Research conducted by scientists at the University of Southampton, UK, has shown that the use of chlorine to decontaminate fresh produce can make foodborne pathogens undetectable but still able to cause illness.

The team, led by Southampton University chair in environmental healthcare and head of the Microbiology Group Dr. Bill Keevil, incubated Listeria monocytogenes and Salmonella enterica cultures on spinach leaves and then washed the spinach in chorine. The two strains became viable but nonculturable (VBNC) by 50 and 100 parts per million (ppm) respectively. VBNC is a form of dormancy wher bacteria are in a state of very low metabolic activity and do not divide, but are alive and have the ability to become culturable once resuscitated.

Additional experiments showed that Caenorhabiditis elegans (roundworms) that ingested the VBNC cells on the leaves had shortened lifespans.

Keevil said of the research: “This important work is a major breakthrough, after 100 years of relying on chlorine to sanitise foods and drinking water, and may explain the many unrecognised or untraceable disease outbreaks relying on the gold standard of culture recovery.”

Gram-positive bacteria, which carry a thick cell wall made of protein, can form tough spores and survive for thousands of years, as a response to conditions such as extreme temperatures, nutrient starvation or desiccation (extreme dryness). One example is Bacilius anthracis, which causes Anthrax. Many other bacteria such as Listeria and Salmonella cannot form spores and survive by entering a dormant-like VBNC state in response to environmental stress.

“VBNC cells cannot be detected by standard laboratory culture techniques, presenting a problem for the food and water supply industries, which uses these techniques to detect pathogen contaminants,” said Keevil.

The study found that chlorine induces the VBNC state in the highlighted foodborne pathogens and was ineffective at killing total populations of these bacteria.

“These data show that VBNC foodborne pathogens can both be generated and avoid detection by industrial practices, while potentially retaining their ability to cause disease,” he added.

Keevil noted that even the modern molecular detection methods PCR and qPCR are unable to detect the dormant pathogen, if the testing laboratory relies on a pre-enrichment step involving chlorine, to increase cell numbers and sensitivity to the testing.

“The problem with fresh produce of course is that many people eat it fresh, so it misses the cooking step which would kill most pathogens, one reason why companies have relied on chlorine washing before sale,” he said.

There is a likely impact on post-Brexit trade agreements for the UK specifically, as countries insist that the UK imports their chlorine treated foodstuffs, which according to Keevil could now be unsafe.

Agricultural company Vitacress Salads recently became the first UK business to obtain approval by supermarkets to sell its chlorine-free, fresh produce.