Submitted to: Journal of Bacteriology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 16, 1999
Publication Date: N/A
Interpretive Summary: As part of our studies on the mechanisms by which bacteria are able to cause plant disease, we are analyzing the behavior of pseudomonas syringae, the agent that causes the brown spot disease of snap bean, an important crop in Wisconsin. We have found two genes (named gacA and gacS) that are important in sensing and responding to environmental stimuli. When these genes are eliminated by mutation of bacteria, the bacteria no longer form discreet, thin colonies. The addition of the product programmed by one of these genes back to the bacterial media restores this behavior termed "bacterial swarming." This observation is the first of swarming in a bacterial plant pathogen. An understanding of the factors controlling the bacterial behavior may lead to new means for controlling the disease caused by the bacteria.
The snap bean pathogen Pseudomonas syringae pv. syringae B728a exhibited bacterial swarming on media that induces genetic pathways that are regulated by the gacS/gacA two component sensor-kinase/response regulator gene pair. When stabbed onto modified SK medium, B728a forms a diffuse dendritic colony that extends to the margins of the petri dish upon incubation for 24 to 48 hours at 28 degrees C. SK media with the addition of skim milk is commonly used for the assay of extracellular protease production by P. syringae pv. syringae, a phenotype that requires a functional gacS and gacA gene. Strains that contained a mutation in either the gacS or the gacA gene lost the ability to swarm on SK media. Addition of partially purified homoserine lactone (HSL) isolated from supernatants of wild-type B728a cultures was able to partially rescue the swarming phenotype of either a gacS or gacA mutant strain. We have previously shown that gacS and gacA are required for the production of HSL by B728a. Thus, gacS/gacA appear to regulate bacterial swarming in B728a primarily by the requirement of this sensor/response regulator for the production of HSL.