Purpose: The purpose of this project is to evaluate the use of a plant growth promoting rhizobacteria, Bacillus subtilis UD1022, as a biocontrol agent to reduce the risk of contamination by L. monocytogenes on caramel apples and stainless steel surfaces.
Methods: Stainless steel coupons were inoculated with UD1022 (5.08±1.18 log CFU/ml) in 10% LB broth solution and incubated for 48 h at 22°C. The coupons were rinsed with sterile water and placed into media inoculated with L. monocytogenes (8.19±0.08 log CFU/ml). After 48 h, coupons were rinsed and then placed in BPW with glass beads and vortexed for 1 min. Apples were co-inoculated at the stem scar with 4.30-4.60 log CFU L. monocytogenes and UD1022 or Pseudomonas fluorescens, air-dried, prepped by stick insertion and then dipped in caramel (85-90°C). Following incubation at 22°C for 0 – 240 h, the apples were placed in 1% BPW, hand massaged for 3min, and bacteria were enumerated on Brilliant Listeria Agar (n=6).
Results: Recovery of L. monocytogenes from stainless steel coupons was 1.20 ± 0.86 log lower when treated with UD1022 compared to L. monocytogenes only controls, significant (P=0.0423). Within the first 24 h, no L. monocytogenes growth was observed on caramel apples co-inoculated with UD1022, apples with L. monocytogenes alone had 2.18 log CFU/ml recovered. By day 5 post-inoculation, L. monocytogenes levels were similar in all samples (4.36-4.96 log CFU/ml), including apples that were co-inoculated with P. fluorescens.
Significance: UD1022 may be of use as a natural biocontrol agent to reduce the risk of contamination of apple and production surfaces by L. monocytogenes.