Evaluation of the bacterial populations present in Spring Mix salad and their interactions with Escherichia coli O157:H7

The native bacterial populations present in three brands of Spring Mix salads (brands A, B, and C) were analyzed by plating and 16S rRNA sequencing during a 15-day 4 oC storage. The interaction between the native bacteria and E. coli O157:H7 (ECO157) and the impact of salad shelf life on the changes of bacterial populations and the survival of ECO157were investigated by artificially inoculating the day-0 and day-15 salad homogenates with ECO157. The surviving ECO157 and the changes of native bacteria were evaluated on day 0 and 5, 10 and 15 days after inoculation. Although the top four phyla present in all three brands of salad consisted Bacterioidetes, Proteobacteria, Actinobacteria, and Firmicutes, their changes (abundance and diversity) during storage were determined by the “brand” and “storage time” factors, with “brand” playing a larger impact. The growth of ECO157 was observed in inoculated day-0 brand A, inoculated day-15 brand A, and inoculated day-15 brand C salad. The linear discriminant analysis effect size analyses showed that the abundance of Arthrobacter, Shewanella, and Brochothrix positively correlated with the increases of ECO157, while Janthinobacterium, Carnobacterium, Erwinia, Enterobacter, and Paenibacillus negatively correlated with ECO157 increase.