The study tested five lab-made versions of natural antimicrobial peptides, including LL-37, to see how well they stop the movement and kill three types of disease‑causing spirochete bacteria. The results showed that the peptides worked very differently depending on the bacterial species and strain, with LL-37 being the least effective and needing much higher concentrations to have any effect.

The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of five different cathelicidin-derived synthetic peptides (SMAP-29, LL-37, PG-1, CRAMP and BMAP-28) for Leptospira interrogans, Borrelia spp. and Treponema pallidum subsp. pallidum were investigated in vitro. The MIC of individual peptides was defined as the lowest concentration able to inhibit the motility of spirochaetes after 2 h of incubation, as detected by dark-field microscopy. The MBC of individual peptides was defined as the lowest concentration at which no spirochaetes were subcultured either in cathelicidin-free medium (leptospires and borreliae) or in hamsters (T. pallidum). The MIC values of peptides for leptospires were highly variable, depending on the compound and the strain used. Of the five cathelicidin-derived peptides, SMAP-29 from sheep and BMAP-28 from cattle were the most active against L. interrogans serovars, with MIC values varying between 3 and 51 mg/L, depending on the strains. The MICs of the remaining synthetic peptides ranged between 4.3 and 224 mg/L. The MIC values of synthetic peptides for T. pallidum ranged between 32.3 mg/L for PG-1 and 449.4 mg/L for LL-37. The MICs of all cathelicidin-derived peptides tested for Borrelia strains ranged between 307 and 449.4 mg/L. The activity of the peptides on the motility of spirochaetes was both dose- and time-dependent. The MBC values of the peptides were the same as the MIC values. The results of this study demonstrate that the activity of cathelicidin-derived peptides against spirochaetes is fast and highly variable, depending on the species and the strain.