Bacteriocins to Thwart Bacterial Resistance in Gram Negative Bacteria

Abstract

An overuse of antibiotics both in human and animal health and as growth promoters in farming practices has increased the prevalence of antibiotic resistance in bacteria. Antibiotic resistant and multi-resistant bacteria are now considered a major and increasing threat by national health agencies, making the need for novel strategies to fight bugs and super bugs a first priority. In particular, Gram-negative bacteria are responsible for a high proportion of nosocomial infections attributable for a large part to Enterobacteriaceae, such as pathogenic Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa. To cope with their highly competitive environments, bacteria have evolved various adaptive strategies, among which the production of narrow spectrum antimicrobial peptides called bacteriocins and specifically microcins in Gram-negative bacteria. They are produced as precursor peptides that further undergo proteolytic cleavage and in many cases more or less complex posttranslational modifications, which contribute to improve their stability and efficiency. Many have a high stability in the gastrointestinal tract where they can target a single pathogen whilst only slightly perturbing the gut microbiota. Several microcins and antibiotics can bind to similar bacterial receptors and use similar pathways to cross the double-membrane of Gram-negative bacteria and reach their intracellular targets, which they also can share. Consequently, bacteria may use common mechanisms of resistance against microcins and antibiotics. This review describes both unmodified and modified microcins [lasso peptides, siderophore peptides, nucleotide peptides, linear azole(in)e-containing peptides], highlighting their potential as weapons to thwart bacterial resistance in Gram-negative pathogens and discusses the possibility of cross-resistance and co-resistance occurrence between antibiotics and microcins in Gram-negative bacteria.

Keywords: Gram-negative bacteria; antibiotics; bacteriocins; enterobacteria; microcins; resistance.

FIGURE 1

A schematic representation of archetypical organization of microcin and microcin-like gene clusters. Arrows indicate individual microcin genes; arrows are not drown to scale and their direction does not necessarily indicate the direction of transcription that can change between homologous specific gene clusters. The A genes code for the precursors. Genes coding for microcin PTM enzymes and for export systems (efflux pumps, ABC exporters) that expel the microcins out of the producers are in blue and in violet, respectively. Genes whose products contribute to self-immunity of the producing strains (either immunity proteins or exporters/efflux pumps) are colored yellow. When genes code for proteins ensuring simultaneously two functions, they harbor the two corresponding colors. The gene coding for RRE, which ensures leader peptide recognition in MccJ25 and MccJ25-like peptides is shown as hatched motif. The functions of the different PTM enzymes are indicated as follows, taking McC, MccB17, MccJ25 and MccE492 as models. McC and analogs: mccB product ensures MccA adenylation, mccD- and mccE-encoded enzymes (MccD and MccE N-terminal domain) are required for phosphate modification with propylamine; MccB17 and analogs: mcbBCD-encoded three-component synthetase catalyzes dehydration and cyclization to form azolines, which are subsequently oxidized to azoles; MccJ25 and analogs: mcjC product acts as a lasso cyclase that closes the macrolactam ring through an isopeptide bond and mcjB product is a leader peptidase; MccE492 and siderophore peptides: mceCDIJ are required for PTM with mceC encoding a glycosyltransferase that ensures glycosylation of enterobactin and mceD an enterobactin esterase that cleaves the glycosylated enterobactin macrolactone ring into its linear derivatives. mceIJ are involved in attachment of the PTM to MccE492 C-terminus. The function of mceE gene (gray) is undefined.

FIGURE 2

Mechanisms of action of antibiotics (A) and microcins (B) against Gram-negative bacteria showing the membrane proteins involved in uptake into sensitive bacteria and the final targets. β-LAC, β-lactams; QNL, quinolones; TET, tetracycline; AMG, aminoglycosides; FOS, fosfomycin; CHL, chloramphenicol; CS, colistin; RIF, rifampicin; ALB, albomycin; P, pore; LPS, lipopolysaccharide. A letter and a number are assigned to each antibiotic and each microcin respectively, which are used in the scheme to identify the path they follow for their killing activity.

FIGURE 3

Mechanisms of cross-resistance and co-resistance of antibiotics and microcins in Gram-negative bacteria. β-LAC, β-lactams; QNL, quinolones; TET, tetracycline; AMG, aminoglycosides; FOS, fosfomycin; CHL, chloramphenicol; CS, colistin; RIF, rifampicin; ALB, albomycin.