Khan, AnjamAbdalkrim, Horeyah2023-07-242023-07-242023-07-19https://hdl.handle.net/20.500.14154/68696Salmonella causes a broad range of diseases in humans ranging from gastroenteritis to typhoid fever. The intestinal microbiota has evolved various strategies, termed colonisation resistance, for restraining the growth of invading enteric pathogens. In addition, probiotic bacteria can strengthen the resistance of the host to invading pathogens, but our knowledge of their direct antagonistic effects on pathogens is limited. Escherichia coli Nissle 1917 is one example of a probiotic bacterium. This study aimed to identify contact-dependent and contact-independent interactions between Salmonella and selected members of the intestinal microbiota, including probiotics. This thesis used Salmonella enterica serovar Enteritidis P125109 as a model pathogen and E.coli Nissle 1917 as the probiotic strain. E.coli is the predominant member of the Gamma proteobacteria in the intestinal microbiota. In the coculture experiments, Salmonella Enteritidis and E.coli Nissle 1917 were grown together, and it was observed that the presence of both strains together did not affect their growth profiles. To explore contact-independent interactions, we utilized cell-free conditioned media from E.coli Nissle 1917. When Salmonella Enteritidis was grown in conditioned media containing molecules released by E.coli Nissle 1917, there were no significant differences in Salmonella growth compared to the control. However, an analysis of the extracellular protein profiles revealed that certain proteins involved in Salmonella's pathogenicity. Using MALDI-TOF MS, the missing proteins were identified as Salmonella pathogenicity island 1 type 3 secretion system (SPI-1 T3SS) effectors SipA and SipC. Further investigations focused on the role of indole, a signalling molecule produced by E.coli Nissle 1917, in regulating Salmonella's behaviour. We knocked out the tryptophanase gene (tnaA) in E.coli Nissle 1917, which eliminated the production of indole. RNA-sequencing analysis of Salmonella Enteritidis grown in conditioned media from either E.coli Nissle 1917 WT and the indole-deficient mutant revealed that the expression of SPI-1 T3SS genes was significantly decreased in the presence of E.coli Nissle 1917 WT and indole mutant conditioned media. Interestingly, even in the absence of indole, the expression of SPI-1 T3SS genes remained downregulated but to a lesser extent than in the presence of indole. This suggested the existence of an additional thermostable and proteinase K-resistant factor, referred to as "Factor X," which contributed significantly to the observed effects on the SPI-1 T3SS phenotype.238enSalmonellaE.coli NissleType 3 secretion systemThesis