Syringopeptin SP25A-mediated killing of gram-positive bacteria and the role of teichoic acid d-alanylation

Abstract

The Pseudomonas syringae syringopeptins are cationic cyclic lipodepsipeptides that inhibit fungi and bacteria. The homolog syringopeptin (SP)25A was strongly inhibitory to several Gram-positive bacteria with minimum inhibitory concentrations ranging between 1.95 and 7.8 microg mL(-1). In contrast, it was not inhibitory to several Gram-negative bacteria. At 5 and 10 microg mL(-1), SP25A rapidly inhibited the growth of Bacillus subtilis and was bacteriocidal. Teichoic acid D-alanylation dltB- and dltD-defective mutant strains of B. subtilis were more susceptible to SP25A compared with the parental wild-type strain. The degree of susceptibility of the parent strain, but not the dltB and dltD mutant strains, increased at alkaline pH (9.0). In contrast, the parental and mutant strains had the same susceptibilities to syringopeptins SP22A and SP508A at pH 7.0 and 9.0. These results suggest that the cell wall anionic teichoic acids modulate SP25A action against B. subtilis, and they provide an explanation for the selective inhibition of Gram-positive bacteria by SP25A.