While eating fruits and veggies is a healthy thing to do, if they’re crawling with the bacterium Listeria monocytogenes (Lm), they may do more harm than good.

Lm contamination of fresh produce can stem from soil, water, and animals, among other sources (in the US, Listeria outbreaks have been traced to cantaloupe melons, Enoki mushrooms, and bean sprouts). The effects of Lm infection, or “listeriosis”, range diarrhea and fever to brain and bloodstream infections.

While proper handling or avoidance of high-risk foods is one way to prevent listeriosis, another is to use biological control tactics to limit Lm growth on food in the first place. 

Biological control involves reducing populations of one organism, often insect pests or invasive weeds, by introducing a natural predator. To this end, a recent study in Applied and Environmental Microbiology sought to identify bacteria capable of limiting Lm colonization and persistence on produce, specifically cantaloupe melons. 

The scientists isolated bacteria from various types of produce, ranging from alfalfa to grapes. They then tested the ability of each of 8,736 isolates to inhibit Lm growth in the laboratory. Of seven highly effective isolates, one called Bacillus amyloliquefaciens ALB65 (BaA) was the best inhibitor.

BaA was able to grow and persist on cantaloupe melon rinds and, importantly, did not inhibit plant growth or fruit production — in fact, cantaloupes colonized by BaA grew twice as fast as cantaloupes without it. Excitingly, BaA significantly reduced Lm growth on whole cantaloupes in the greenhouse, and completely inhibited growth at post-harvest refrigerator conditions (this is key, as Lm can survive cold temperatures, making it difficult to control). 

While the researchers did not determine exactly how BaA inhibits Lm, they did identify genes in BaA’s genome that likely encode compounds known to limit growth of other bacterial species; these compounds may be responsible for the observed effects on Lm. This study points to a novel, effective biological control agent for reducing Lm growth on cantaloupes, with potential applications for other produce types as well.

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