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The University of Southampton
Global Network for Anti-Microbial Resistance and Infection Prevention

New insights into detecting microbials in fresh produce

Published: 23 April 2018
Ensuring watercress is safe to eat

Recently published research, based on the PhD of NAMRIP member Callum Highmore (supervised by Professor Bill Keevil), shows that chlorine, commonly used to clean fresh produce such as salad and spinach, can actually cause bugs to go undetected.

The paper Viable-but-nonculturable Listeria monocytogenes and Salmonella enterica Serovar Thompson induced by chlorine stress remain infectious (by Callum Highmore, Jennifer Warner, Sandra Wilks, Steve Rothwell and Bill Keevil) is published in the current issue of mBio, the lead journal for the American Society for Microbiology.

The research was carried out to investigate the danger of foodborne pathogens in fresh produce entering a state called viable but nonculturable (VBNC). 


Callum Highmore in the laboratory
Callum Highmore in the laboratory

As Callum explained: “A wide range of bacteria are known to go into the VBNC state when exposed to a growing number of physical or chemical stresses, however there is very little consensus on what this means for their survival and pathogenicity. This could be a particular problem for fresh produce such as salads, as they often aren’t cooked and rely on factory processes for their decontamination. We have shown that the commonly used sanitiser chlorine is not wholly effective in killing foodborne pathogens Listeria monocytogenes and Salmonella enterica, and any survivors become VBNC. This could allow them to go undetected using standard microbiological methods, and reach consumers via food products."

Describing how VBNC pathogens can then go on to cause harm, Callum explained: “Our paper shows that the pathogens made VBNC through chlorine exposure are still infectious to the animal model Caenorhabditis elegans. This confirms not only that chlorine is ineffective at killing pathogenic bacteria, but chlorine treatment can mask the detection of VBNC pathogens that are capable of causing disease.”

Callum added: "I am very grateful for the opportunity to research this topic for my PhD alongside such a great team”.

Professor Bill Keevil, Head of the Microbiology Group here at the University of Southampton added: “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. The problem with fresh produce, such as lettuce and spinach, 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.”

Vitacress Salads, who partly sponsored the study, wash their produce in spring water, avoiding the consequences of VBNC pathogens caused by cleaning with chemicals. Dr Steve Rothwell from Vitacress Salads added “This latest publication further validates our adoption of an environment and consumer friendly wash process, suggesting that whilst chlorine washed salads may sometimes yield lower microbial counts, the chlorine could simply be hiding potential problems that we would see and address.”

Further research is required to fully understand the VBNC state and develop a method to detect VBNC pathogens in fresh produce that can be used within the agricultural industry.



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