Investigation of quorum sensing process in Bacillus licheniformis

Quorum sensing is a well-established system of communication adopted by a number of bacterial, and some fungal, populations. This cell density dependent phenomenon is based on the accumulation of small diffusible molecules, termed as quorum sensing molecules, in the extracellular milieu until a threshold concentration triggers alteration in the expression of specific genes culminating in variety of responses including virulence, bioluminescence, sporulation, biofilm formation and secondary metabolites production.

In Bacilli, quorum sensing is mediated by small peptides that control competence, sporulation, and the production of certain secondary metabolites in a cell density dependent fashion. Two divergent pathways, triggered by the ComX pheromone and the Competence and Sporulation Factor (CSF), are engaged in the control of these processes.

B. licheniformis NCIMB 8874 is a bacterium with industrial relevance for the production of the antimicrobial agent bacitracin. This organism is genetically related to Bacillus subtilis, whose quorum sensing is regulated by the comQXPA operon. This study aimed to investigate the role of the comQXPA locus in B. licheniformis NCIMB 8874 and the production of potential signalling molecules in this bacterium.

Production of signalling molecule/s in B. licheniformis NCIMB 8874 was confirmed by the significant increase (p>0.05) in srfA expression in response to the addition of supernatants of B. licheniformis NCIMB 8874 cultures in their late exponential phase to low cell density cultures of B. subtilis reporter strains, carrying a srfA-lacZ fusion. The investigation of quorum sensing-regulated secondary metabolites production established production of lichenysin, -polyglutamic acid and extracellular proteases, whose biosynthesis is impaired in bacteria with disrupted comQXPA clusters. Bioinformatics studies on B. licheniformis NCIMB 8874 genome sequence confirmed the presence of essential quorum sensing-related genes, such as the comQXPA gene cluster, comK, mecA and comS. Moreover, in silico analysis allowed the identification of members of the Rap and Phr families, which aid the regulation of cell density dependent phenomena in B. subtilis.

The results presented in this work positively indicate that B. licheniformis NCIMB 8874 cell-cell communication operates in analogy with the well established comQXPA-controlled pathway of B. subtilis.