QUANTITATIVE STUDIES OF INVASION OF RABBIT ILEAL MUCOSA BY SALMONELLA-TYPHIMURIUM STRAINS WHICH DIFFER IN VIRULENCE IN A MODEL OF GASTROENTERITIS

An asymmetric organ culture system in which ileal tissues, freshly rem oved from rabbits, can be maintained structurally and functionally for up to 4 h has been developed. The composition of the solutions used t o maintain ileal tissue in vitro were as follows. The serosal surface was bathed in the World Health Organization (WHO) rehydration formulat ion: NaCl, 60 mM; NaHCO3, 30 mM; KCl, 20 mM; and glucose, 111 mM. The mucosal surface was bathed in the same solution with tao important cha nges: all the sodium was replaced by choline, which is not absorbed, a nd tissue culture medium (consisting of commercial minimal essential m edium to which was added fetal calf serum and glutamine to final conce ntrations of 10% [vol/vol] and 2.0 mM, respectively) was added to the choline-containing medium to a final concentration of 10% (vol/vol). T he initial invasiveness (first 2 h) of seven strains of Salmonella typ himurium differing in virulence (defined in terms of clinical origin o r the ability to induce fluid loss in monkeys or rabbit ileal loops) w as assessed quantitatively in an in vitro invasion assay with the orga n culture system. The virulent strains (TML, W118, and WAKE) were foun d to be about 25- to 100-fold more invasive than the avirulent strains (SL1027, M206, LT7, and Thax-1). Thus, a clear correlation between in itial mucosal invasion and virulence of S. typhimurium in a model whic h is relevant to human gastroenteritis was established. This is the fi rst time, to our knowledge, that quantitative studies of invasiveness have been carried out in vitro on freshly isolated functioning gut.