Purpose: The objectives of this study were to (1) isolate HPB from onions and celery to determine their role in contamination of commercially-produced tuna salad and (2) evaluate Hst production by HPB during storage of tuna salad containing added ingredients.
Methods: Bacteria were isolated from onion and celery using MRS or TSB+1% histidine broths and characterized by real-time PCR, API 20E, 16S sequencing, and/or Hst production. Twenty-five g of tuna salad (3:1 tuna:mayonnaise) and tuna salad with added ingredients (sanitized onion or celery, or 20% vinegar-based product) were inoculated with 2 log CFU/g Pantoea/Erwinia, Erwinina persicina, Erwinia spp, or Enterobacter pyrinus isolated from celery (30°C, 3 d). Tuna salad preparations were also inoculated with a four-strain cocktail of Morganella morganii, previously isolated from raw scombroid fish (Mm; 18°C or 30°C, 3 d). Plate counts and MPN-PCR were performed on all inoculated samples; Hst was determined fluorometrically.
Results: HPB from celery were 0.7-4.3 log CFU/g higher in the presence of sanitized celery and onions vs plain tuna salad; only E. pyrinusproduced significant Hst levels (513-2046 ppm; 30°C, 3 d). Mm increased by 2-3 log CFU/g in plain tuna salad and tuna salad with onion and by 4-5 log CFU/g in tuna salad with celery held at 18°C for 3 d. Mm produced 1315-3083 ppm Hst and increased by 2.5-4 and 5-6 log CFU/g in tuna samples after 1 and 3 days at 30°C, respectively. Vinegar-based product inhibited growth of all isolates by 1-5 log CFU/g.
Significance: Introduction of raw celery into commercially-prepared tuna salad can introduce HPB that may cause Hst poisoning if the product is temperature abused. We established that addition of a vinegar-based product to tuna salad can inhibit growth of HPB and/or production of Hst.