According to a study in the United Kingdom, imported chicken and salmon were more likely to be contaminated than domestic products.

Researchers said the work, published in the journal Food Microbiology, demonstrated the diverse pathogens present in various foods of animal and plant origin.

They looked at the prevalence and co-occurrence of E. coli as an indicator organism, Klebsiella, Salmonella, and Vibrio in chicken, pork, shrimp, salmon, and leafy greens. The role of Klebsiella from food as a risk to human health is not known.

imported frozen chicken was 6.4 times more likely to contain Salmonella than domestic chilled chicken, and imported salmon was 5.5 times more likely to be contaminated with E. coli. Factors linked to the presence of individual bacteria are relevant for food safety risk assessments and the design of surveillance programs, according to the study.

E. coli and Salmonella results
Seasonality was associated with E. coli and Klebsiella contamination in leafy greens, with higher detection in summer and autumn. Scientists said contributing factors might be the growing conditions, water sources, or weather patterns in different growing regions throughout the year.

The odds of Klebsiella contamination were higher in summer in chicken and pork samples. 

Between May 2018 and November 2019, 203 retail outlets were visited in Norfolk, England, and 1,369 food samples were purchased. In total, 311 raw chicken and raw pork, 157 raw salmon, 217 raw shrimp, 62 cooked shrimp, and 311 leafy greens were bought. The method used indicated presence rather than overall bacterial load.

E. coli was detected in the majority of raw chicken and raw pork samples and at a lower frequency in leafy greens, salmon, and shrimp. The odds of detection were higher in imported salmon than in domestic or unlabeled products.

Packs of washed leafy greens had a significantly higher prevalence of E. coli than unlabeled wash status packages but there was no difference between washed and unwashed items.

Salmonella was detected in chicken, pork, and raw shrimp. The frozen chicken was more often contaminated than chilled. All frozen chicken contaminated was imported and packed in the same country by nine suppliers.

Four samples of domestically produced pork were positive for Salmonella. Of the eight raw shrimp positive, seven were black tiger shrimp of which five were from conventional aquaculture.

Vibrio and co-detection
In the seafood tested, raw shrimp mainly were contaminated with Vibrio followed by cooked shrimp and salmon. Contamination of raw shrimp varied between imported and unknown origin with no domestic products sampled.

“The prevalence of Vibrio detected in this study warrants further inclusion of Vibrio as a microbial hazard into food risk assessments, a sentinel species of climate change effects on food systems and public health surveillance systems,” said researchers.

Klebsiella was found in all commodities, most often in washed leafy greens and least often in cooked shrimp. Risk factor analysis did not identify food presentation, store type, or origin of a product for any commodity as associated with its presence.

“The relatively high contamination observed in this study and the possibility of hypervirulent and multidrug-resistant strains in ready-to-eat foods such as leafy greens and other fresh produce warrants further investigation,” said researchers.

More than 30 percent of samples contained at least two target bacteria in chicken, pork, and raw shrimp. Salmonella was always detected with other bacteria, primarily E. coli, and to a lesser extent with other organisms.

The most common co-occurrence was E. coli – Klebsiella. In raw shrimp, the top co-occurrence profiles included Klebsiella – Vibrio and E. coli – Vibrio. Five samples of raw shrimp contained all four tested bacteria.