Listeria poisoning episodes are frequent in many western countries and foods worth millions of dollars have to be recalled from the market because Listeria scare. That Western consumers are more susceptible to Listeria poisoning is a puzzle which does not have answer still but it is possible that many past episodes are linked to consumption of cold foods or foods which have not been pasteurized allowing the pathogen to be dormant and strike under favorable conditions. Listeria is rarely a threat in most tropical countries as foods are invariably over cooked and share of processed/refrigerated foods is minuscule in the daily diet of the population. One of the mysteries surrounding Listeria pathogenesis is why it affects some people while others escape from its infection. Some idea on this issue seems to have been gained by recent studies being carried out in UK and the reason attributed this phenomenon is the remarkable adaptability of this microorganism to different host environment and food composition.
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). 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 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." "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 Listeriabacteria 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."
The role of glutamate in the manifestation of Listeriosis is also quite significant and the unpredictability of the consequences of Listeria infection can be a worrisome factor for the food industry in general. Any abstract number of CFU of Listeria spp in foods, considered dangerous, may have little relevance if the unique ability of this bacteria to survive and navigate through the stomach environment without any significant destruction is taken into consideration. Future standards will have to keep in mind this factor and probably no one standard may fit all the foods. A food for thought for safety agencies!
V.H.POTTY
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