Antagonistic activity of colombian yeasts against Salmonella Typhimurium and Escherichia coli O157

The antimicrobial activity of 52 yeast isolates was evaluated against the pathogenic bacteria Salmonella enterica subsp. enterica serovar Typhimurium and Escherichia coli O157. The hypothesis was that culture conditions of both the yeasts and the pathogenic bacteria will influence the antimicrobial activity of yeast. The yeasts were grown on modified yeast extract-malt agar (MYM) and modified yeast extract–peptone–glycerol agar (MYPG). While the enteropathogens were cultured on iron-deficient media (IDM) and Reasoner's 2A semisolid agar (R2A). At least three biological replicates consisting of 3 technical replicates were carried out for all treatments. Considering the results of the 31 yeast accessions displaying antimicrobial activity and their taxonomic identity, 8 isolates were selected for further experiments. The anti-biofilm activity was evaluated using polystyrene microplates. Cultures of enteropathogenic strains were prepared and mixed with lyophilized microbial extracts ((100 mg/mL) in the microplate wells. The stained biofilms were measured at 600 nm using a microtiter plate reader. Each treatment had eight replicates, and the experiment was repeated three times. To exhibit the presence of quorum quenching (QQ) compounds, Chromobacterium violaceum was used as biosensor. Violacein reduction was assessed by measuring optical density in liquid culture, following a protocol by Rajivgandhi et al. (2018). The supernatant was measured at 540 nm to compare violacein production with the control. Three biological replicates, each consisting of one technical replicate, were carried out. The growth of yeast under varying temperatures, pH levels, and osmotic shock was assessed using 96-well microplates. Yeast cell suspensions were prepared and added to Sabouraud dextrose broth (SDB). For temperature tests, the plates were incubated at 4, 25, 30, 35, and 40°C. For pH tests, SDB was adjusted to pH levels from 3 to 9. The osmotic shock was evaluated by supplementing SDB with glucose to create 50% and 60% solutions. Growth was measured at 405 nm after 48 hours of incubation, with experiments conducted in a completely randomized design with four replicates per treatment and repeated three times. To select the most competent yeasts for bioproduct development, data of all evaluated phenotypes were integrated into a composite index using a three-step process: (1) A univariate ANOVA was performed to evaluate yeast responses under specific conditions; (2) A factorial A