the growth of a B. bacteriovorus strain expressing the fluorescent reporter Tdtomato in mutant pools to reveal predation-sensitive variants. Two independent loci were identified in mutant strains exhibiting significant levels of susceptibility to the predator. Genes in the two loci were analysed using both protein sequence homology and protein structure modeling. Both were secretion-related proteins and thus associated to the bacterial cell wall. Successful complementation of gspK, a gene encoding for a minor pseudopilin protein confirmed the involvement of the type II secretion system in A. citrulli M6 resistance. This proof of concept study shows that our approach can identify key elements of the BALO-prey interaction, and it validates the hypothesis that mutational changes in a single gene can drastically impact prey resistance to BALO predation.