Single strains of Lactobacillus acidophilus and Lact. fermentum, isolated from chicken intestine, were used to study in vitro interactions with Salmonella enteritidis, Salm. pullorum or Salm. typhimurium in an ileal epithelial cell (IEC) radioactive assay. Exclusion, competition and displacement phenomena were investigated by respectively incubating (a) lactobacilli and IEC together, prior to addition of salmonellae, (b) lactobacilli, IEC and salmonellae together, and (c) salmonellae and IEC, followed by the lactobacilli. Lactobacilli were selected for study because of their strong ability to adhere to IEC and poor aggregation with salmonellae. The results demonstrated that Lact. acidophilus significantly reduced (P < 0.05) the attachment of Salm. pullorum to IEC in the tests for exclusion and competition, but not in the displacement tests. Lactobacillus fermentum was found to have some ability to reduce the attachment of Salm. typhimurium to IEC under the conditions of exclusion (P < 0.08), competition (P < 0.09), but not displacement. However, both Lact. acidophilus and Lact. fermentum were unable to reduce the adherence of Salm. enteritidis to IEC under any of the conditions.
Specific Escherichia coli isolates lysogenised with prophages that express Shiga toxin (Stx) can be a threat to human health, with cattle being an important natural reservoir. In many countries the most severe pathology is associated with enterohaemorrhagic E. coli (EHEC) serogroups that express Stx subtype 2a. In the United Kingdom, phage type (PT) 21/28 O157 strains have emerged as the predominant cause of life-threatening EHEC infections and this phage type commonly encodes both Stx2a and Stx2c toxin types. PT21/28 is also epidemiologically linked to super-shedding (>103 cfu/g of faeces) which is significant for inter-animal transmission and human infection as demonstrated using modelling studies. We demonstrate that Stx2a is the main toxin produced by stx2a+/stx2c+ PT21/28 strains induced with mitomycin C and this is associated with more rapid induction of gene expression from the Stx2a-encoding prophage compared to that from the Stx2c-encoding prophage. Bacterial supernatants containing either Stx2a and/or Stx2c were demonstrated to restrict growth of bovine gastrointestinal organoids with no restriction when toxin production was not induced or prevented by mutation. Isogenic strains that differed in their capacity to produce Stx2a were selected for experimental oral colonisation of calves to assess the significance of Stx2a for both super-shedding and transmission between animals. Restoration of Stx2a expression in a PT21/28 background significantly increased animal-to-animal transmission and the number of sentinel animals that became super-shedders. We propose that while both Stx2a and Stx2c can restrict regeneration of the epithelium, it is the relatively rapid and higher levels of Stx2a induction, compared to Stx2c, that have contributed to the successful emergence of Stx2a+ E. coli isolates in cattle in the last 40 years. We propose a model in which Stx2a enhances E. coli O157 colonisation of in-contact animals by restricting regeneration and turnover of the colonised gastrointestinal epithelium.