A HOMOSERINE LACTONE AUTOINDUCER REGULATES VIRULENCE OF AN INSECT-PATHOGENIC BACTERIUM, XENORHABDUS-NEMATOPHILUS (ENTEROBACTERIACEAE)

N-beta-Hydroxybutanoyl homoserine lactone (HBHL), the autoinducer of t he luminescent system of Vibrio harveyi, has been identified as the fi rst small compound to restore virulence to avirulent mutants of Xenorh abdus nematophilus. HBHL stimulated the level of lipase activity excre ted by avirulent X. nematophilus and lowered the phenoloxidase activit y in the hemolymph of insects infected with X. nematophilus, parameter s that are both associated with insect pathogenesis. Moreover, mortali ty of the insects infected with avirulent X. nematophilus was restored upon injection with HBHL. Chloroform extraction of medium conditioned with wild-type but not avirulent X. nematophilus led to the isolation of a compound with the same chromatographic mobility as HBHL as well as the ability to stimulate the luminescence of a dim autoinducer-depe ndent mutant of V. harveyi, Transfer of the V. harveyi lux operon into avirulent and wild-type,X. nematophilus generated dim and bright lumi nescent strains, respectively,,which responded to HBHL and an agonist and antagonist in a manner analogous to their effects on the luminesce nce of dim autoinducer-deficient and bright wild-type strains of V. ha rveyi, indicating that similar HBHL-dependent regulatory systems exist in these two bacterial species.