Synthesis and Antimicrobial Specificities of Halogenated Tryptophan-Containing Nisin Variants

Antimicrobial peptides, and in particular ribosomally produced and post-translationally modified peptides (RiPPs), are a potentially important class of candidate antibiotics for combating multidrug-resistant bacteria. Introduction of a halogenated Trp residue into a RiPP can possibly enhance antimicrobial efficacy and alter specificity, but this modification has hardly been explored. This study employs an efficient expression system utilizing a tryptophan auxotrophic Lactococcus lactis strain to biosynthetically and efficiently incorporate halogenated tryptophan analogues, namely 5-fluoro-tryptophan (5FW), 5-chloro-tryptophan (5CW), 5-bromo-tryptophan (5BW), as well as 5-methyl-tryptophan (5MW) at position 1 of I1W nisin A. Wild-type nisin and Trp-containing I1W nisin show a high and broad activity against four tested pathogens. However, the activity spectrum of the three different halogen atom containing nisin variants became more strain specific, as both increased and decreased activities were measured against the four tested pathogens. No trend between the chemical properties of the halogen atom (e.g., electronegativity, size) and the bioactivity of the nisin variants toward each of the four pathogens could be detected, suggesting strain specific antimicrobial activity mechanisms. These findings demonstrate that halogenated tryptophan analogues can be successfully incorporated into a bioactive RiPP produced by an auxotrophic L. lactis strain and underscore the utility of peptide halogenation for discovering novel antimicrobial agents with tailored pathogen specificity.