Last updated April 19, 2018 at 3:59 pm
A common method used by the agricultural industry to make fruit and vegetables safe for human consumption might instead be producing hidden killers.
A listeria outbreak in Australia in February 2018 was traced back to one rockmelon farm. Credit: iStock
Commercial fruit and vegetable crops around the world are frequently sprayed with a dilute chlorine mix with the intent of destroying any bacteria such as Listeria or Salmonella which may be clinging their surfaces. In some places, the technique has been used for more than a century.
Outbreaks of food poisoning caused by fruit can be lethal. The rockmelon listeria outbreak in Australia in February this year, for example, claimed six lives.
While that outbreak was traced to a single farm in New South Wales, investigations into outbreaks often find that a breakdown in hygiene standards somewhere along the production process is to blame.
Sometimes, however, no such obvious problem is found and the reason Listeria or Salmonella bacteria entered the food chain remains mysterious.
Invisible and undetectable
Now, however, one study suggests that chlorine is to blame.
Instead of killing all foodborne pathogens, microbiologist Bill Keevil from the University of Southampton in the UK has shown, the chemical might in fact induce bacteria to become effectively invisible and undetectable.
In a paper published in the journal mBio, Keevil and several colleagues demonstrate how industry-standard chlorine sprays fail to kill all Listeria monocytogenes or Salmonella enterica bacteria in a given population.
Instead, a proportion of the microbes enter a state known as “viable-but-nonculturable” (VBNC). In effect, the bacteria shut down, entering a dormant mode because of the environmental stress caused by the chlorine.
This makes them undetectable. In lab tests, bacteria are discovered and identified through the process of “culturing” – providing them with a nutrient base on which to grow. Culturing is the standard method by which pathologists identify microbes present on foodstuffs.
Wake-up call for food producers
Bacteria in a VBNC state might be inactive, but they are not dead. This means that given the appropriate stimulus – the warm, welcoming broth of a human intestine, for instance, they can wake up and cause havoc.
Keevil and colleagues found that Listeria populations entered VBNC when sprayed with a solution containing chlorine at 50 parts per million (ppm).
Salmonella bugs did the same when exposed to 100 ppm. In such a state, their presence on fresh food would remain undetected by all common testing procedures.
“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,” says Keevil.
As well as constituting a wake-up call for food producers and health inspectors, the research may also mean that manufacturers of many common consumer products will have to rethink their marketing or potentially face devastating legal claims.
“This has big implications for many household and industrial cleaning agents, paints, cosmetics and so on, sold with an antimicrobial claim,” says Keevil.
