Listeria develops reduced sanitizer sensitivity but not resistance at recommended sanitizer use levels

Survival and persistence of Listeriaspp. and Listeria monocytogenes in packing houses and fresh cut produce processing facilities continues to be a major concern. While “Listeria issues” in these facilities can typically be traced back to survival of these organisms in locations that cannot be (easily) reached by sanitizers (so called niches), there is also a concern about emergence of sanitizer resistant Listeria spp. and L. monocytogenes. The importance of sanitizer resistance in Listeria is also supported by recent reports of Listeria isolates that showed reduced susceptibility to quaternary ammonium compounds (“quats”) and that carry genes (e.g., bcrABC, qacH) that confer reduced sensitivity to quats. In addition to known sanitizer resistance mechanisms, there are concerns that additional unknown mechanisms can lead (i) to resistance to sanitizers other than quats or (ii) to resistance to higher levels of sanitizers, as compared to the current quat resistance mechanisms, which typically only confer reduced sensitivity to quat levels considerably below recommended use levels. Importantly, even reduced susceptibility to
sanitizers (to concentrations considerably below recommended use levels) can be of practical concern as it may not be unusual for Listeria to be exposed to reduced sanitizer concentrations in produce operations, for example due to high organic loads, which reduces sanitizer efficacy even if applied at appropriate concentrations. This proposal aims to provide industry with better tools (i) to assess the risk of reduced sanitizer susceptibility in Listeria, (ii) to more rapidly and reliably identify Listeria with reduced sanitizer susceptibility, and (iii) to control Listeria with reduced sanitizer susceptibility. To achieve these goals, we will complete the following three objectives: Obj. 1: Screen > 500 Listeria spp. and L. monocytogenes isolates from packing houses and fresh cut operations for reduced sensitivity to key sanitizers, including quaternary ammonium compounds, sodium hypochlorite and peroxyacetic acid. Obj. 2: Perform whole genome sequencing of Listeria spp. and L. monocytogenes strains identified as showing reduced sanitizer sensitivity to identify mutations and gene acquisitions responsible for reduced sanitizer sensitivity. Obj. 3: Expose selected strains with reduced sanitizer sensitivity to increasing sanitizer concentrations to determine the potential of these strains to become resistant to sanitizer levels close to the recommended use levels (e.g., 200 ppm quat or above).