The survival of Listeria monocytogenes during long term desiccation is facilitated by sodium chloride and organic material

Abstract

One specific DNA-subtype, as determined by RAPD, of Listeria monocytogenes persisted in a fish slaughterhouse for years, even during months with no production where the plant was cleaned and kept dry. We hypothesised that tolerance to desiccation could be a factor in explaining the persistence of L. monocytogenes in food processing environments and the purpose of the present study was to determine ability of L. monocytogenes to survive desiccation on stainless steel under simulated food processing conditions. Viable counts of eight different L. monocytogenes strains exposed to different soils and relative humidities (RHs) during desiccation decreased significantly ( p < 0.05) during the first week but subsequently remained constant at a plateau for weeks or even months thereafter. Desiccation in physiological peptone saline (PPS) reduced survivors by 3–5 log units whereas bacterial cells suspended in bacteriological growth substrates (tryptone soy broth with 1% glucose, TSB-glu) or PPS with 5% NaCl only were reduced by 1–3 log units. At RHs of 2, 43 and 75%, surfaces were visibly dry after 1, 3 and 5 days of incubation, respectively. The lowest RH resulted in the most significant loss of viability, however, 10 3–10 4 CFU/cm 2 remained viable regardless of the desiccation treatment (i.e., presence of TSB-glu and/or salt). At 75% RH, the bacterial counts remained almost constant when desiccated in TSB-glu. When bacteria were grown and desiccated (15 °C, 43% RH) in salmon or smoked salmon juice, survivors decreased slowly resulting in low numbers (10 2–10 3 CFU/cm 2) from all eight strains remaining viable after 3 months. Whilst conditions during desiccation had a pronounced influence on inactivation kinetics and the number of survivors, persistent L. monocytogenes were not more tolerant to desiccation than presumed non-persistent isolates. Our study shows that the ability to survive for months during desiccated conditions may be a factor explaining the ability of L. monocytogenes to persist in food processing environments.