Investigation of bacterial social interactions in two species biofilms composed of Salmonella enterica serovar Typhimurium and Bacillus subtilisPodnar, Eli (Avtor) Mandić Mulec, Ines (Mentor) Danevčič, Tjaša (Komentor) B. subtilisS. Typhimuriumbiofilmcompetitionsecondary metabolitespolysaccharidessporulationprobioticspathogensSalmonella enterica serovar Typhimurium is one of the most common foodborne pathogens affecting humans and animals. Its pervasive nature allows it to thrive in environments beyond its host. The emergence of antimicrobial resistance within Salmonella strains is a major concern, prompting the exploration of alternative strategies to curtail infections. Probiotic bacteria, such as Bacillus subtilis, are a promising new strategy to combat foodborne pathogens. However, existing literature on B. subtilis-Salmonella interactions mostly focuses on probiotic effects in live animals leaving a gap in understanding the mechanisms of their interplay. In this thesis, we tested the competition between B. subtilis PS-216 and S. Typhimurium SL1344 under different environmental conditions, focusing on nutrient availability. The results show that under nutrient-rich conditions, B. subtilis PS-216 inhibits the growth of S. Typhimurium SL1344 and reduces its adhesion and biofilm thickness. The B. subtilis antagonistic potential is lost in the Δpks mutant, in which the synthesis of the polyketide antibiotic bacillaene was impaired. S. Typhimurium increased the PpksC activity, which controls bacillaene production. This suggests that B. subtilis senses and responds to a Gram-negative competitor. However, nutrient limitation reduced B. subtilis antagonism against S. Typhimurium and inhibited the efficacy of B. subtilis sporulation in coculture. The results also showed that sporulation inhibition by the pathogen is dependent on iron limitation and sigma B (SigB)-controlled general stress response in B. subtilis. Furthermore, interactions under nutrient limitation were dependent on cell-cell contact and the type VI secretion system (T6SS) of S. Typhimurium. The thesis reveals novel molecular determinants of competition between the two bacteria but also underscores the pivotal role played by environmental conditions and cell-cell contact. The thesis also highlights the need to assess probiotic strains in conditions resembling their intended use, which is crucial for enhancing strategies to control pathogens.[E. Podnar]20232023-12-10 07:15:52Doktorsko delo/naloga152875UDK: 579.22/.26:579.842.1/.2:579.852.1VisID: 236063COBISS_ID: 176990979sl