INCLINE VILLAGE, NEV. — Freezing a food does not always kill all the bacteria in it. Covering boiling water helps kill all the organisms in the pot. And houseflies can carry pathogenic Escherichia coli.
Those are some of the intriguing findings of recent studies about food-borne infectious disease, said Robert W. Derlet, M.D., at an annual emergency medicine meeting sponsored by the University of California, Davis.
U.S. Department of Agriculture researchers looked at whether refrigerating or freezing chicken at either 4° C or −20° C would kill Campylobacter jejuni, which is prevalent in U.S. poultry, said Dr. Derlet, chief of emergency medicine at the University of California, Davis.
They found that when the chicken was frozen for 1 week, about 10% of the C. jejuni population survived, and when it was frozen for 2 weeks, 5% survived.
However, most home refrigerators cannot achieve −20° C. In addition, he said, “Other studies have shown that even with months of freezing, some bacteria that are hardy survive. E. coli, as well as salmonella, tend to be hardy environmental organisms.”
The researchers concluded that freezing is not a substitute for proper cooking (Appl. Environ. Microbiol. 2004; 70:7103–9).
Researchers from the Centers for Disease Control and Prevention, who are studying Bacillus anthracis because of its potential use as a bioterrorism agent, investigated whether boiling water contaminated with the organisms would sterilize the water. They found that when the water was covered and boiled for 3 minutes or 5 minutes, all the organisms were killed. However, when the water was boiled uncovered, high numbers of the organisms survived (Emerg. Infect. Dis. 2004;10:1887–8).
Some organisms can encapsulate into spores and survive intense temperatures, Dr. Derlet said. Clostridia, for example, form temperature-resistant capsules that break down when they cool, which is why there are instances of people becoming sick after eating soup that has cooled.
Researchers at Kansas State University collected flies from a cattle farm in that state to see if the insects could be contributing to dissemination of E. coli O157:H7, which may be present in up to 40% of beef that comes from a feedlot or passes through a stockyard.
They found that 6% of the flies collected harbored the organism, which can cause hemolytic-uremic syndrome (Appl. Environ. Microbiol. 2004;70:7578–80).
Flies can also carry shigella, salmonella, and cholera, and a fly needs only a second to get its mouth parts onto your food—though it is not known if in that time the fly can deposit enough organisms to make someone ill.
An investigation of an outbreak of about 3,000 cases of salmonellosis contracted from raw tomatoes served at a fast-food restaurant chain in 1999 demonstrated that Salmonella enterica can be transferred from hands and grow rapidly in tomatoes, Dr. Derlet noted. A recent study has now shown that it can survive on vegetables for 200 days, at least under experimental conditions (Appl. Environ. Microbiol. 2004;70:2497–502).