The survival and virulence of Listeria monocytogenes was assessed during storage on three low-moisture foods (LMFs), chocolate liquor, corn flakes and shelled, dry-roasted pistachios (a w 0.18, 0.27, 0.20). The LMFs were inoculated with a 4-strain cocktail of L. monocytogenes at 8 log CFU/g, dried, equilibrated and then stored at 4°C, 25–81% relative humidity (RH) and 23°C, 30–35% RH for at least 336 days. At 4°C, L. monocytogenes remained stable on the LMFs for at least 336 days. At 23°C, L. monocytogenes levels declined on the chocolate liquor, corn flakes and pistachios at initial rates of 0.84, 0.88 and 0.32 log CFU/g/month, respectively. After 8 months at 23°C, L. monocytogenes concentrations on the chocolate liquor and corn flakes decreased to below the limit of detection (i.e., 0.48 log CFU/g). Relative populations of each strain were assessed before (i.e., day 0) and after 6 and 12 months of storage at 23°C and 4°C, respectively. Generally, a decline in the relative abundance of the serotype 1/2a strain was observed during storage, coupled with the relative increase of other strains, depending on the LMF and storage temperature. The total viable populations of L. monocytogenes quantified by PMAxx-qPCR after 12-plus months of storage at 4°C were significantly higher than that obtained by plating on TSA-YE by 1.8 to 3.7 logs. Decreases in the culturable population of L. monocytogenes during storage on the LMFs were the result of both cellular inactivation and transition to a viable-but-non-culturable state. The surviving cells, specifically after long-term storage at 4°C on the chocolate liquor and pistachios, remained infectious and capable of intracellular replication in Caco-2 enterocytes. These results have great relevance for predictive modeling used in microbial health risk assessments and support the addition of LMFs to food safety questionnaires conducted during listeriosis outbreaks.
This work was supported by the IAFNS Food Microbiology Committee.