Purpose: The purpose of this study is to assess the ability of Lm, including the outbreak strains, to grow in the mayonnaise-based chicken salad and in celery.
Methods: Chicken salads and celery were purchased at retail or prepared using the recipe involved in the outbreak. An antibiotic-resistant cocktail of 1/2a, 1/2b and 4b Lm strains were used to inoculate celery and chicken salad at 3-6 x 103 CFU per 5 grams. The inoculated samples were stored at 5°, 10° and 25°C. Samples were withdrawn at different intervals and plated onto BHI agar containing selective antibiotics and incubated at 37°C for 24-48 hours before colonies were counted.
Results: The growth of all three serotypes in celery and in both retail and lab-made chicken salads were comparable. Lm grew slowly at 5°C. Final counts reached 8.0 x 107 /5g at 5°C after 30 days of incubation. The doubling time varied from 30 to 60h at 5°C, 36 – 48h at 10°C and 1.3 – 10.6h at 25°C. The salads had a water activity of 0.99 and average pH ranged from 5.9 to 5.3. Growth in diced celery reached 104 -105 CFU/5g at 5°C, after 20 days.
Significance: These data show that celery and chicken salad, retail as well as homemade, can support Lm growth. The outbreak investigation found that the implicated salad was properly refrigerated and stored for only a few days, indicating limited growth of Lm in the implicated outbreak salad. This means that the infective dose was almost entirely due to the number of Lm cells contributed by the contaminated celery.