Membrane cyclopropane fatty acid content is a major factor in acid resistance of Escherichia coli

Cyclopropane fatty acid (CFA) formation is a postsynthetic modification of the lipid bilayer that occurs as cultures of Escherichia coli and many othe r bacteria enter stationary phase. We report the first distinct phenotype f or this membrane modification; early stationary phase cultures of strains l acking CFA (as a result of a null mutation in the cfa gene) are abnormally sensitive to killing by a rapid shift from neutral pH to pH 3. This sensiti vity to acid shock is dependent on CFA itself because resistance to acid sh ock is restored to cfa mutant strains by incorporation of CFAs from the gro wth medium or by introduction of a functional cfa gene on a plasmid. The sy nthesis of CFA depends in part on the RpoS sigma factor, but the role of Rp oS in resistance to acid shock involves additional factors because strains with null mutations in both cfa and rpoS are more sensitive to acid shock t han either single mutant strain. Exponential phase cultures of E. coli are much more sensitive to acid shock than stationary phase cultures, but survi val is greatly increased if the exponential phase cultures are exposed to m oderately acid conditions (pH 5) before shift to pH 3. We show that exposur e to moderately acid conditions gives a marked increase in cfa transcriptio n. The efficiency of the survival of acid shock is extremely strain depende nt, even among putative wild-type strains. Much, but not all, of this varia bility can be explained by the partially or totally defective RpoS alleles carried by many strains.