Survival and Virulence of Listeria monocytogenes during Storage on Chocolate Liquor, Corn Flakes, and Dry-Roasted Shelled Pistachios at 4 and 23°C

Abstract

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.

Heat Inactivation of Listeria monocytogenes on Pecans, Macadamia Nuts, and Sunflower Seeds

“…Salazar et al ( 13 ) studied the survival of L. monocytogenes in pine nuts and sesame seeds and reported viable counts of more than 4 Log CFU/g (initial inoculum of 9 Log CFU/g) after 180 days of storage at different relative humidities. A recent study corroborated the prolonged survival of L. monocytogenes in pistachios and reported approximately 2 Log CFU/g from an initial 6 Log CFU/g inoculum after 195 days at 23°C and 5 Log CFU/g after 336 days at 4°C ( 14 ). These findings clearly support the idea that L. monocytogenes can remain viable in nuts and seeds during storage.…”

Quantification of Viable Brochothrix thermosphacta in Cold-Smoked Salmon Using PMA/PMAxx-qPCR

“…Few studies have been conducted with this new dye so far (Truchado et al, 2020) used it to detect and quantify VBNC L. monocytogenes cells in wash water from processed fruit and vegetables. It has also been used to quantify L. monocytogenes in chocolate liquor, corn flakes, and dryroasted, shelled pistachios (Ly et al, 2020), quantify viable and non-viable Salmonella from a poultry environment (Zhang et al, 2020), quantify viable Campylobacter in raw milk (Wulsten et al, 2020), detect Clavibacter michiganensis in a viable but nonculturable state in tomato (Han et al, 2018), discriminate between viable and membrane-damaged cells of the plant pathogen Xylella fastidiosa (Sicard et al, 2019) or quantify VBNC Vibrio parahaemolyticus in raw shrimp (Cao et al, 2019). Among all these studies, three of them have compared this new dye with other available dyes, notably EMA or PMA.…”