Environmental signals induce major changes in virulence of Shigella spp.

Growth conditions play a major role in expression of virulence by Shigella spp. both in vitro (adherence and internalization in eukaryotic host cells) and in vivo (keratoconjunctivitis). Optimized expression of virulence required anaerobic growth to log phase in particular media such as brain heart infusion broth. Kinetic studies of guinea pig eye infections showed that as few as 2 x 10(5) S. dysenteriae CG097 or S. flexneri M90T, grown under these optimized conditions, produced keratoconjunctivitis in 15 h. In vitro studies demonstrated that adherence to and invasion of Henle 407 cells, at 37 degrees C, by organisms grown under these optimized conditions, were significantly greater than when organisms were grown aerobically under the same conditions.