Listeria monocytogenes Survival and Growth in Milkshakes Made from Artificially- and Naturally-contaminated Ice Cream

Introduction: A foodborne listeriosis outbreak spanning from 2010-2014 was linked to consumption of ice cream contaminated with Listeria monocytogenes.  The outbreak resulted in 10 listeriosis cases, leading to 100% hospitalizations and 33% mortality rate. The deaths were attributed to consumption of milkshakes made with pre-portioned ice cream scoops in a healthcare setting. Work by other FDA investigators during the outbreak demonstrated uniform contamination of the scoops at approximately 10 MPN/g. 

Purpose: To evaluate the survival and growth of L. monocytogenes in milkshakes prepared with artificially- and naturally-contaminated scoops.

Methods: For each trial, half of the scoops were inoculated with 10 CFU/g of a rifampicin-resistant L. monocytogenes 4b cocktail (F2365, LS806, R2-502, and ScottA) directly with a pipet tip inserted into the sample. The other half of the scoops remained as naturally-contaminated with multiple L. monocytogenes 4b strains. Milkshakes were prepared according to the outbreak-associated recipe (1 scoop blended with 2 oz 1% milk). For this study, UHT milk was used and milkshakes were stored at 25°C for 0, 6, 9, 12, 24, and 48 h, followed by enumeration of L. monocytogenes via MPN according to the FDA-BAM. Twenty-four and 48 h enrichments were streaked onto PCA^Rif or Brilliance Listeria agar for detection. 

Results: The artificially-contaminated L. monocytogenes achieved significantly higher (P<0.05) populations than the naturally-contaminated at both 12 and 48 h. At 12 h, the populations of naturally- and artificially-inoculated L. monocytogenes were 1.26±0.45 and 2.22±0.29 log MPN/g, respectively. At 24 h, both populations were approximately 4.25 log CFU/g and not significantly different. At 48 h, populations increased to 4.82±0.18 and 6.65±0.11 log CFU/g, respectively.

Significance: This study highlights the difference in survival between an artificial and natural contamination of L. monocytogenes. These results may help to inform risk assessment, which often depends on studies using artificial contamination.