Optimization and spectrum characterization of the antibacterial activity of lugdunin

Staphylococcus lugdunensis, an emerging coagulase-negative Staphylococcus (CoNS) pathogen, has garnered increasing interest due to its production of lugdunin, a thiazolidine-containing antimicrobial peptide. However, standardized protocols for directly assessing lugdunin activity produced by S. lugdunensis remain lacking. In this study, we examined the effects of pH and incubation duration on lugdunin activity and evaluated the antibacterial spectrum of lugdunin produced by S. lugdunensis isolates against a panel of gram-positive and gram-negative bacterial strains. The optimal conditions for lugdunin antibacterial activity of isolate CGMH-SL85 were identified as pH 7.5 and a 72-h incubation period. Under the tested conditions, the lugdunin produced by CGMH-SL85 exhibited antimicrobial activity against five gram-positive strains, including Staphylococcus aureus ATCC29213 and Staphylococcus haemolyticus CGMH-SH53, followed by Enterococcus faecium EF029 and EF081-2 and Listeria monocytogenes ATCC10403S. However, no antibacterial activity was observed against any of the 11 tested gram-negative bacterial species. Furthermore, four distinct lugdunin susceptibility phenotypes were observed among 47 lugdunin-nonproducing S. lugdunensis strains (14 sequence type (ST)4, 27 ST27, and 6 ST29 strains), including Type A characterized by large, clear inhibition zones; Type B with smaller, clear zones; Type C displaying halo-like inhibition zones; and Type D showing no detectable activity. Moreover, 20 S. lugdunensis strains (42.6 %) exhibited the Type C phenotype. Notably, all six ST29 strains displayed the Type C phenotype, while the Type A phenotype was observed only among ST27 strains (3 strains). In conclusion, we developed a standardized protocol for evaluating lugdunin activity, using pH 7.5 and a 72-h incubation period, and found that different S. lugdunensis strains exhibited distinct lugdunin susceptibility phenotypes.