The Lipid Lysyl-Phosphatidylglycerol Is Present in Membranes of Rhizobium tropici CIAT899 and Confers Increased Resistance to Polymyxin B Under Acidic Growth Conditions

Abstract

Lysyl-phosphatidylglycerol (LPG) is a well-known membrane lipid in several gram-positive bacteria but is almost unheard of in gram-negative bacteria. In Staphylococcus aureus, the gene product of mprF is responsible for LPG formation. Low pH-inducible genes, termed lpiA, have been identified in the gram-negative α-proteobacteria Rhizobium tropici and Sinorhizobium medicae in screens for acid-sensitive mutants and they encode homologs of MprF. An analysis of the sequenced bacterial genomes reveals that genes coding for homologs of MprF from S. aureus are present in several classes of organisms throughout the bacterial kingdom. In this study, we show that the expression of lpiA from R. tropici in the heterologous hosts Escherichia coli and Sinorhizobium meliloti causes formation of LPG. A wild-type strain of R. tropici forms LPG (about 1% of the total lipids) when the cells are grown in minimal medium at pH 4.5 but not when grown in minimal medium at neutral pH or in complex tryptone yeast (TY) medium at either pH. LPG biosynthesis does not occur when lpiA is deleted and is restored upon complementation of lpiA-deficient mutants with a functional copy of the lpiA gene. When grown in the low-pH medium, lpiA-deficient rhizobial mutants are over four times more susceptible to the cationic peptide polymyxin B than the wild type.

Low pH Enhances the Action of Maximin H5 against Staphylococcus aureus and Helps Mediate Lysylated Phosphatidylglycerol-Induced Resistance

“…Low pH can exert a protective effect by promoting the membrane anchoring of cell wall enzymes and thereby shield these organisms from AMPs 108,109 . Acid conditions have also been shown to promote resistance to AMPs in soil bacteria 92 and to inhibit the antibacterial activity of endogenous human peptides, such as defensins and LL-37, to organisms found in lungs 110,111 . On the other hand, PMF dependent efflux pumps also appear able to import porcine, bovine and other AMPs, thereby promoting their activity [112][113][114] .…”

Resistance Phenotypes Mediated by Aminoacyl-Phosphatidylglycerol Synthases

“…One important bacterial response to such compounds is the aminoacylation of PG resulting in a reduction of the overall net negative charge of the membrane. The resulting aminoacylphosphatidylglycerol (aaPG) molecules can either be zwitterionic (A-PG) or alternatively they carry an overall positive net charge, as is the case for lysyl-phosphatidylglycerol (L-PG) (20,34,43,56,75,82). The resulting charge characteristics of the bacterial envelope were also proposed to have a profound impact on biophysical properties like membrane fluidity and lipid head group interaction (72,76,89).…”

“…L-PG formation under low-pH conditions has been described for Staphylococcus aureus, Enterococcus faecalis (formerly known as Streptococcus faecalis) (35), and Rhizobium tropici (82). The enzyme responsible for the formation of L-PG in S. aureus was identified during studies of the bacterial immune escape mechanisms (65).…”

Sinorhizobium melilotiMutants Deficient in Phosphatidylserine Decarboxylase Accumulate Phosphatidylserine and Are Strongly Affected during Symbiosis with Alfalfa

“…Additionally, both PE-deficient mutants have a drastically increased proportion of anionic membrane lipids. This might change the surface characteristics of the bacteria and make them more susceptible to antimicrobial cationic peptide of the host's defense, as has been described for membrane lipid biosynthesis mutants in S. aureus and R. tropici (40,52). Initiation of nodule formation can be observed in the wild type and the mutant CS111 around the same day, whereas nodule formation starts about 20 days later in the mutant MAV01.…”

“…Recently PC has been shown to be required for pathogenesis of Legionella pneumophila, Brucella abortus, and Agrobacterium tumefaciens on their hosts (7,8,9,59). The cationic membrane lipid lysyl-phosphatidylglycerol is involved in conferring resistance to cationic antimicrobial peptides of the host's innate immune system to Staphylococcus aureus (40), and the presence of LPG in Rhizobium tropici also increases resistance to the cationic peptide polymyxin B (52).…”