Bacillus Genotypes Exhibit Antagonistic Effects on Lettuce-Based Enterobacter Pathotypes

Globally, among ready-to-eat (RTE) vegetables, lettuce is well-known as a potential host for Enterobacteriaceae, which theatens public health. However, in addition to E. coli, pathogenic Enterobacter species associated with RTE lettuce is poorly investigated, as well as their genetic relationship with sources of contamination has not been determined. This study investigated the evolutionary relationship between Enterobacter species and their antibiotic resistance attributes in RTE lettuce, irrigation water, and the soil of lettuce farms. Enterobacter species had the highest occurrence in the irrigation water (38%), followed by both the RTE lettuce (31%) and the soil of the lettuce farm (31%). The 16S rRNA-identified the bacteria isolates as Enterobacter species. Phylogenetic analysis revealed that Enterobacter species from RTE lettuce and irrigation water maintained a strong evolutionary relationship in comparison with those from the soil of the lettuce farm. In addition, irrespective of the source of contamination, the isolates demonstrated resistance to 67% of the 12 antibiotics tested. To overcome these challenges, efforts have been made to identify novel antimicrobial agents, especially from eco-friendly soil bacteria. Among the 76 soil bacterial isolates that were assessed via the antagonist‒pathogen agar plug method, only two (IBT42 and VFK47) exhibited outstanding (P = 0.05) antagonism against > 50% of the Enterobacter pathotypes in comparison with the control. The potency of IBT42 and VFK47 were validated via agar well diffusion, the minimum inhibitory concentration, and the minimum bactericidal concentration. The 16S rRNA and phylogenetic analysis revealed that the identities of IBT42 and VFK47 were Bacillus mycoides and Bacillus cereus, respectively.