Ten recent clinical isolates of serotype Typhimurium from man that were tested for their invasiveness in rabbit ileal explants , were compared with Typhimurium strain TML, a well-characterised invasive strain isolated from a case of human gastro-enteritis. Nine of the 10 strains showed invasiveness that was comparable to that of strain TML. One isolate (GM3) was apparently substantially less invasive; electron microscopy showed this strain to be histotoxic - the probable reason for its reduced recovery from ileal mucosa and thus apparent ‘low’ invasiveness. serotype Choleraesuis strain A50, isolated from a case of systemic salmonellosis in pigs, and serotype Dublin strain 3246, isolated from a case of systemic salmonellosis in calves, were also examined. Dublin strain 3246, when grown at 37C and used immediately in the invasion assay, damaged the mucosa in a manner similar to that of Typhimurium strain GM3, whereas Dublin strain 3246 grown at 37C and stored overnight at 4C did not. This was reflected in an apparently lower invasiveness of freshly grown organisms compared with that of organisms stored at 4C. In contrast, the histotoxicity of Typhimurium strain GM3 was not affected by storage at 4C. When stored at 4C, the levels of invasiveness of Choleraesuis strain A50 and Dublin strain 3246 were not significantly different from each other or from Typhimurium strain TML.


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