Fatal Bacteria May Have Survived Pasteurization
The bacteria found in Mexican-style cheese and linked to 31 deaths in Southern California possess an unusual ability to live as parasites inside the white blood cells of animals and humans where they may be protected from the heat of the pasteurization process, scientists at the federal Centers for Disease Control believe.
Although researchers at the U.S. Food and Drug Administration, which sets pasteurization standards, are skeptical of this hypothesis, they are studying the possibility that Listeria monocytogenes can, under certain circumstances, be an exception to their belief that proper pasteurization kills all disease-causing bacteria.
Preliminary results of a study being conducted in Yugoslavia under FDA contract and monitoring are at least a month away, according to agency officials.
Preliminary checks of the milk-pasteurizing equipment at Jalisco Mexican Products Inc., the maker of the suspect cheese, indicated that it was working properly, according to state food and agriculture officials in Sacramento.
But, according to CDC epidemiologists in Atlanta, knowledge that has been gleaned in the last several years from previous outbreaks--especially one that occurred in Massachusetts two years ago--raises questions about the ability of pasteurization to kill all of the L. monocytogenes in milk if there is a large number of organisms to begin with.
“All of this is so new, and much of it hypothetical, that it may be hard for people whose field is pasteurization to accept the possibility that pasteurization might be overwhelmed under certain conditions,” Dr. Stephen Cochi, an epidemiologist in the CDC’s special pathogens branch said in a telephone interview.
“The more that people look at Listeria, the more they realize it has heat-resistant properties,” he said.
Although the existence of the organism has been known since 1929, detailed knowledge about it and how it enters the human body is scanty.
Although the bacterium is known to infect more than 50 different animal species, it has been only in the last several years that researchers have acquired some understanding of how humans might acquire it from contaminated raw vegetables and fruits and milk products, for example.
The possibility that the Listeria can contaminate pasteurized milk products first arose only two years ago after a CDC investigation of the Massachusetts outbreak, in which 14 people died out of 49 reported cases of listeriosis. The report of that finding was not published in the scientific literature until February of this year.
As with the preliminary report by California officials on Jalisco’s pasteurization machinery, the federal researchers found no reason to suspect faulty pasteurization to be the cause of the Massachusetts outbreak.
Instead, Dr. David W. Fleming and others blamed the outbreak on the unusual ability of L. monocytogenes to exist as parasites inside white blood cells. An infected cow’s white blood cells are known to be in raw milk, where they remain--dead or alive--after pasteurization, unless an additional procedure known as clarification is performed to remove them.
Fleming and the other authors of the report believe that L. monocytogenes hidden inside the cow’s white blood cells in the raw milk may be protected enough for some of them to survive the heat pasteurization process. This speculation is based on the assumption that a larger than usual number of bacteria had been present in the raw milk.
The speculation received added weight when it was found that the dairy involved in the Massachusetts outbreak had not clarified the milk to remove the cow’s white blood cells.
There was no clarification done at the Jalisco cheese plant, although some of the milk used had been clarified elsewhere, according to Richard Tate, chief of milk and dairy foods for the California Food and Agriculture Department.
Jalisco officials declined to confirm or deny whether milk used to make the contaminated cheese had been clarified.
Clarification is not required by the FDA, and Tate said that although he has no exact figures, he believes that a “high percentage” of milk producers clarify milk. He added that “at one time it was completely standard practice.” Federal and California regulations call for Grade A pasteurized milk to undergo a heat treatment of 161 degrees (F.) for a minimum of 15 seconds. According to Tate, Jalisco exceeded that standard by exposing the milk to 164 degrees for 17 seconds.
The federal Food and Drug Administration, which sets pasteurization standards, is certain that Listeria do not survive pasteurization.
“I can confidently say that (California’s) state pasteurization process would adequately eliminate Listeria, " said Dr. Robert Twedt, chief of bacterial physiology at the FDA’s laboratories in Cincinnati, in a telephone interview.
Twedt said his statement is based partly on a study he reported only last week at a scientific meeting. In that study Twedt added to raw milk, containing no bacteria of any kind, a dose of L. monocytogenes that had been obtained from the cows in the Massachusetts outbreak. The milk was then pasteurized and after pasteurization it was found to contain no live Listeria.
But in the laboratory experiment, Twedt admits, the bacteria were free-floating organisms and not ones that are inside the protective coating of cows’ white blood cells, as occurs in nature.
A study of pasteurization’s effects on naturally infected milk is being conducted in Yugoslavia under U.S. Food and Agriculture Department contract and FDA monitoring, he said. No results have been reported yet, the scientist said, but the next monitoring report is expected in a month.
Another factor that may enter into the organisms’ survival from pasteurization is the observation by CDC researchers that any bacteria that survive the process may actually proliferate during the refrigeration that is standard procedure following pasteurization.
California standards allow a maximum of 50,000 bacteria per milliliter of raw milk to be present before Grade A pasteurization. Following pasteurization, a maximum of 15,000 bacteria are still allowable but on the assumption that all the disease-causing ones have been killed and the remaining ones are nonpathogenic, according to Tate, the state milk official.
Tate said he does not know whether current tests are capable of detecting pathogenic bacteria that may be hidden inside white blood cells in the milk.
SURVIVING PASTEURIZATION: A HYPOTHESIS NORMAL PROCEDURE: 1. A cow’s white blood cells, which are present in raw milk, are natural enemies of bacteria, attacking and killing some bacteria even in raw milk. 2. Raw milk is heated to kill bacteria. California law specifies that there can be up to 50,000 bacteria per milliliter of raw milk, and up to 15,000 bacteria per milliliter of pasteurized milk. 3.After being heated, pasteurized milk usually is cooled rapidly, a process that is supposed to kill more bacteria. 4. The product is passed along to the consumer in the belief that pasteurization has removed harmful microorganisms. SUSPECT PROCEDURE 1. Rather than being destroyed by white cells, the Listeria Monocytogenes strain of bacteria is thought to survive attack from white blood cells by “hiding” inside the white cell. 2. Some scientists speculate that by hiding inside the white blood cells, some Listeria survive the heat. 3. Unlike other strains of bacteria, Listeria may actually multiply during the quick cooling process. 4. Because the bacteria survived, the hypothesis suggests that pasteurization failed to eliminate the hazards in the cheese product made by Jalisco and marketed under four brand names.