Microbiological analysis of strawberries for the presence of Salmonella spp. and VTEC in primary production, processing and trade in Belgium

Abstract

Introduction: Fresh produce are an important component of a healthy diet, yet the last decades an increase was observed in the number of foodborne outbreaks associated with the consumption of fresh produce. Outbreaks included leafy vegetables, sprouted seeds, soft red fruit and melons. Since strawberries are an important fruit in Belgium and little information is available about microbial risks and how to control these risks, strawberries were sampled during primary production (6 farms: 3 open field cultivation and 3 hydroponic cultivation), processing (1 company) and trade (2 companies). Materials and methods: At farm level, samples were taken of strawberries, water, hands and substrate/soil during 4 visits, whereas at the processing level, wash water, strawberries (fresh, cut and in fruit salad), hands and contact surfaces were sampled during 6 visits. In the case of the trading companies 41 batches of strawberries were sampled. The samples were distributed over strawberry batches coming from Belgium/The Netherlands (20), Spain (13) and Egypt (8). Hands and contact surfaces were analysed for E. coli and Enterobacteriaceae by using a petrifilm and VRBL agar respectively. Strawberries, water and substrate/soil were analysed for E. coli and the presence or absence of Salmonella spp. and Verocytotoxin producing E. coli (VTEC) by the use of a petrifilm and the GeneDisc Cycler (Multiplex PCR). Results and Discussion: In general, no Salmonella spp. (0/201; 95% CI 0,00 – 1,88) or VTEC (0/201; 95% CI 0,00 – 1,88) was detected in any of the strawberry samples analysed nor at the level of processing and trade, nor in primary production. Neither was E. coli detected (< 10/g) as a hygiene indicator in any of the strawberry samples at processing or trade level. In primary production, E. coli was present in 2 strawberry samples (1,0 x 101 and 9,8 x 102 cfu/g). Generic E. coli were enumerated in 4/57 hand samples (2,0 ± 1,5 log cfu/25 cm²), 44/78 samples of irrigation water (1,5 ± 1,1 log cfu/100 ml), 4/24 soil samples (2,5 ± 0,5 log cfu/1000 cm²) and 7/24 substrate samples (1,8 ± 1,3 log cfu/g). Salmonella was not isolated from any of the environmental samples in primary production. On the other hand, 11/78 samples of irrigation water and 2/24 substrate samples were PCR-positive for VTEC, meaning that stx1 and/or stx2 and eae genes were detected. VTEC isolates from this irrigation waters or substrate samples were obtained in 4 out of 13 PCR-positive samples and confirmed as E. coli O26 (2 x water, 2 x substrate; the 4 isolates from these samples being obtained at one time interval from one farm). After further statistical analysis, it was found that there was a higher probability to find VTEC PCR signals in water in primary production if water sources can receive surface run-off water, are not disinfected or are located adjacent to farm animals. In addition, elevated E. coli numbers are an indicator for the presence of PCR VTEC signals (respectively 0/16, 3/13 en 8/15 samples with 1-10, 10-100 and ≥ 100 E. coli cfu/100 ml, respectively, showed PCR VTEC positive signals)