Abstract
Nisin, a 3.4 kDa antimicrobial peptide produced by some Lactococcus lactis strains is the most prominent member of the lantibiotic family. Nisin can inhibit cell growth and penetrates the target Gram-positive bacterial membrane by binding to Lipid II, an essential cell wall synthesis precursor. The assembled nisin-Lipid II complex forms pores in the target membrane. To gain immunity against its own-produced nisin, Lactococcus lactis is expressing two immunity protein systems, NisI and NisFEG. Here, we show that the NisI expressing strain displays an IC50 of 73±10 nM, an 8–10-fold increase when compared to the non-expressing sensitive strain. When the nisin concentration is raised above 70 nM, the cells expressing full-length NisI stop growing rather than being killed. NisI is inhibiting nisin mediated pore formation, even at nisin concentrations up to 1 µM. This effect is induced by the C-terminus of NisI that protects Lipid II. Its deletion showed pore formation again. The expression of NisI in combination with externally added nisin mediates an elongation of the chain length of the Lactococcus lactis cocci. While the sensitive strain cell-chains consist mainly of two cells, the NisI expressing cells display a length of up to 20 cells. Both results shed light on the immunity of lantibiotic producer strains, and their survival in high levels of their own lantibiotic in the habitat.
Highlights
Since the 1920s the heterogeneous group of bacteriocins have become an interesting research topic for different applications e.g., as food preservatives or as antibiotic alternatives [1]
Within the group of bacteriocins, there is a large family called lantibiotics [6]. They contain characteristic thioether bridges, called lanthionine rings, which are post-translationally introduced. These lanthionine rings provide a high level of protection against peptide-digesting enzymes, and more importantly ensure high antimicrobial activity against mainly Gram–positive bacteria, which is reflected by the low nanomolar concentration needed to fulfill their activity [7,8]
Activity of NisI and NisID22 in L. lactis The nisI gene was cloned in a pNZ-SV vector, which was complemented with an origin of replication for E. coli, allowing rapid cloning and mutagenesis in all standard E. coli laboratory strains
Summary
Since the 1920s the heterogeneous group of bacteriocins have become an interesting research topic for different applications e.g., as food preservatives or as antibiotic alternatives [1]. Bacteriocins are small, heat stable ribosomally synthesized peptides showing antimicrobial activity [2,3]. They are mostly produced by Grampositive bacteria and mainly act against other Gram–positive species. Within the group of bacteriocins, there is a large family called lantibiotics [6] They contain characteristic thioether bridges, called lanthionine rings, which are post-translationally introduced. These lanthionine rings provide a high level of protection against peptide-digesting enzymes, and more importantly ensure high antimicrobial activity against mainly Gram–positive bacteria, which is reflected by the low nanomolar concentration needed to fulfill their activity [7,8]
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