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Abstract
Cyclic lipopeptides act against a variety of plant pathogens and are thus highly efficient
crop-protection agents. Some pesticides contain Bacillus subtilis strains that produce
lipopeptide families, such as surfactins (SF), iturins (IT), and fengycins (FE). The
antimicrobial activity of these peptides is mainly mediated by permeabilizing cellular
membranes. We used a fluorescence-lifetime based leakage assay to examine the effect
of individual lipid components in model membranes on lipopeptide activity. Leakage
induction by FE was strongly inhibited by cholesterol (CHOL) as well as by phosphatidylethanolamine
(PE) and -glycerol (PG) lipids. Already moderate amounts of CHOL increased the tolerable
FE content in membranes by an order of magnitude to 0.5 FE per PC + CHOL. This indicates
reduced FE-lipid demixing and aggregation, which is known to be required for membrane
permeabilization and explains the strong inhibition by CHOL. Ergosterol (ERG) had
a weak antagonistic effect. This confirms results of microbiological tests and agrees
with the fungicidal activity and selectivity of FE. SF is known to be much less selective
in its antimicrobial action. In line with this, liposome leakage by SF was little
affected by sterols and PE. Interestingly, PG increased SF activity and changed its
leakage mechanism toward all-or-none, suggesting more specific, larger, and/or longer-lived
defect structures. This may be because of the reduced energetic cost of locally accumulating
anionic SF in an anionic lipid matrix. IT was found largely inactive in our assays.
B. subtilis QST713 produces the lipopeptides in a ratio of 6 mol SF: 37 mol FE: 57 mol
IT. Leakage induced by this native mixture was inhibited by CHOL and PE, but unaffected
by ERG and by PG in the absence of PE. Note that fungi contain anionic lipids, but
little PE. Hence, our data explain the strong, fungicidal activity and selectivity
of B. subtilis QST713 lipopeptides.