Gene expression kinetics governs stimulus-specific decoration of the Salmonella outer membrane

Abstract

Lipid A is the innermost component of the lipopolysaccharide (LPS) molecules that occupy the outer leaflet of the outer membrane in Gram-negative bacteria. It 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 now 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-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 post-translationally, resulting in expression of PmrA-activated genes including pmrR, encoding an LpxT inhibitor, and those 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 increase in lipid A negative charge. Our findings demonstrate how bacteria tailor their cell surface to different stresses such as those faced inside phagocytic cells.

Intestinal epithelial NAIP/NLRC4 restricts systemic dissemination of the adapted pathogen Salmonella Typhimurium due to site-specific bacterial PAMP expression

“…Such compartmentspecific down regulation has been observed previously in various infection models. 25,[80][81][82] S. Tm requires TTSS-1 and flagella (composed of flagellin subunits) for the initial colonization and establishment of gut infection. 16,72 Especially for invasion of IECs, which strongly express Naip1, 2, 5, 6, and Nlrc4 ( Fig.…”

Overexpression of lpxT Gene in Escherichia coli Inhibits Cell Division and Causes Envelope Defects without Changing the Overall Phosphorylation Level of Lipid A

“…Finally, we observed that tris -phosphorylated lipid A was not enhanced in lpxT overexpressing cells, as shown by MALDI-TOF MS, probably because of feedback regulation mechanisms that limit LpxT activity when the lpxT gene is induced [ 17 , 18 , 21 ]. This result rules against the hypothesis that LpxT toxicity may be due to hyperphosphorylation of lipid A. Contextually, an LpxT H190A variant devoid of catalytic activity [ 19 ] is even more detrimental for growth, at least in solid medium, than the wt protein.…”

“…In E. coli , LpxT is encoded by the lpxT gene and it is subject to a complex regulation. Indeed, lpxT transcription initiation is downregulated at 37–42 °C vs. 30 °C, in stationary phase and at high Mg 2+ concentration [ 15 , 17 , 18 ]. The lpxT mRNA stability is also modulated, as the mRNA is quickly destabilized after a temperature upshift from 30 to 37 °C [ 17 ].…”

Genetic variation in the MacAB-TolC efflux pump influences pathogenesis of invasive Salmonella isolates from Africa

“…We took a reductive approach to quantify the impact of macAB genotype on antimicrobial peptide resistance. We reasoned that the contribution of MacAB to antimicrobial peptide resistance could be obscured in vitro by the profound PhoP-induced membrane modifications that dramatically slow antimicrobial peptide interactions with the cell envelope [ 59 ]. Furthermore, the constitutively expressed AcrAB-TolC pump effluxes a wide variety of compounds and can mask the contributions of other efflux pumps like MacAB during in vitro tests of antibiotic resistance [ 34 , 60 ].…”

“…Typhimurium replication in macrophages [ 38 , 56 ], a niche where PhoP/Q signaling is active and important for Salmonella survival [ 55 , 61 ]. In addition to the different Salmonella isolates used across studies, our results suggesting PhoP promotes macAB transcription may arise from differences in experimental conditions, the importance of regulatory kinetics such as PhoP turnover [ 38 ], changes in ribosomal RNA levels in response to low magnesium [ 84 ], or coordinated activity with other transcriptional regulators [ 59 ]. We conclude that PhoP-inducing signals generally serve to maximize macAB transcription in Salmonella .…”