2. NCIM Resource Center, CSIR- National Chemical Laboratory, Pune-411 008, India.
3. Department of Veterinary Public Health and Epidemiology, Madras Veterinary College, Chennai-600 007, India.
4. Soybean Genomics and Improvement Laboratory, Henry A. Wallace Beltsville Agricultural Research Center, Agricultural Research Service, USDA, 10300 Baltimore Avenue, BARC-E, Beltsville, MD 20705-2350, USA.
5. Diet, Genomics and Immunology Laboratory, Henry A. Wallace Beltsville Human Nutrition Research Center, Agricultural Research Service, USDA, 10300 Baltimore Avenue, BARC-E, Beltsville, MD 20705-2350, USA.
Background: Salmonella enterica serovar Typhimurium SL 1344 migrates on moist surfaces by swarming motility. S. enterica serovar Typhimurium synthesized osmoregulated periplasmic glucans (OPG) using opgGH bicistronic operon under low osmolarity conditions (<70 mMos Mol l-1). OPG were not detected when cells were grown in swarm motility-promoting media which were typically iso- or hyperosmotic (>400 mMos Mol l-1).
Findings: We observed that an opgGH-deletion mutant was defective in swarm motility. Swarm motility was complimented by a plasmid bearing a wild type copy of opgGH. Since synthesis of OPG is below the detection limits at medium osmolarity >400 mMos Mol l-1 the requirement of opgGH operon for swarm motility appears counter intuitive. We observed that in wild-type cells, transcripts of opgGH genes remained high even at 600 mMos Mol l-1, the highest osmolarity at which swarm motility occurred. Truncated and in-frame deletion copies of opgGH (carrying deletions in transmembrane domains) as well as plasmid expressing catalytically dysfunctional active site of OpgH (OpgHD346G, D348G) failed to restore swarm motility.
Conclusions: Thus full-length opgGH gene products were needed to support swarm motility even though no OPG synthesis was detected in swarm motility growth media. The requirement of OPG for swarm motility appears to be indirect, since in the opgGH mutant, several class-II and -III flagella regulatory genes were down-regulated specifically under swarm growth conditions. It is postulated that the lack of OPG results in cells incapable of transducing surrounding environmental stimuli, possibly due to increased transcript levels of cyclic di-GMP (secondary messenger) modulator gene (ydiV) in the opgGH mutant under swarm growth conditions.
Keywords: Food borne pathogens, flagella function, pathogen transport, salmonella