The bacterium that caused the nation`s biggest salmonella epidemic may be a ''super superbug,'' raising fears among public health officials that similar widespread outbreaks could occur again, according to preliminary findings.
One of the chief findings so far is that the Salmonella typhimurium strain that caused the milk-borne infection, which has affected nearly 16,000 Illinois residents, may be more dangerous than other types of salmonella.
The findings also raise new doubts about the practice of feeding low levels of antibiotics to farm animals, which some experts believe increases the risk of developing antibiotic-resistant bacteria.
The term superbug has been used to describe bacteria that have developed resistance to antibiotics. But the Illinois epidemic strain may be a ''super superbug,'' because it appears to be a more deadly cause of disease than doctors thought, said Dr. Russell Martin, a communicable-disease expert at the Illinois Department of Public Health.
Not only is the Salmonella typhimurium bacterium resistant to five different antibiotics, but people who took antibiotics and then drank tainted milk were more prone to get sick, said Dr. Mitchell Cohen, a salmonella expert at the federal Centers for Disease Control in Atlanta.
The early findings also indicate that the organism is very effective in causing disease at doses much lower than previously thought, he said.
''The potential exists for there to be more problems like this, because there are many products that can harbor salmonella,'' said Cohen. ''This is an important public health problem, and substantial efforts must be made to resolve it and to learn how we can prevent future episodes.''
Genetic analyses of the bacterium show that it has been found only in infections centered in Illinois, said Cohen. The same bacterium has now been positively identified in four episodes of contaminated milk produced at Jewel Companies Inc.`s, Hillside Dairy in Melrose Park, he said.
These include milk produced on Aug. 6, 1984, which caused a small outbreak, and on March 20, March 30 and April 8 of this year. Milk produced on March 20 and 30 is blamed for the present epidemic. The April 8 milk was never sold.
The fact that all four contaminations were caused by the same strain of salmonella indicates there is a common source of the bacterium, said Cohen.
However, it has not yet been determined whether the source of infection is inside the dairy or from outside, such as a herd of dairy cattle in Wisconsin, he said.
A federal and state task force investigating the plant said it has not found the exact cause of the contamination, but it suspects that a crossover line may have allowed contaminated raw milk to mix with pasteurized milk.
Though federal investigators have found salmonella contamination in some Wisconsin herds, these organisms are not the same strain that caused the Illinois epidemic.
The puzzling thing is that if there was a leak in the plant, it would probably lead to a number of milk contaminations caused by a variety of salmonella organisms, said Cohen.
One explanation for this may be that, indeed, salmonella-contaminated raw milk regularly leaked into pasteurized milk, but the doses were so low that they caused little or no infection, he said.
It may be that infections occurred only when raw milk coming from a specific herd infected with Salmonella typhimurium was mixed with pasteurized milk, because this bacterium is more effective at causing disease in extremely small doses, he said.
''This bacterium represents a unique situation,'' said Martin. ''It is not a garden variety salmonella, and it raises a lot of concerns.''
One of the major concerns is whether the nation`s food supply is being increasingly contaminated with antibiotic-resistant salmonella, he said.
Resistant bacteria can be caused by the inappropriate use of antibiotics in humans, but there is growing worry that the widespread use of antibiotics in animal feed may be producing superbugs. Low doses of antibiotics are fed to many farm animals to stimulate growth.
The first documented case of a human infected by a superbug coming from animals was made by a team of scientists from the Centers for Disease Control and the Minnesota State Health Department in a report last September in the New England Journal of Medicine.
They traced to cows in South Dakota a Salmonella newport outbreak in Minnesota that affected people who ate contaminated hamburger. In this outbreak, people who became the most severely ill ate the contaminated meat first and then took antibiotics.
The reason antibiotic-resistant bugs are dangerous is that they rapidly multiply when a person takes an antibiotic. The antibiotic drug kills the bacteria that normally exist in the intestinal tract, thereby eliminating the competition that usually keeps disease-causing bacteria in check.
An estimated 5 percent to 8 percent of the people in the U.S. are taking antibiotics at any given time. People taking antibiotics who are exposed to superbugs seem to get more severely ill than those not taking antibiotics, said Cohen.
The evidence indicates that the number of superbugs is increasing and that the risk of widespread infections is becoming greater, said Cohen.
The U.S. Food and Drug Administration attempted to ban antibiotics in animal feed in 1977, but Congress insisted on more studies as a result of heavy pressure from the pharmaceutical industry. The meat industry also opposes the ban, on the grounds it would create economic hardships.
Because of the growing controversy, Margaret Heckler, secretary of health and human services, is considering a petition to ban antibiotic feeding to livestock as an imminent health hazard.