The bacterium Campylobacter jejuni is a major cause of food poisoning in humans. It is normally transmitted from contaminated chicken meat, as it is frequently found in the intestines of chickens, where it apparently does not result in any symptoms. Campylobacter jejuni is well adapted to life in the intestines of animals -- and humans -- so it is surprising that it is able to survive on the surface of meat, which is generally stored in a much more oxygen-rich atmosphere.
Researchers at the University of Veterinary Medicine, Vienna have now solved the puzzle, showing that Campylobacter can survive ambient oxygen levels thanks to the presence of other bacteria, species of Pseudomonas. The interaction between the different species seems to be a mechanism for Campylobacter to remain viable on chicken meat and thus to infect humans. The results are published in the current issue of Applied and Environmental Microbiology and provide important clues to combating enteritis in humans.
Many a holiday is ruined by food poisoning, frequently caused by the bacterium Campylobacter jejuni. Although Campylobacter infections are rarely life-threatening they are extremely debilitating and have been linked with the development of Guillain-Barré syndrome, one of the leading causes of non-trauma-induced paralysis worldwide.
Campylobacter jejuni is well adapted to life in the guts of animals and birds, where it is often found in very high levels. However, to infect humans it also needs to be able to survive outside the gut, on the surface of meat that will be eaten by humans. It is known that C. jejuni cannot grow under normal atmospheric conditions -- the levels of oxygen are too high for it -- so how it survives was until recently unknown. The mystery has now been solved by Friederike Hilbert and colleagues at the Institute of Meat Hygiene, Meat Technology and Food Science of the University of Veterinary Medicine, Vienna.
The surface of meat harbours a number of species of bacteria that -- fortunately -- are rarely harmful to humans, although they are associated with spoilage. It seems possible that the various species interact and Hilbert hypothesized that such interactions might help bacteria such as Campylobacter jejuni survive under hostile, oxygen-rich conditions. She thus tested the survival of C. jejuni in the presence of various meat-spoiling bacteria. When incubated alone or together with bacteria such as Proteus mirabilis or Enterococcus faecalis, Campylobacter survived atmospheric oxygen levels for no longer than 18 hours. However, when incubated together with various strains of Pseudomonas, Campylobacter were found to survive for much longer, in some cases over 48 hours, which would be easily long enough to cause infection.
There were differences in the extent of prolonged survival depending on the sources of the Campylobacter analysed but all isolates of all strains clearly survived significantly longer in the presence of Pseudomonas bacteria than when cultured alone. And the Campylobacter cells did not change shape when cultured together with Pseudomonas under oxygen-rich conditions, unlike when they were cultured alone, providing further indications of an interaction between the species. Interestingly, there is no evidence that the Pseudomonas benefit at all from the interaction, although they effectively save the lives of the Campylobacter.
Hilbert's findings show clearly that the presence of Pseudomonas bacteria is responsible for significantly enhanced survival of the disease-causing Campylobacter bacteria on the surface of meat. The results have implications for the control of meat, especially poultry, destined for human consumption. As Hilbert says, "On the basis of this study it should be possible to elucidate new mechanisms for limiting the level of Campylobacter on chicken meat and thus the incidence of food poisoning could be much reduced."
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