Contribution of the Lipopolysaccharide to Resistance of Shigella flexneri 2a to Extreme Acidity

Abstract

Shigella flexneri is endemic in most underdeveloped countries, causing diarrheal disease and dysentery among young children. In order to reach its target site, the colon, Shigella must overcome the acid environment of the stomach. Shigella is able to persist in this stressful environment and, because of this ability it can initiate infection following the ingestion of very small inocula. Thus, acid resistance is considered an important virulence trait of this bacterium. It has been reported that moderate acid conditions regulate the expression of numerous components of the bacterial envelope. Because the lipopolysaccharide (LPS) is the major component of the bacterial surface, here we have addressed the role of LPS in acid resistance of S. flexneri 2a. Defined deletion mutants in genes encoding proteins involved in the synthesis, assembly and length regulation of the LPS O antigen were constructed and assayed for resistance to pH 2.5 after adaptation to pH 5.5. The results showed that a mutant lacking O antigen was significantly more sensitive to extreme acid conditions than the wild type. Not only the presence of polymerized O antigen, but also a particular polymer length (S-OAg) was required for acid resistance. Glucosylation of the O antigen also contributed to this property. In addition, a moderate acidic pH induced changes in the composition of the lipid A domain of LPS. The main modification was the addition of phosphoethanolamine to the 1′ phosphate of lipid A. This modification increased resistance of S. flexneri to extreme acid conditions, provide that O antigen was produced. Overall, the results of this work point out to an important role of LPS in resistance of Shigella flexneri to acid stress.

IntracellularShigellaremodels its LPS to dampen the innate immune recognition and evade inflammasome activation

“…Genetically modified Y. pestis harboring fully potent LPS is completely avirulent, suggesting that the evasion of LPS-TLR4-mediated inflammation is critical for Y. pestis virulence (52,53). Very recently, two reports have highlighted in S. flexneri serotype two adaptation mechanisms involving LPS modifications and amount, respectively (54,55). LPSs contribute to the capacity of Shigella to resist acid conditions, like those encountered in the stomach, with modifications in the lipid A region, which is added with a phosphoethanolamine residue to decrease the negative charge at the bacterial surface (54).…”

“…Very recently, two reports have highlighted in S. flexneri serotype two adaptation mechanisms involving LPS modifications and amount, respectively (54,55). LPSs contribute to the capacity of Shigella to resist acid conditions, like those encountered in the stomach, with modifications in the lipid A region, which is added with a phosphoethanolamine residue to decrease the negative charge at the bacterial surface (54). Furthermore, LPS layer from Shigella grown at 30°C appeared thicker than that of cells grown at 37°C.…”

Unique Structural Modifications Are Present in the Lipopolysaccharide from Colistin-Resistant Strains of Acinetobacter baumannii

“…Recent work has shown that modification of lipid A is a strategy employed not only in A. baumannii strains but in many other Gram-negative bacteria. Recent reports have shown Shigella flexneri lipid A to be modified by pEtN groups and involved in resistance to extreme acidity (32). Loss of pEtN addition in lptA null strains (lptA encodes the pEtN transferase for the 4= position of lipid A) of Neisseria gonorrhoeae modulates resistance to complement killing and polymyxin resistance (33).…”

Bacteriophage-mediated Glucosylation Can Modify Lipopolysaccharide O-Antigens Synthesized by an ATP-binding Cassette (ABC) Transporter-dependent Assembly Mechanism

“…Although glucosylation does not appear to change this role in Salmonella and Shigella (43,45,46), other cellular properties are certainly impacted in a species-specific manner. In Shigella, glucosylation leads to greater invasion, which is correlated to enhanced function and exposure of the type 3 secretion system (43), and elevated acid tolerance (47). In Salmonella, glucosylation is associated with virulent isolates (48) but is not a stable property leading to antigenic (form) variation in the O serotype (45,46,49).…”