Temperature-dependent expression of virulence genes in Shigella species

The pathogenicity of Shigella spp. involves the ability of the bacteria to penetrate and replicate within the epithelial cells of the large intestine. Model systems for examining the virulence of shigellae employ Henle intestinal epithelial cells in tissue culture and an in vivo assay for virulence in guinea pig eyes (Sereny test). Using these systems, we studied the genetic and physiological bases for the ability of shigellae to invade epithelial cells. We found that expression of virulence in Shigella spp. is dependent on the temperature at which the bacteria are grown. When grown at 37 degrees C, strains of Shigella flexneri 2a, Shigella sonnei, and Shigella dysenteriae 1 were fully virulent and invaded Henle cells. They also produced keratoconjunctivitis in guinea pigs. When grown at 30 degrees C, the bacteria neither penetrated Henle cells nor produced conjunctivitis in the Sereny test and were phenotypically avirulent. Strains grown at 33 degrees C were only partially invasive in the Henle assay, whereas strains grown at 35 degrees C were as invasive as strains grown at 37 degrees C. Using the Henle cell assay, we determined that the loss of ability to penetrate epithelial cells was completely reversed by shifting the growth temperature from 30 to 37 degrees C. The percentage of Henle cells invaded by bacteria increased with increasing time of growth at 37 degrees C. Restoration of invasiveness after growth at 30 degrees C required protein synthesis. When shigellae were grown at 30 degrees C and shifted to 37 degrees C for 2 h in the presence of chloramphenicol, the bacteria remained noninvasive. Similarly treated bacteria grown at 37 degrees C were still invasive. These results suggested that expression of one or more genes required for virulence of Shigella spp. are subject to regulation by growth temperature.