Bacillus subtilis Increases Crop Yield and Reduces Contamination by Listeria monocytogenes on Cantaloupes

Introduction: Cantaloupes serve as one of the major crops in the state of Delaware accounting for approximately $850,000 in estimated annual sales. Preliminary data has shown the ability of Bacillus subtilis UD1022 to help increase crop yield through complex plant-microbe interactions that increase plant defenses toward plant pathogens. Growth of L. monocytogenes is inhibited in culture and on Romaine lettuce plants when UD1022 is inoculated onto the roots of plants.

Purpose: The purpose of this project is to evaluate the use of a plant growth promoting rhizobacteria, Bacillus subtilis UD1022, to reduce the contamination of cantaloupes by L. monocytogenes in the pre-harvest environment as well as in the packinghouse environment.

Methods: In this study, pieces of cantaloupe rind (3.20 cm x 2.50 cm) were treated with UD1022 supernatant or sterile water immediately following inoculation with L. monocytogenes (9.05 ± 0.7 log CFU/ml) or after an 8 hour incubation at 37°C, 22°C or 4°C. Three cantaloupe rind pieces were pooled into one sample (n = 6 samples). Bacterial enumeration was performed by plating samples onto Brilliant Listeria Agar.

Results: At 22°C, recovery of L. monocytogenes was ~2.5 log lower when treated with UD1022 supernatant at 8 hours post-inoculation (hpi) and ~3 log lower at 24 hpi compared to controls, which was statistically significant (P = 0.0024). Recovery of L. monocytogenes was also ~2.5 log lower at 22°C when UD1022 supernatant was applied at 8 hpi and L. monocytogenes was enumerated at 24 hpi. Although recovery of L. monocytogenes was lowest for all temperatures when treated with UD1022, this reduction was not significant at 37°C or 4°C. 

Significance: These results indicate that UD1022 may be used as a natural biocontrol agent to reduce the risk of contamination by L. monocytogenes on cantaloupe rind during times of temperature abuse in the packinghouse, storage, or transport.