P2-106 Investigating the Use of Bacillus subtilis as a Biocontrol Agent for Listeria monocytogenes on Caramel Apples and Stainless Steel Surfaces

Tuesday, August 2, 2016
America's Center - St. Louis
Shani Craighead, University of Delaware, Newark, DE
June Teichmann, University of Delaware, Newark, DE
Paula Thomas, University of Delaware, Newark, DE
Sarah Markland, University of Florida, Gainesville, FL
Harsh Bais, University of Delaware, Newark, DE
Kali Kniel, University of Delaware, Newark, DE
Introduction: L. monocytogenes persists in diverse environments including food matrices and production surfaces. In 2014, L. monocytogenes contamination of caramel apples resulted in 7 deaths and 35 illnesses. Unique control measures are needed to reduce the risk of L. monocytogenes contamination, like Bacillus subtilis UD1022 which has been shown to inhibit the growth of L. monocytogenes in culture.

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.