Intra- and interspecies regulation of gene expression by Actinobacillus actinomycetemcomitans LuxS

The cell density-dependent control of gene expression is employed by many b acteria for regulating a variety of physiological functions, including the generation of bioluminescence, sporulation, formation of biofilms, and the expression of virulence factors. Although periodontal organisms do not appe ar to secrete acyl-homoserine lactone signals, several species, e.g., Porph yromonas gingivalis, Prevotella intermedia, and Fusobacterium nucleatum, ha ve recently been shown to secrete a signal related to the autoinducer II (A I-2) of the signal system 2 pathway in Virbrio harveyi. here, we report tha t the periodontal pathogen Actinobacillus actinomycetemcomitans expresses a homolog of V. harveyi luxS and secretes an AI-2-like signal. Cell-free con ditioned medium from A. actinomyceterncornitans or from a recombinant Esche richia coli strain (E. coli AIS) expressing A. actinomycetemcomitans luxS i nduced luminescence in V. harveyi BB170 > 200-fold over controls. AI-2 leve ls peaked in mid-exponential-phase cultures of A. actinomycetemcomitans and were significantly reduced in late-log- and stationary-phase cultures. Inc ubation of early-log-phase A. actinomycetemcomitans cells with conditioned medium from A. actinomycetemcomitans or from E. coli AIS resulted in a thre efold induction of leukotoxic activity and a concomitant increase in leukot oxin polypeptide. In contrast, no increase in leukotoxin expression occurre d when cells were exposed to sterile medium or to conditioned broth from E. coli AIS(-), a recombinant strain in which luxS was insertionally inactiva ted. A. actinomycetemcomitans AI-2 also induced expression of afuA, encodin g a periplasmic iron transport protein, approximately eightfold, suggesting that LuxS-dependent signaling may play a role in the regulation of iron ac quisition by A. actinomycetemcomitans. Finally, A. actinomycetemcomitans AI -2 added in trans complemented a luxS knockout mutation in P. gingivalis by modulating the expression of the luxS-regulated genes uvrB and hasF in thi s organism. Together, these results suggest that LuxS-dependent signaling m ay modulate aspects of virulence and the uptake of iron by A. actinomyceter ncornitans and induce responses in other periodontal organisms in mixed-spe cies oral biofilm.