In this master’s thesis, we investigated the molecular mechanisms controlling interactions between Bacillus subtilis and Salmonella enterica. Microbial interactions can be cooperative (beneficial to both partners) or competitive (beneficial to one and detrimental to the other). Competition is especially strong in multi-species biofilms. To test the role of specific biofilm components of S. enterica in the interaction with B. subtilis, we used different mutants of S. enterica with inactivated biofilm production master regulatory gene (ΔcsgD), cellulose synthesis gene (⠆bcsA), and fimbriae synthesis gene (⠆csgB). First, we determined the surface properties of the strains used, i.e. auto-aggregation, co-aggregation and hydrophobicity. Then we observed the competitive interaction in a liquid medium under different environmental conditions, and analysed the consequences of co-culturing of specific mutants of S. enterica with B. subtilis in mixed biofilms. We found that nutrient deficiency affects competition more at higher temperatures than at lower temperatures and that S. enterica adhesion decreases in the presence of B. subtilis. We also investigated the susceptibility of mono-species and multi-species biofilms to various antibiotics and determined the minimal inhibitory concentration. The susceptibility of S. enterica in co-culture with B. subtilis increased most with the addition of the antibiotic tetracycline. From the results, we concluded that the potential probiotic strain B. subtilis PS-216 is a suitable candidate in the treatment of S. enterica infections, because it increases the susceptibility of pathogenic bacteria to the antibiotic tetracycline.
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