Interference of virulence in Escherichia coli O157:H7 by molecule(s) secreted by probiotic lactic acid bacteria
Enterohemorrhagic 'E. coli' (EHEC) O157:H7 is a clinically important food- and waterborne pathogen that causes bloody diarrhea and hemolytic uremic syndrome (HUS) worldwide. Expression of virulence-related genes in EHEC O157:H7 is controlled by quorum sensing (QS). EHEC O157:H7 can sense and act in response to environmental signals, including signals produced by commensal microbiota and the human host. As many bacteria in the human gastrointestinal tract (GIT), including probiotic bacteria, utilize QS mechanisms to coordinate a range of processes, it should not be surprising that microbiota from the GI tract could manage to alter or interfere with bacterial QS. The purpose of this study was to investigate the effects of molecule(s) secreted by selected probiotic lactic acid bacteria (LAB) on EHEC O157:H7 virulence and to determine their role in QS. This thesis shows that EHEC O157:H7 grown in the presence of cell-free spent medium (CFSM) from 5 of the 6 strains of probiotic LAB studied decreased significantly the expression of virulence-related genes in EHEC O157:H7 and interfered with the production or the detection of the signaling molecule, autoinducer 2, (AI-2) involved in QS by EHEC O157:H7. Furthermore the capacity of EHEC O157:H7 to attach to epithelial cells in tissue culture and in mice was attenuated when the probiotic 'Lactobacillus acidophilus' strain La-5 CFSM was present. The 'in vivo' study showed that 'L. acidophilus' La-5 CFSM increased resistance against infection and colonization with EHEC O157:H7. Taken together, these results propose another possible mode of action by which probiotic bacteria might keep bacterial pathogens in check within the GIT. In addition, we undertook separation and purification analysis in order to determine the nature of the molecules responsible for impairing the virulence of EHEC O157:H7. After several separation steps, we collected four chromatographic peaks and showed that the molecules involved in the interference of QS regulation of EHEC O157:H7 O157 virulence are likely to be small peptides secreted by the probiotic 'L. acidophilus' La-5.