Researchers have found a mixed salad contaminated with the same type of Listeria that caused a multi-country outbreak with 54 cases and 10 deaths several years ago.

Salad and other fresh produce were collected in England from retail and catering businesses from September 2020 to December 2021 and were tested for Salmonella, Shiga toxin-producing E. coli (STEC), Listeria, Bacillus cereus, and E. coli.

From more than 600 samples, 84 percent were judged satisfactory, 12 percent were borderline, and 4 percent were unsatisfactory, found the study just published in the Journal of Food Protection.

Results do not suggest an improvement in microbiological quality since studies over the past 20 to 25 years, said researchers.

Sampled products included salad leaves, tomatoes, fresh herbs, cucumber, carrot, pepper, onion, and cabbage. The majority of items with a known origin were from the UK or Europe. 

STEC and Listeria findings
A sample of prepacked leaves, cucumber, and tomato from a caterer was unacceptable because STEC O76 was detected but no human infections in the United Kingdom matched this isolate. A sampling of the same salad product from the same premises four weeks later found no STEC.

No Salmonella was detected, but Listeria monocytogenes was recovered from 11 samples. Bacillus cereus was at borderline levels in 9 percent of samples and at an unsatisfactory level in one sample. E. coli was found in 3 percent of samples at borderline levels and in 4 percent at unsatisfactory levels.

One Listeria isolate matched human cases and was from a prepacked mixed salad from a catering business in 2021. It was the same strain that was behind a multi-country outbreak from 2015 to 2018 associated with frozen sweetcorn produced in Hungary by Greenyard but no link in the supply chain was established. There was no information on countries of origin for components in the salad product tested in 2021. A dozen people were sick in the UK from 2015 to 2021.

The strain that clustered with isolates from the outbreak of listeriosis was recovered from a bag of bistro salad containing lettuce and beetroot collected from a catering site that had purchased it from a national supermarket chain in August 2021. Despite a recall in 2018, the outbreak strain was detected in England at the importer in 2019, in a patient in 2019, and in another sick person in 2020.  

Listeria species other than Listeria monocytogenes were detected in 19 samples. One sample was contaminated at borderline levels of 100 colony Forming Units per gram (CFU/g) with Listeria innocua, and another with Listeria monocytogenes was also positive for Listeria seeligeri. Among the 11 samples in which Listeria monocytogenes was detected, five were recovered from loose products taken from two premises.

COVID-19 impact
Elevated levels of E. coli were mostly in loose products from the UK from caterers in the summer or autumn of 2021 and may have resulted from the relaxation of COVID-19 restrictions, said, scientists. 

The study showed 1.7 percent contamination with Listeria monocytogenes and 4 percent with E. coli at levels above 100 CFU/g.

Higher levels of Bacillus cereus occurred in a range of salad types and were in similar proportions among loose and prepacked products, as well as in samples from catering and retail settings and different countries of origin.

An association was observed between E. coli and Listeria in salad products. Cross-contamination and poor hygiene in catering environments are likely to be major factors, said researchers.