Diep D.
Universiteit for Miljoog Biovitenskap, Norway

Bacteriocins have been extensively studied during the last 50 years for their potential in antimicrobial applications. Still they are in general very much under-exploited for commercial use in food safety and in medicine, and they are at best in infancy with regard to detailed knowledge of their mode of action. The latter is crucial in order to develop bacteriocins into effective and safe antimicrobial interventions. The nature of the receptor is the holy grail in bacteriocins' mode of action research. It reveals the Achilles’ heel where the bacteriocin attacks and the answers how cells trigger adaptive response or resistance mechanisms. This set of information is invaluable to help design strategies optimized for the individual bacteriocins and the type of applications.

We have developed a simple but powerful strategy to identify receptor genes. Briefly, this approach is composed of three steps: step 1, generation of resistant mutants by random mutations; step 2, the mutations are revealed by whole genome sequencing and SNP identification; and step 3, receptor candidate genes are confirmed by complementation or by heterologous expression that renders a resistant host sensitive to the bacteriocin.

By this approach many receptor genes are now identified, including a maltose-transporter for the circular bacteriocin garvicin ML; UppP, a protein involved in lipid II synthesis-for the two-peptide bacteriocin lactococcin G; RseP, a Zn-dependent protease-for the LsbB-like bacteriocins; APC, an amino acid transporter-for another two-peptide bacteriocin plantaricin S; and PspC, a stress responsive protein-for two unrelated bacteriocins.

In general we have observed that related bacteriocins employ the same receptors on target cells. This is true for the pediocin-like bacteriocins which employ the sugar permease man-PTS as receptor on target cells, and the LsbB-like bacteriocins which employ RseP, a membrane-bound Zn-dependent protease. But there are also cases where bacteriocins belonging to different sunclasses attacking the same receptors. We see a great diversity in the nature of the receptors, implying that bacteriocins have different modes of action and that there are likely different mechanisms to develop resistance. All the receptors identified have one feature in common, in that they all are membrane-bound proteins, supporting the general notion that bacteriocins are membrane-active peptides.

Two interrelated aspects connected to the mode of action have been the major bottlenecks in the field for decades: (i) the nature of the receptor on which bacteriocins bind specifically to target cells, and (ii) the molecular nature of the damage triggered by the interaction between a bacteriocin and its receptor. Now we have now cracked the code of the first aspect. The second aspect is more challenging but necessary to gain a full insight as to how a bacteriocin kills a target cell, i.e., whether it forms pores causing leakage of cellular solutes that disrupts the membrane integrity and proton motive force, or it destroys an enzyme or a metabolic pathway important for cell survival, etc. This second aspect directly links to the mechanisms by which bacteria develop resistance.

Keywords: Bacteriocin, Resistance development, Bacteriocin receptor, Antimicrobial development, Antibiotics, Probiotics

Diep D. (2016). A breakthrough identification of bacteriocin targets. Conference Proceedings of IPC2016. Paper presented at the International Scientific Conference on Probiotics and Prebiotics, Budapest (p. 23.). IPC2016

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