Surfactin production enhances the level of cardiolipin in the cytoplasmic membrane of Bacillus subtilis

Surfactin is a cyclic lipopeptide antibiotic that disturbs the integrity of the cytoplasmic membrane. In this study, the role of membrane lipids in the adaptation and possible surfactin tolerance of the surfactin producer Bacillus subtilis ATCC 21332 was investigated. During a 1-day cultivation, the phospholipids of the cell membrane were analyzed at the selected time points, which covered both the early and late stationary phases of growth, when surfactin concentration in the medium gradually rose from 2 to 84μmol·l(-1). During this time period, the phospholipid composition of the surfactin producer's membrane (Sf(+)) was compared to that of its non-producing mutant (Sf(-)). Substantial modifications of the polar head group region in response to the presence of surfactin were found, while the fatty acid content remained unaffected. Simultaneously with surfactin production, a progressive accumulation up to 22% of the stress phospholipid cardiolipin was determined in the Sf(+) membrane, whereas the proportion of phosphatidylethanolamine remained constant. At 24h, cardiolipin was found to be the second major phospholipid of the membrane. In parallel, the Laurdan generalized polarization reported an increasing rigidity of the lipid bilayer. We concluded that an enhanced level of cardiolipin is responsible for the membrane rigidification that hinders the fluidizing effect of surfactin. At the same time cardiolipin, due to its negative charge, may also prevent the surfactin-membrane interaction or surfactin pore formation activity.