Enterohemorrhagic E. coli, EHEC, is a verotoxin producing, zoonotic pathogen, which causes diseases in humans such as bloody or watery diarrhea. Microorganisms compete for limited living space, nutrients and other resources and therefore other microorganisms are EHECs biggest competitors. To avoid outbreaks and infections with EHEC, one possible approach is to use harmless but competitive bacteria as probiotics. Therefore, the aim of this study was to evaluate three probiotic E. coli strains and their ability to outcompete EHEC in bovine feces.     Ten different cattle fecal samples from three different farms were used to mix with the three probiotic and EHEC strains. The mixture was diluted and cultivated at 0 h as a control and then incubated for 48 h at 20°C and 37°C before dilution and cultivation on CT-SMaC. Colonies was counted and ratios between EHEC and probiotic E. coli before and after incubation were calculated. Kruskal-Wallis test with Dunn’s test as post hoc test were used to see if observed reductions of EHEC were significant or not.     In 37°C, strain 10 was the only strain producing a significant reduction of EHEC. In contrast, no significant reduction was observed at 20°C in any of the strains.     Future research studying other factors and performed on live cattle models are necessary to confirm the usefulness of the studied probiotic candidates. However, these results indicate probiotics can be a useful tool to avoid infections and big outbreaks of EHEC in the future.

Escherichia coli are bacteria found in bowels of warm blooded animals. Most subspecies are harmless and part of the normal gut flora. However, E. coli have the ability to exchange genetic material with other bacteria, and some E. coli have acquired genes coding for virulence factors. VTEC, E. coli with the ability to produce verotoxin are commonly found in cattle, but certain types can cause severe disease in humans, known as enterohaemorrhagic E. coli, EHEC.     In this study, isolates of E. coli and other bacteria in the family Enterobacteriaceae from calves were subtyped and clustered using MALDI-TOF. Ten strains were selected for experimental competition analysis against E. coli MG1655.     The aim of the study was to identify strains of bacteria with the potential to outcompete VTEC in the cattle host and decrease the risk of human infections.     Three of the bacterial strains were able to outcompete the laboratory strain, and in future studies these strains can be analysed when competing against VTEC. The rest of the strains were outcompeted. Four known strains of VTEC were analysed competing the laboratory strain, showing weak ability to compete. Finally, a highly pathogenic strain of VTEC was analysed against Escherichia coli Nissle 1917, known for its ability to outcompete many strains of bacteria. Nissle could not outcompete the tested VTEC strain under the tested conditions.     In conclusion the majority of the bacterial strains isolated from calves were identified as E. coli and three of the isolates showed good ability to compete against the laboratory strain.