We purified osmoregulated periplasmic glucans (OPGs) from Salmonella enterica serovar Typhimurium and found them to be composed of 100 % glucose with 2-linked glucose as the most abundant residue, with terminal glucose, 2,3-linked and 2,6-linked glucose also present in high quantities. The two structural genes for OPG biosynthesis, opgG and opgH, form a bicistronic operon, and insertion of a kanamycin resistance gene cassette into this operon resulted in a strain devoid of OPGs. The opgGH mutant strain was impaired in motility and growth under low osmolarity conditions. The opgGH mutation also resulted in a 2 log increase in the LD 50 in mice compared to the wild-type strain SL1344. Inability to synthesize OPGs had no significant impact on the organism's lipopolysaccharide pattern or its ability to survive antimicrobial peptides-, detergent-, pH-and nutrient-stress conditions. We observed that the opgGH-defective strain respired at a reduced rate under acidic growth conditions (pH 5.0) and had lower ATP levels compared to the wild-type strain. These data indicate that OPGs of S. Typhimurium contribute towards mouse virulence as well as growth and motility under low osmolarity growth conditions.

We recently demonstrated that osmoregulated periplasmic glucans (OPGs) of Salmonella enterica serovar Typhimurium are required for optimal mouse virulence. However, lack of OPGs also generated pleiotropic phenotypes such as reduced motility and slower growth rate under hypoosmotic growth conditions. Whether the observed suboptimal virulence of opg mutants was due to reduced motility was investigated by isolating fully motile revertants of opgGH mutants. Motility revertants remained defective in OPGs synthesis and restitution of motility did not restore mouse virulence. In Escherichia coli, inactivation of rcsB, rcsD, and rcsF lead to restoration of motility in opg mutants, while in Salmonella strains, inactivation of the Rcs pathway is known to attenuate virulence. DNA sequence analysis revealed that except for two silent mutations no other changes in the DNA sequences of Rcs pathway genes were detected in the motility-revertant strain. Moreover, transcripts of all the Rcs phosphorelay pathway genes were detected in opgGH mutants and revertant strain. The data suggest that Salmonella may have distinctive regulatory elements in addition to Rcs phosphorelay genes to rescue motility of opg mutants and affecting also mouse virulence.

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