We recently designed cationic, hydrophobic, antimicrobial peptides based on the human salivary protein PSP. The peptide GL13NH2 causes bacterial agglutination while GL13K is bactericidal to oral and non-oral bacteria. We hypothesize that these PSP-peptides could prevent the transfer of plasmids by competitively blocking agglutination or signaling in E. faecalis .
Methods: Donor bacteria, OG1RF-pCF10 (plasmid containing), were cultured under inducing or non-inducing conditions. Induced donor cells are expected to have a high transfer rate (10-1-10-2) due to the presence of pheromone. Non-induced donors have a lower rate (~10-5) due to the presence of an inhibitor peptide. Induced and un-induced donor cultures were incubated without PSP-peptides or with a sublethal concentration (32µg/ml) of GL13K or GL13NH2 and incubated 2h/35°C. Donor cultures were then combined 1:10 with recipients (OG1SSp-plasmid free) to form mating mixtures. After 10-min incubation to allow conjugation, bacterial mixtures were plated on selective media for enumeration of CFUs. The efficiency of plasmid transfer was determined by the transconjugant/donor ratio.
Results: In induced cultures, the transconjugant/donor ratio was 2.5×10-2 (control) and 5.5×10-3 in the presence of GL13K. With GL13NH2 the ratio was 1.1×10-2 vs. 1.8x10-2 for the peptide-free control. Uninduced cultures yielded similar qualitative results. The transconjugant/donor ratio did not differ significantly for any mating combination (P>0.2).
Conclusion: The PSP-peptides GL13K and GL13NH2 had no effect on the conjugation system of E. faecalis and are not suited to prevent plasmid transfer.
Keywords: Antimicrobials, Bacterial, Endodontics and Microbiology