Scientists at University College in Cork, Ireland, have deciphered at least part of the puzzle behind Listeria’s virulence and unpredictability.
Although many foods can be at least mildly contaminated with Listeria, its sometimes deadly outbreaks are relatively rare. Researchers have long wondered why it is that some people can eat Listeria-tainted food and remain healthy, while others become ill. The answer, the Irish scientists found, may lie in Listeria’s ability to adapt itself to its host environment (Hill, C. Presentation to the autumn meeting of the Society for General Microbiology, September 6, 2010. Nottingham, UK).
If we both sat down and had some mildly acidic food contaminated with Listeria, and you also ate a food high in glutamate and I didn’t, you’d become ill and I wouldn’t.
—Colin Hill, PhD, DSc, University College, Ireland
“If you take Listeria growing at neutral pH, in a neutral food, and drop it into stomach acid, it’s wiped out instantly,” said Colin Hill, PhD, DSc, professor of microbial food safety at University College. “But we found that if you take the exact same strain and shifted it to a mildly acidic environment, not so acid that it would kill it, the bacteria sensed the change, and it changed.”
In fact, the bacteria became completely resistant to stomach acid. Through what Dr. Hill called a “clever set of chemical reactions,” the Listeria combined one molecule of glutamate with one molecule of acid and effectively neutralized the acid.
“If we both sat down and had some mildly acidic food contaminated with Listeria, and you also ate a food high in glutamate and I didn’t, you’d become ill and I wouldn’t,” Dr. Hill said. “This does seem to fit with some of the foods that have been correlated with listeriosis, but of course we can’t deliberately prove the theory.”
These findings make controlling Listeria much more complex. “It’s difficult to set an absolute number of Listeria bacteria that is safe in food,” said Dr. Hill. “One level may be perfectly safe in one food and not so safe in another. This is a very rare disease, so we’re not suggesting that people avoid foods with higher levels of glutamate just to protect themselves from listeriosis. One possibility may be to develop less ‘risky’ diets for high-risk individuals, like young babies, pregnant women, and immunocompromised people.”