Temperature-dependent expression of virulence genes in Shigella species

Abstract

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°C, strains of Shigella flexneri 2a, Shigella sonnei, and Shigella dvsenteriae 1 were fully virulent and invaded Henle cells. They also produced keratoconjunctivitis in guinea pigs. When grown at 30°C, the bacteria neither penetrated Henle cells nor produced conjunctivitis in the Sereny test and were phenotypically avirulent. Strains grown at 33°C were only partially invasive in the Henle assay, whereas strains grown at 35°C were as invasive as strains grown at 37°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°C. The percentage of Henle cells invaded by bacteria increased with increasing time of growth at 37°C. Restoration of invasiveness after growth at 30°C required protein synthesis. When shigellae were grown at 30°C and shifted to 37°C for 2 h in the presence of chloramphenicol, the bacteria remained noninvasive. Similarly treated bacteria grown at 37°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. Bacteria of the genus Slhigella are capable of penetrating and multiplying within the epithelial cells of the colon (15). The infection produces ulcerative lesions in the colonic mucosa and results in the bloody mucous diarrhea characteristic of bacillary dysentery (7). Reliable model systems that measure the pathogenic potential of Shigella spp. have been developed, and these include infection of guinea pig conjunctiva (Sereny test [25]) and in vitro infection of epithelial cells in culture (9). In both assay systems, virulent strains of Shigella spp. which are capable of producing dysentery in humans can invade the epithelial cells and multiply within them, whereas avirulent strains do not invade these cells. These assay systems have been useful in defining genetic determinants required for virulence. For example. Sansonetti et al. have demonstrated that virulence is associated with the presence of a 120-megadalton plasmid in Shigella sonnei and a 140-megadalton plasmid (pSf2al40) in Shigellaflexneri 2a (23, 24). We are interested in examining the genetics of pathogenesis of S. flexneri 2a and in determining how expression of

IpaB of Shigella flexneri causes entry into epithelial cells and escape from the phagocytic vacuole.

“…Analysis of the adhesive capacity of M9OT and SC403, following growth at 30°C, a non-permissive temperature for the expression of virulence polypeptides, resulted in a total absence of binding similar to the non-invasive plasmidless derivative BS176. This observation indicated that the expression of this function, like the majority of Shigella virulence determinants is temperature regulated (Maurelli et al, 1984). Destruction of the ipaB gene therefore allowed identification of a temperature regulated adhesion step preceding the entry step.…”

Host age modulates parasite infectivity, virulence and reproduction

“…The tradeoff hypothesis also assumes that virulence and parasite transmission are genetically correlated and linked positively with within-host exploitation. Thus, the virulencetransmission relationship is assumed to be the same for all hosts in the population, even though parasites can express context-dependent virulence due to ecological conditions such as shortage of food (Jokela et al 1999;Brown, Loosli & Schmid-Hempel 2000;Restif & Kaltz 2006), temperature (Maurelli, Blackmon & Curtiss 1984;Blanford et al 2003), stressful hibernation (Brown, depends on the shape of the relationship between virulence and parasite transmission.…”

The Proteome of Shigella flexneri 2a 2457T Grown at 30 and 37 °C

“…For example, a temperature of 37°C is a particularly important environmental signal. Maurelli et al (5) found that S. flexneri cultivated at 37°C produced keratoconjunctivitis in guinea pigs and was able to penetrate and replicate in intestinal epithelial cells, but bacteria grown at 30°C were phenotypically avirulent and non-invasive. The mechanism regulating virulence gene expression in S. flexneri has been well studied (for a review, see Ref.…”