Survival of Salmonella and E. coli O157 on strawberries and basil during storage at different temperatures

Abstract

Limited information about the survival of Salmonella and E. coli O157 on strawberries and basil is available. As strawberries and basil are often consumed raw, they can pose a potential risk of foodborne illness for consumers. Therefore, the survival of both pathogens was assessed during one week storage at cold and ambient temperatures. Strawberries (100 ± 5 g in a perforated box) and basil leaves (25 g in a closed tray), were inoculated with a mix of 2 strains of E. coli O157 or Salmonella (Thompson and Typhimurium) to obtain an initial inoculum of 104 – 105 cfu/g. Strawberries’ samples were stored at 4°C, 10°C, 15°C, and 22°C and basil leaves at 7°C, 15°C and 22°C for 7 days (or less if visual spoilage occurred before or the pathogens dropped below the detection limit i.e. 50 cfu/g). For each temperature/time condition 2 independent storage experiments were performed (study A and B) (with triplicate sampling at defined time points). Samples were analysed by plating on Chromocult agar (+ 50 µg/ml nalidixic acid; E. coli O157) and XLD (Salmonella). In addition, the visual quality was assessed during storage. No growth of E. coli O157 or Salmonella was observed in case of strawberries stored at 4°C, 10°C, 15°C and 22°C. E. coli O157 and Salmonella survived during 7 days at 4°C but with a decrease of 2,5 to 3,8 log units being observed and a final recovery rate from 2 and 5 out of 6 samples respectively. At 10°C, no E. coli O157 was detected anymore after 6 days (0/6), in contrast to Salmonella which could be detected after 7 days in 5/6 samples with a similar decrease as for 4°C (2,5 to 3,9 log units). The numbers of pathogens dropped below the detection limit after 4 and 5 days at 15°C for E. coli O157 and Salmonella in study A respectively, while in study B a reduction of 3,0 to 4,0 log units was observed after 6 days, which was the end of shelf life due to the growth of molds. Storage of strawberries at 22°C resulted in growth of molds from day 2 (A) or 3 (B) whereby a reduction of pathogens was noticed of 1 to 2 log units after 2 days. In contrast, reduction of pathogens below the detection limit was not yet reached after 7 days storage of basil at different temperatures and only at 22°C some samples needed to be discarded due to the growth of molds and quality defects. Storage of basil leaves for 7 days at 7°C resulted in a decline of maximum 1,8 log cfu/g for both pathogens. Similar results were obtained at 15°C for Salmonella, whereas no decline was observed in case of E. coli O157. Also at 22°C restricted increase/decrease (≤ 1 log cfu/g) was observed for both pathogens after 7 days. Thus, avoiding contamination in particular at cultivation (and (post-)harvest) is important as both pathogens survive during storage, washing has only a limited effect and both strawberries and basil are consumed after minimal processing, excluding an inactivation treatment