A single regulatory gene is sufficient to alter bacterial host range

A gene for host selection An understanding of the beneficial relationships between bacteria and animals has become increasingly important to several areas of biology, including pathogenic microbiology. Many pathogens display specificity for a single host or tissue, but the molecular basis for such specificity is largely unknown. A comparative genomic study of host specificity in the mutualism between the sepiolid squid Euprymna scolopes and the bioluminescent bacteria Vibrio fischeri now demonstrates that the presence of a single bacterial gene, the regulatory gene rscS , can modify host range. When this gene is expressed in strains of V. fischeri that normally colonize the Japanese pinecone fish, it produces a symbiotic biofilm that is critical for squid colonization. This work raises the possibility that the specificity of human pathogens might be similarly manipulated for therapeutic purposes. Here it is shown that the absence or presence of a regulatory gene determines whether the bioluminescent bacterium Vibrio fischeri colonizes squid or fishes. Microbial symbioses are essential for the normal development and growth of animals 1 , 2 , 3 . Often, symbionts must be acquired from the environment during each generation, and identification of the relevant symbiotic partner against a myriad of unwanted relationships is a formidable task 4 . Although examples of this specificity are well-documented, the genetic mechanisms governing it are poorly characterized 5 . Here we show that the two-component sensor kinase RscS is necessary and sufficient for conferring efficient colonization of Euprymna scolopes squid by bioluminescent Vibrio fischeri from the North Pacific Ocean. In the squid symbiont V. fischeri ES114, RscS controls light-organ colonization by inducing the Syp exopolysaccharide, a mediator of biofilm formation during initial infection. A genome-level comparison revealed that rscS , although present in squid symbionts, is absent from the fish symbiont V. fischeri MJ11. We found that heterologous expression of RscS in strain MJ11 conferred the ability to colonize E. scolopes in a manner comparable to that of natural squid isolates. Furthermore, phylogenetic analyses support an important role for rscS in the evolution of the squid symbiosis. Our results demonstrate that a regulatory gene can alter the host range of animal-associated bacteria. We show that, by encoding a regulator and not an effector that interacts directly with the host, a single gene