Investigation of a quorum sensing peptide in bacillus licheniformis and its novel antifungal property

Quorum sensing molecules (QSMs) are involved in the regulation of complicated processes helping bacterial population benefit from their cell-density. This phenomenon has been recently studied in some fungal populations. Prokaryotes and Eukaryotes’ co-evolution raises the prospect of the existence of inter-kingdom signalling pathways. The involvement of hormone-like molecules such as QSMs in microbial cells communication promise potential role of QS process in inter-kingdom cross-talk.

Bacterial antagonistic activity against fungi is considered as an important bio-control opportunity to control fungal invasion of plants. Several bacterial species such as Bacillus spp. have shown the ability to inhibit fungal growth. During the screening of antagonistic bacteria against Aspergillus flavus (A. flavus), Bacillus subtilis (B. subtilis) was identified as having high antifungal activity. The bacterium, Bacillus licheniformis (B. licheniformis) is related to B. subtilis genetically and is used at industrial-scale for production of the antimicrobial compound bacitracin. Although the comQXPA cluster involved in QS development has been identified in the genome sequence of B. subtilis and different B. licheniformis strains, the QS system in B. licheniformis was not previously investigated in detail, and its QSM (ComX pheromone) was not identified.

In this context, and given the importance of this antagonistic bacterium as an industrial workhorse, this study was aimed to use B. licheniformis NCIMB-8874 as a model antagonistic bacterium to investigate its effect, and the effect of its ComX pheromone on potential inhibition of fungal growth.

The results obtained from bioinformatics studies on B. licheniformis NCIMB 8874 genome sequence presented in this project confirmed the presence of essential quorum sensing-related genes, such as the comQXPA gene cluster. The cell-cell communication of B. licheniformis NCIMB-8874 was investigated through further elucidation of QS process in this bacterium. The detection of the QSM, ComX pheromone, was achieved through molecular biology and biochemical studies including over-production, purification and partial identification.

Subsequently, the potential influence of ComX pheromone and Bacillus cells on the growth of A. flavus was examined and concluded that the QSM could cause a significant reduction in the growth of A. flavus strains (NRRL 3357 and ESP 15).

This work reports for the first time the amino acid sequence of the purified ComX pheromone and its novel antifungal property. Pheromone as a QSM is a potential signal for communication of cells between kingdoms and could be applied for bio-control purposes. Identification of new antifungal peptides against A. flavus could lead to the development of biotechnological strategies which facilitate control of aflatoxin contamination.