Screening of Biocontrol Agents against Listeria monocytogenes, Salmonella, and Escherichia coli O157:H7 and Antimicrobial Efficacy on Iceberg Lettuce

Introduction: Foodborne disease outbreaks continue to plague the produce industry, indicating a need for alternative or additional microbial control methods.  The use of biological control agents postharvest could provide an additional hurdle to challenge pathogen growth.  Many bacterial species, including Lactic Acid Bacteria (LAB) and Bacillus species, have demonstrated antimicrobial activity and thus are candidates for postharvest biocontrol agents.

Purpose: The purpose of this study was to screen 22 bacterial isolates for antimicrobial activity against Listeria monocytogenes, Salmonella species, and Escherichia coli O157:H7 in vitro, then to assess antimicrobial efficacy of select isolates against L. monocytogenes on iceberg lettuce.

Methods: The antimicrobial activity of the LAB isolates was determined using a seeded-overlay method and all other isolates were evaluated by spot inoculating the isolate on pathogen-seeded TSA; antimicrobial activity was determined by the size of the clearing around the isolate. Antimicrobial efficacy on iceberg lettuce was assessed by spraying a cocktail of the three LAB isolates (10⁷ - 10⁸ CFU/g) onto lettuce spot-inoculated with L. monocytogenes (10² - 10³ CFU/g), then incubating at 10°C for 14 days.

Results: Three LAB isolates and six Bacillus isolates suppressed L. monocytogenes, Salmonella, and E. coli O157:H7 in vitro.  LAB isolates Lactobacillus plantarum, Pediococcus acidilactici, and Pediococcus pentosaceus were chosen for use in the iceberg lettuce challenge study.  L. monocytogenes levels were 1.84 logs lower on lettuce treated with LAB cocktail than untreated lettuce after 14 days incubation at 10°C.

Significance: This study has identified nine bacterial isolates capable of inhibiting L. monocytogenes, Salmonella, and E. coli O157:H7 in vitro.  Three LAB isolates suppressed L. monocytogenes on iceberg lettuce and merit additional testing to determine commercial applicability.