The Structural Effects On Biofilm Inhibition and Dispersion by Trp/Arg Antimicrobial Peptides

Biofilms are sessile microbial communities that attach to a variety of surfaces. Due to significantly enhanced tolerance to antibiotics, biofilms cause serious chronic infections with high morbidity and mortality. In order to develop more effective approaches for biofilm control, a series of linear cationic antimicrobial peptides (AMPs) with varying tryptophan (Trp or W) and arginine (Arg or R) repeats ([RW]_n-NH₂, where n = 2, 3, or 4), as well as the dendrimeric peptide [RW]_4D were rigorously compared to correlate their structures with antimicrobial activities on the planktonic growth and biofilm formation of Escherichia coli.

The antimicrobial activities of peptides against planktonic growth were found to increase significantly with the size of peptide molecules; e.g., the dendrimer exhibited almost complete growth inhibition at 25 µM. In addition, all AMPs except tetramer significantly reduced E. coli biofilm surface coverage and viability of biofilm cells when added at inoculation. Among the peptides tested, dendrimer exhibited the strongest activity; e.g., E. coli biofilm formation was reduced by 89.2 ± 3.1% by 25 μM dendrimer based on surface coverage compared to peptide-free control. These results are corroborated by the finding that E. coli motility was significantly reduced by hexamer, octamer, and dendrimer at 25 μM.

In addition to biofilm prevention, significant killing of biofilm cells was observed after a 3-h treatment of mature biofilms by hexamer and dendrimer. Interestingly, treatment with octamer caused significant biofilm dispersion without apparent killing of biofilm cells; e.g., the biofilms were removed by 47.4 ± 6.2%, 71.5 ± 9.2%, 87.9 ± 8.0%, and 91.5 ± 3.5% by 25, 50, 100, and 200 μM octamer, respectively. Overall, these results suggest that hexamer, octamer, and dendrimer are potent inhibitors of both planktonic growth and biofilm formation, while octamer can also disperse mature biofilms. The possible mechanisms will be discussed to explain these observations. These results are helpful for designing novel biofilm inhibitors and developing more effective therapeutic methods.