Studies on early association of Salmonella typhimurium with intestinal mucosa in vivo and in vitro: relationship to virulence.

The abilities of six strains of Salmonella typhimurium to associate with rabbit ileal mucosa have been measured in vitro. Two were "virulent" strains (TML and W118 which are invasive and inducers of fluid secretion in rabbit ileal loops); four were "avirulent" (LT7, M206 and SL1027 which are invasive but induce negligible fluid secretion, and Thax-1 which is neither invasive nor an inducer of fluid secretion). A special organ-culture apparatus was designed to expose only the luminal surface of the mucosa to organisms. Viable counts of washed homogenised tissue taken 30 min after challenge showed that virulent strains TML and W118 and avirulent strains LT7 and M206 could not be distinguished from each other. Avirulent strain SL1027 associated less well than the other four strains, and Thax-1 associated less well than SL1027; both these strains were non-motile whereas the other four were motile. Thus, early association with gut mucosa did not discriminate all avirulent strains from the virulent strains. Qualitative examination of tissues by scanning electronmicroscopy did not detect strains LT7 and M206 on the mucosal surface whereas strains TML and W118 were readily seen, suggesting that the nature of association of virulent and avirulent strains was different. Qualitative examination by transmission electronmicroscopy of tissues challenged in vivo for 120 min showed virulent strains TML and W118 invading epithelial cells; similar events were reproduced after 120-min challenge in vitro. In contrast, invasion by avirulent strains was observed only very rarely.