Effect of the O-Antigen Length of Lipopolysaccharide on the Functions of Type III Secretion Systems in Salmonella enterica

Abstract

The virulence of Salmonella enterica critically depends on the functions of two type III secretion systems (T3SS), with the Salmonella pathogenicity island 1 (SPI1)-encoded T3SS required for host cell invasion and the SPI2-T3SS enabling Salmonella to proliferate within host cells. A further T3SS is required for the assembly of the flagella. Most serovars of Salmonella also possess a lipopolysaccharide with a complex O-antigen (OAg) structure. The number of OAg units attached to the core polysaccharide varies between 16 and more than 100 repeats, with a trimodal distribution. This work investigated the correlation of the OAg length with the functions of the SPI1-T3SS and the SPI2-T3SS. We observed that the number of repeats of OAg units had no effect on bacterial motility. The interaction of Salmonella with epithelial cells was altered if the OAg structure was changed by mutations in regulators of OAg. Strains defective in synthesis of very long or long and very long OAg species showed increased translocation of a SPI1-T3SS effector protein and increased invasion. Invasion of a strain entirely lacking OAg was increased, but this mutant strain also showed increased adhesion. In contrast, translocation of a SPI2-T3SS effector protein and intracellular replication were not affected by modification of the OAg length. Mutant strains lacking the entire OAg or long and very long OAg were highly susceptible to complement killing. These observations indicate that the architecture of the outer membrane of Salmonella is balanced to permit sufficient T3SS function but also to confer optimal protection against antimicrobial defense mechanisms.Salmonella enterica is a remarkable pathogen with strategies for adaptation to different lifestyles in the environment as well as within various host organisms. The requirements for life within the host can vary dramatically, for example, after transition from extracellular life within the intestine to an intracellular life within a special organelle formed inside infected host cells (reviewed in reference 23). The presence of an outer membrane is an important structural feature that enables commensal as well as pathogenic bacteria to adapt to the intestine and to resist bile salts and various molecules of the host innate immune system (44). Of specific importance is the lipopolysaccharide (LPS), the major constituent of the outer leaflet of the outer membrane. LPS is composed of (i) the lipid A portion, consisting of acyl chains linked to phosphorylated N-acetylglucosamine; (ii) the inner and outer core moieties, consisting of rather conserved sugars; and (iii) a highly variable O antigen (OAg). The LPS of S. enterica serovar Typhimurium and several other serovars has special characteristics, with extreme heterogeneity in the length of the OAg repeats. LPS species with a short OAg (S-OAg) consisting of about 16 repeats of OAg units can be found. In addition, LPS species with long and very long OAg (L-OAg and VL-OAg, respectively) are present, containing about 35 and more than 100 repeat...

Design of Glycoconjugate Vaccines against Invasive African Salmonella enterica Serovar Typhimurium

“…Since S. Typhimurium OAg size and structure impact the immune response (25,48,49), we investigated both aspects by producing strain-specific conjugates, with OAg populations differing only for the average MW and OAg/CRM 197 ratio (D23580 OAg vaccines) and strain-variant conjugates, with OAg differing for both O-acetylation (position and amounts of O-acetyl groups) and glucosylation levels.…”

Protein Export According to Schedule: Architecture, Assembly, and Regulation of Type III Secretion Systems from Plant- and Animal-Pathogenic Bacteria

“…Notably, additional studies of Shigella and Salmonella spp. also revealed an influence of the length of extracellular lipopolysaccharide molecules on effector protein translocation (226,596).…”

Requirement of the Lipopolysaccharide O-Chain Biosynthesis Gene wxocB for Type III Secretion and Virulence of Xanthomonas oryzae pv. Oryzicola

“…Possibly, the O chain creates a distance between the bacterial and plant cells that positively influences the ability of the type III needle to penetrate the plant cell wall. The effect of O-chain length on efficiency of SPI1-T3S but not SPI2-T3S in Salmonella supports this concept, since the SPI system (in contrast to the SP2 system) plays its role when the pathogen is outside the host cell (26). Testing of the possible structural dependency of the X. oryzae pv.…”

“…Glucosylation maintains a coiled configuration for LPS and allows the T3S pilus to extend beyond it and efficiently deliver effectors (25). Related to this, shorter O-chain length increases the efficiency of the SPI1-T3S system and invasiveness of Salmonella (26). Regulatory dependencies of T3S on LPS have been reported for Pseudomonas aeruginosa (27) and Yersinia enterocolitica (28,29).…”