Purpose: Isolation of VT11 after cleaning and sanitizing indicated the need to evaluate the efficacy of the quaternary ammonium chloride (QAC) sanitizer used in the facility as well as the ability of VT11 to attach to surfaces.
Methods: Strain differences for QAC tolerance were therefore compared by determining minimum inhibitory concentrations (MIC) and decimal reduction times (D200ppm). Mean log density (MLD) values at 48 h for each strain were compared for surface attachment on stainless steel (SS) and concrete surfaces.
Results: MIC values for all VTs were 4.5 ppm and D200ppm ranged from 0.76 to 2.10 s. No significant differences in D200ppmwere found for 20 h cells; although, the mean value for 7 d long term survival (LTS) VT11 cells was significantly higher than other VTs (P ≤ 0.05). Overall, MLD values were significantly higher (P ≤ 0.05) on concrete (7.10 ± 0.49) compared to SS (5.21 ± 0.81) surfaces. MLD values on concrete were significantly lower (P ≤ 0.05) for VT105 (6.41 ± 0.25) compared to VT11 (7.27 ± 0.33), VT107 (7.22 ± 0.26) and VT56 (7.32 ± 0.33). SS adherence was significantly higher (P ≤ 0.05) for VT11 (5.73 ± 0.30) and VT107 (5.87 ± 0.68) than VT105 (4.47 ± 0.69) and VT56 (4.76 ± 0.44). These results suggest that the observed predominance and persistence of VT11 over long periods of time may at least be partially explained by increased tolerance of the LTS phase of this strain to QAC.
Significance: These results are useful for providing guidance on targeted interventions to reduce L. monocytogenes contamination risks in mushroom and other fresh produce facilities.