Pre-growth conditions and strain diversity affect nisin treatment efficacy against Listeria monocytogenes on cold-smoked salmon

Listeria monocytogenes is a human pathogen that is commonly found in environments associated with cold-smoked salmon. Nisin is a natural antimicrobial that can be used as a food preservative. While nisin is active against a number of Gram-positive bacteria, including L. monocytogenes, environmental stresses encountered in cold-smoked salmon processing facilities might affect L. monocytogenes' nisin susceptibility. The objective of this study was to investigate the effect of seafood-relevant pre-growth conditions and L. monocytogenes strain diversity on nisin treatment efficacy on cold-smoked salmon. Six L. monocytogenes strains representing serotypes most commonly associated with cold-smoked salmon (1/2a, 1/2b, and 4b) were initially pre-grown under a number of seafood-relevant conditions and challenged with nisin in growth media modified to represent the characteristics of cold-smoked salmon. The pre-growth conditions with the lowest mean log reduction due to nisin and the highest strain-to-strain variability were selected for experiments on cold-smoked salmon; these included: (i) 4.65% w.p. NaCl (“NaCl”); (ii) pH = 6.1 (“pH”); (iii) 0.5 μg/ml benzalkonium chloride (“Quat”); and a control (“BHI”). Cold-smoked salmon slices with or without nisin were inoculated with L. monocytogenes pre-grown in one of the conditions above, vacuum-packed, and incubated at 7 °C. L. monocytogenes were enumerated on days 1, 15, and 30. A linear mixed effects model was constructed to investigate the effect of pre-growth condition, day in storage, serotype, source of isolation as well as their interactions on nisin efficacy against L. monocytogenes. Compared to pre-growth in “BHI”, significant reduction (P < 0.05) in nisin efficacy was induced by pre-growth in “pH” and “Quat” on both days 15 and 30, and by pre-growth in “NaCl” on day 30, indicating a time-dependent cross-protection effect. Additionally, an effect of L. monocytogenes' serotype on the cross-protection to nisin was observed; pre-growth in “pH” significantly reduced nisin efficacy against serotype 1/2a and 4b strains, but not against 1/2b strains. In conclusion, pre-exposure to mildly acidic environment, high salt content, and sublethal concentrations of quaternary ammonium compounds, is likely to provide cross-protection against a subsequent nisin treatment of L. monocytogenes on cold-smoked salmon. Therefore, challenge studies that use pre-growth in “BHI”, as well as more susceptible L. monocytogenes strains, may overestimate the efficacy of nisin as a control strategy for cold-smoked salmon.