Influence of habituation to inorganic and organic acid conditions on the cytoplasmic membrane composition of Listeria monocytogenes

• Mild proton stress decreased membrane FA ratios of anteiso:iso and C15:C17.
• Organic acids additionally increased most membrane BCFAs and lowered C15:C17 ratio.
• Acid and bile resistance tied to changes in membrane FAs plus unknown adaptations.

This study followed changes in the fatty acid (FA) composition of the membrane of Listeria monocytogenes habituated to organic acids known to induce resistance to acid and bile. Four human-pathogenic strains were separately grown in broth adjusted with HCl to pH 7.4 (baseline) or pH 6.0 (pH control) or broth at pH 6.0 containing 4.75 mM of L-lactic, levulinic or acetic acid. Membrane FAs were extracted from log phase cells and identified using GCMS. Habituation at pH 6.0 versus pH 7.4 decreased (P < 0.01) levels of four of five detected straight chain FAs, while levels of five of six branched chain FAs (BCFAs) increased leading to lowered FA ratios of anteiso:iso and C15:C17. Habituation in organic acids versus the pH control increased levels of most BCFAs and decreased levels of 15-carbon FAs leading to further reduction in the C15:C17 ratio. Those overall effects on membrane composition were largely mirrored in the individual strains known to develop variable resistance to acid and bile in response to habituation in the organic acids. Taken together, these observations suggest exposure of L. monocytogenes to organic acids induces resistance to acid and bile both through generalized changes in membrane composition plus other not-yet identified mechanisms.