PhD (Microbiology) / Department of Microbiology / Diarrhoea is still a major problem worldwide. Enteric pathogens such as Enteroaggregative E. coli (EAEC), Enteropathogenic E. coli (EPEC) and Enterotoxigenic E. coli (ETEC) have been reported to cause diarrhoea in children under the age of 5 years. The incidences of these pathogens are due to factors such as poor water quality, sanitation and hygiene practices. Infections with these pathogens result in diarrhoea and have been reported to result in severe disease outcomes more especially in children under 2 years of age. EPEC infections have been well studied using in vitro analyses, with studies highlighting the adherence traits, proteins and virulence genes involved in pathogenesis and inflammatory responses. EPEC is characterized by localized adherence with microcolony formation at the site of infection. In vivo studies have reported on human EPEC infection. However, the current animal models have not been able to replicate clinical outcomes (such as diarrhoea and weigh loss) of EPEC infection similar to humans. Therefore, there is still a need for a suitable small animal model that mimic clinical outcomes of human EPEC infections in vivo. Children living in poor environmental conditions are more susceptible to diarrhoeal pathogens. Furthermore, the incidences of children being exposed to co-infections (more than one pathogen at the same time) is relatively high. The EAEC/EPEC (A/P) and EPEC/ETEC (P/T) co-infections have been increasingly detected in children with and without diarrhoea. It has been suggested that patients infected with these co-infections might result in severe disease outcome than those infected with single pathogens. Pathogens are constantly evolving and the microbe-microbe interaction in the host can result in these pathogens competing for the same niche and thus result in increased virulence. Interaction of co-infections can lead to increased inflammatory responses thus affecting the infected host. The first objective of this study was to develop an EPEC murine model using weaned C57BL/6 mice that have been pretreated with antibiotic cocktail. Mice were orally infected with wild-type (WT) typical EPEC, bfp- and escN mutant strains. The WT had transient weight loss and wet stools with mucous; and the bfp- infected mice also had transient weight loss and bloody stool appearance. Increase in inflammatory biomarkers MPO, LCN-2, CRP, IL-6 and SAA were observed in the WT and bfp- infected mice. The mice infected with escN mutant did not exhibit any weight changes and the stools were similar to the uninfected mice. Furthermore, no inflammatory biomarkers were observed in mice infected with the escN mutant. Metabolic perturbations were observed in WT EPEC infected mice at day 3 post infection with the TCA cycle metabolites (reduced succinate, citrate, fumarate, cis-aconitate) being excreted at lower quantities indicating that the energy production in the infected mice was greatly affected. The second objective of this study was to determine the interaction between the P/T coinfections using in vitro and in vivo analyses. In vitro, human colorectal tumour 8 (HCT-8) cells were infected with single strains of ETEC, EPEC and both the pathogens and incubated for 3 hours. After infection the cells were analysed for bacterial adherence using real-time PCR. The single strains adhered at the same rate similar to the P/T coinfected cells. IL-8, as a marker of inflammatory response, was measured using ELISA. The results indicated that the P/T co-infected cells had a significant increase in IL-8 response higher than the single infections. The P/T co-infections were further analysed in vivo using the EPEC murine model developed in this study. Interestingly, mice infected with P/T co-infections developed severe diarrhoea accompanied with significant increased weight loss and some mice died during the 3-day infection period. The inflammatory responses MPO, LCN-2 and SAA were higher in the co-infected mice indicating a synergistic effect. The bfp and eltA virulence genes were significantly increased in the P/T co-infections. The third objective of this study was to determine the interaction between A/P coinfections using in vitro and in vivo analyses. HeLa cells and HCT-8 cells were infected with EAEC, EPEC and both the pathogens at the same time in order to determine adherence and inflammatory responses. EAEC adherence was higher than EPEC and A/P co-infections adherence. A/P co-infections did not have increased IL-8 response in HCT-8 cells when compared to EAEC alone. The virulence genes involved in EPEC adherence and Type 3 Secretion System (bfp, eae, tir, ler, per, espB and espA) were significantly reduced in A/P co-infected cells. An interesting adherence trait was observed between the A/P co-infections in HeLA cells, EAEC was found to adhere around EPEC altering the localized adherence pattern. The A/P co-infections were further analysed using the EPEC murine model developed in this study. The A/P infected mice had diminished weight changes and EAEC shedding was enhanced when EPEC was present. Faecal inflammatory biomarkers MPO and LCN-2 in A/P infected mice did not have any additive effect. The findings of this study contributed significantly to the knowledge of human EPEC infection in weaned C57BL/6 mice, highlighting clinical outcomes, inflammatory responses and metabolic perturbations. Furthermore, this study also highlighted the interaction of P/T and A/P co-infections using in vitro and in vivo analyses in order to determine the disease severity and outcomes. It was observed in this study that coinfections can result in either synergistic or antagonistic effects. Further studies are therefore, required in order to understand the underlying mechanisms that are involved during co-infections and this can further assist in the development of therapeutic interventions. / NRF