Verminephrobacter eiseniae type IV pili and flagella are required to colonize earthworm nephridia.

The bacterial symbiont Verminephrobacter eiseniae colonizes nephridia, the excretory organs, of the lumbricid earthworm Eisenia fetida. E. fetida transfers V. eisenia into the egg capsule albumin during capsule formation and V. eiseniae cells migrate into the earthworm nephridia during embryogenesis, where they bind and persist. In order to characterize the mechanistic basis of selective tissue colonization, methods for site-directed mutagenesis and colonization competence were developed and used to evaluate the consequences of individual gene disruptions. Using these newly developed tools, two distinct modes of bacterial motility were shown to be required for V. eiseniae colonization of nascent earthworm nephridia. Flagella and type IV pili mutants lacked motility in culture and were not able to colonize embryonic earthworms, indicating that both twitching and flagellar motility are required for entrance into the nephridia.

Interference of quorum sensing in Pseudomonas syringae by bacterial epiphytes that limit iron availability.

Leaf surfaces harbour bacterial epiphytes that are capable of influencing the quorum sensing (QS) system, density determination through detection of diffusible signal molecules, of the plant-pathogen Pseudomonas syringae pv. syringae (Pss) which controls expression of extracellular polysaccharide production, motility and other factors contributing to virulence to plants. Approximately 11% of the bacterial epiphytes recovered from a variety of plants produced a diffusible factor capable of inhibiting the QS system of Pss as indicated by suppression of ahlI. Blockage of QS by these interfering strains correlated strongly with their ability to limit iron availability to Pss. A direct relationship between the ability of isogenic Escherichia coli strains to sequester iron via their production of different siderophores and their ability to suppress QS in Pss was also observed. Quorum sensing induction was inversely related to iron availability in culture media supplemented with iron chelators or with FeCl(3). Co-inoculation of interfering strains with Pss onto leaves increased the number of resultant disease lesions over twofold compared with that on plants inoculated with Pss alone. Transposon-generated mutants of interfering strains in which QS inhibition was blocked did not increase disease when co-inoculated with Pss. Increased disease incidence was also not observed when a non-motile mutant of Pss was co-inoculated onto plants with QS interfering bacteria suggesting that these strains enhanced the motility of Pss in an iron-dependent manner, leading to an apparent increase in virulence of this pathogen. Considerable cross-talk mediated by iron scavenging apparently occurs on plants, thereby altering the behaviour of bacteria such as Pss that exhibit important QS-dependent traits in this habitat.

Acyl-homoserine lactone-mediated cross talk among epiphytic bacteria modulates behavior of Pseudomonas syringae on leaves.

The leaf surface harbors a host of bacterial epiphytes that are capable of influencing the quorum sensing (QS) system of the plant pathogen Pseudomonas syringae pv. syringae (Pss). Pss uses QS to regulate expression of genes conferring extracellular polysaccharide production, motility and factors contributing to virulence to plants. About 7% of bacterial epiphytes isolated in this study produce the Pss cognate signal, 3-oxohexanoyl-homoserine lactone (3OC6HSL), often in amounts more than 10-fold higher than Pss. Premature induction of QS in Pss by these 3OC6HSL-producing epiphytes suppressed swarming motility and subsequent disease of the leaf. Co-inoculation of 3OC6HSL-producing strains with Pss reduced the number of lesions when inoculated together onto leaves compared with that of plants inoculated with Pss alone. Strains in which 3OC6HSL accumulation was quenched by expression of an N-acyl-homoserine lactonase did not decrease disease when co-inoculated with Pss. Disease incidence caused by a nonmotile mutant of Pss was not affected by 3OC6HSL-producing bacteria, suggesting that exogenous 3OC6HSL signal that altered the motility of Pss was responsible for reducing the apparent virulence of this pathogen. Thus, considerable cross talk involving exogenous 3OC6HSL occurs on leaves and this process can be exploited for disease control.