Researchers from the Quadram Institute at Norwich Research Park have used genome sequencing to discover that a significant amount of uncooked prawns bought from UK retailers have been contaminated with bacteria called Vibrio.

Though not harmful in the vast majority of cases, it means that in the future they now have a proven technique that could be able to identify dangerous bacteria before they reach human consumption. Scientists will be able to better understand the bacteria’s potential contribution to human disease and resistance to antibiotics.

The researchers found that 46% of the uncooked prawns they bought in the UK were contaminated with Vibrio bacteria. The specific strains of bacteria identified pose no immediate risk to food safety as they do not cause severe disease in humans. Contamination was far lower in cooked prawns.

Vibrio bacteria are commonly found in estuary or marine environments. Most species are harmless, but some can cause gastroenteritis if people eat seafood that is contaminated with it.

The findings of this study have led to calls for better surveillance to protect the public against an increased risk of contamination, as climate change could introduce more dangerous types of Vibrio bacteria to the UK’s food chain.

Researchers used whole genome sequencing to analyse the Vibrio bacteria DNA, revealing its entire genetic code. It is the first time this technique has been used to analyse the genomic diversity of Vibrio bacteria in prawns. It provides a much broader overview than previous approaches, offering a sound basis for public health authorities to assess the degree of risk to consumers.

The study’s findings show the diversity of Vibrio species present in prawns consumed in the UK. In all, a third of prawns carrying Vibrio had multiple species or diverse strains of the same species.

The method of processing prawns was found to have had an impact on the presence of Vibrio. Shell-on prawns were more commonly found with Vibrio than peeled prawns. Contaminated prawns tended to be raw, confirming that cooking was effective in minimising risks to food safety. However, the presence of Vibrio in a small number of cooked samples shows that contamination is still possible.

Whole genome sequencing can also identify genes that are associated with disease and resistance to antibiotic drugs. The use of antibiotics is a common practice in prawn farming. The study found that antibiotic-resistant genes were identified in three-quarters (77%) of Vibrio, with some found to be resistant to multiple drugs important for protecting human health.

Scientists are already studying Vibrio as likely indicators of climate change, as they are affected by water temperatures and rises in sea level. This could have an impact on the profile of Vibrio strains prevalent in the environment where prawns are grown and harvested. Globally, Vibrio infections are on the rise and they follow a seasonal pattern attributed to an increase in sea temperatures.

With the UK importing the vast majority of its prawns from abroad, this research will be critical in monitoring the arrival of more dangerous strains. Whole genome sequencing will help to track trends and trace emerging and potentially dangerous strains in our food system.

Dr Nicol Janecko, group leader at the Quadram Institute, said: “Our survey found prawns purchased from retailers in the UK carry a diversity of Vibrio species and antibiotic-resistant genes. It has not only provided clarity into the current level of Vibrio contamination in prawn products in the UK, but it has also created a framework to direct further efforts into protecting food safety in the future."