Plantaricins as a novel group of antibacterial compounds and enhancers of antibiotics

Abstract: Antibiotics have revolutionized medicine, however, the rapid development of an-tibiotic resistance among bacteria is diminishing their efficacy. Antimicrobial pep-tides produced by Lactobacillus plantarum, i.e., plantaricins, are considered prom-ising alternatives to antibiotics against infections. In this thesis, the antimicrobial activities of different plantaricins (Pln A, Pln EF, Pln JK, and PLNC8 αβ) were investigated against antibiotic-resistant and susceptible strains of Staphylococcus spp, biofilm-forming strains, as well as clinical isolates of ESKAPE pathogens, and Escherichia coli. Moreover, the stability, cytotoxicity, and immunomodulatory effects of PLNC8 αβ were characterized. The results show that Pln EF and Pln JK have potent antimicrobial activity against Staphylococcus epidermidis and effectively enhance the effects of various antibiotics. Furthermore, PLNC8 αβ shows potent antibacterial effects against different Gram-positive and Gram-negative bacteria, including vancomycin- and methicillin-resistant strains. The antibacterial effects and stability following peptide truncation and D-amino acid substitution were investigated. D-amino acid substitution did not change the antimicrobial activity of PLNC8 αβ, however, it increased the stability of the peptide as it was more resistant to proteolysis by trypsin compared to the native L-enantiomer. Moreover, among the truncated peptides, α1–22, β7–34, and β1–20 retained bacteriostatic effects without displaying bactericidal activity. L-PLNC8 αβ peptides were tested for their antibiofilm properties and displayed rapid disruption of surface-associated S. epidermidis. Electron microscopy shows that PLNC8 αβ targets bacterial cell membranes, ultimately resulting in rapid permeabilization and altered homeostasis, including ATP release. PLNC8 αβ does not show any cytotoxic or hemolytic effects on human cells in vitro. Furthermore, PLNC8 αβ counteracted the cytotoxic effects and expression of inflammatory mediators that were induced by S. aureus, including MMPs and growth factors that are essential in cell regeneration. Pathogen recognition receptors (TLR2, TLR4, and PAR2), intracellular signaling events (c-Jun, c-Fos), and inflammatory mediators (IL-1β, IL-6, CXCL-8), that facilitate pathogen recognition, cell survival, and cellular communication, were all enhanced by the peptides. At sub-MIC concentrations, PLNC8 αβ enhanced the activity of various antibiotics against both Gram-positive and Gram-negative ESKAPE bacteria. In conclusion, plantaricins efficiently impede bacterial pathogens and enhance the activity of antibiotics and thereby constitute a therapeutic option to counter the threatening situation with severe antibiotic-resistant infections.