Occurrence of Selected Foodborne Pathogens on Frozen Berries Collected at Retail Level in Switzerland

Berries are globally appreciated for their health benefits, particularly due to antioxidants, and have grown in popularity over recent decades. However, several foodborne outbreaks-mainly viral, caused by norovirus and hepatitis A-have been linked to both fresh and frozen berries. In contrast, the role of frozen berries in transmitting bacterial pathogens such as Salmonella spp., Escherichia coli, and antimicrobial-resistant bacteria (AMRB) remains underexplored. To address this, we investigated both the qualitative and quantitative presence of selected pathogens. Over a three-month period (Nov 2024-Jan 2025), 100 frozen berry samples from Swiss retailers were analyzed. One-third (n = 32) were labeled organic; 96% contained imported berries. Samples included raspberries (n = 33), blueberries (n = 18), strawberries (n = 18), blackberries (n = 3), redcurrants (n = 1), and berry mixes (n = 17), with some containing blackcurrants or sour cherries (n = 10). All samples were tested qualitatively for Salmonella spp., Shiga toxin-producing E. coli (STEC), and Listeria monocytogenes, and quantitatively for E. coli and members of the Bacillus cereus group. Extended-spectrum β-lactamase-producing Enterobacterales (ESBL-E) were also screened. Salmonella, STEC, and L. monocytogenes were not detected. E. coli counts remained below detection limits. B. cereus group members were found in 12% of samples (2.0-3.41 log CFU/g), with several strains genetically matching B. cereus s.s. Thuringiensis biopesticide strains. ESBL-E were found in 2% of samples (both Egyptian strawberries), identified by WGS as Enterobacter hormaechei carrying bla_CTX-M-15 and E. coli with bla_SHV-12.These findings support defining microbiological criteria for frozen berries in HACCP and routine food safety testing.