LAUSR

Activation of the same genes with different kinetics elicits distinct modifications of the Salmonella outer membrane. Same genes, different phenotypes The lipopolysaccharide (LPS) coat of Gram-negative bacteria can be modified to evade the host immune response or confer resistance to antimicrobial compounds. Modifications that reduce the negative charge of the LPS component lipid A increase the resistance of Salmonella to the antibiotic polymyxin B and other cationic antimicrobial compounds. Hong et al. found that different stimuli elicited distinct lipid A modification profiles in Salmonella enterica serovar Typhimurium, although they promoted expression of the same set of genes encoding lipid A–modifying enzymes. Different stimuli caused the genes to be activated with different kinetics, thus altering the temporal order in which the lipid A–modifying enzymes were produced. This resulted in distinct lipid A profiles and different effects on resistance to polymyxin B. These findings illustrate how bacteria can use a limited set of enzymes to generate a range of adaptations to different stimuli. Lipid A is the innermost component of the lipopolysaccharide (LPS) molecules that occupy the outer leaflet of the outer membrane in Gram-negative bacteria. Lipid A is recognized by the host immune system and targeted by cationic antimicrobial compounds. In Salmonella enterica serovar Typhimurium, the phosphates of lipid A are chemically modified by enzymes encoded by targets of the transcriptional regulator PmrA. These modifications increase resistance to the cationic peptide antibiotic polymyxin B by reducing the negative charge of the LPS. We report the mechanism by which Salmonella produces different lipid A profiles when PmrA is activated by low Mg2+ versus a mildly acidic pH. Low Mg2+ favored modification of the lipid A phosphates with 4-amino-4-deoxy-l-aminoarabinose (l-Ara4N) by activating the regulatory protein PhoP, which initially increased the LPS negative charge by promoting transcription of lpxT, encoding an enzyme that adds an additional phosphate group to lipid A. Later, PhoP activated PmrA posttranslationally, resulting in expression of PmrA-activated genes, including those encoding the LpxT inhibitor PmrR and enzymes responsible for the incorporation of l-Ara4N. By contrast, a mildly acidic pH favored modification of the lipid A phosphates with a mixture of l-Ara4N and phosphoethanolamine (pEtN) by simultaneously inducing the PhoP-activated lpxT and PmrA-activated pmrR genes. Although l-Ara4N reduces the LPS negative charge more than does pEtN, modification of lipid A phosphates solely with l-Ara4N required a prior transient increase in lipid A negative charge. Our findings demonstrate how bacteria tailor their cell surface to different stresses, such as those faced inside phagocytes.