A rare bacterium discovered in Mexican-style cheese and tentatively linked to the deaths of 43 people in the West has led federal investigators to fear that the germ may survive normal purification processes.

Alarmed by similarities to another outbreak of Listeria monocytogenes that killed 14 people in Massachusetts two years ago, officials of the federal Centers for Disease Control (CDC) in Atlanta said they have sent scientists here to assist local health officials investigating the new contamination.

Jalisco Mexican Products Inc., the Pasadena-based firm that produced the tainted cheese, has laid off the 102 employes at its nearby Artesia plant. The firm has also hired Burson-Marsteller, the public relations firm that handled the aftermath of the Union Carbide gas leak in Bhopal, India, that killed more than 2,000 people in December and the cyanide-tainted Tylenol capsules that killed at least seven Illinois residents in 1982.

Investigators say they have found no failure at the Artesia plant or in the pasteurization of the milk used there to explain the bacteria's survival. The bacterium is particularly dangerous to unborn and newborn babies, new mothers, elderly persons and anyone with a weak immune system.

No flaws were found in the Massachusetts system to screen such bacteria, raising "the possibility that it may be able to survive pasteurization," said Dr. David Fleming, a CDC medical epidemiologist.

Although many scientists remain skeptical, the Listeria bacteria are known to live as parasites inside cows' white blood cells. Fleming said the U.S. Food and Drug Administration has funded a study of infected milk in Yugoslavia to determine if the white cells protect the bacteria from the heat of pasteurization. He said he does not expect results for several months.

Many producers use a process called "clarification" to remove the white blood cells from milk. The process is not required by the FDA, and the Massachusetts milk was not clarified. Jalisco officials have declined to say whether the milk used in the contaminated cheese had been clarified.

Fleming said about 1,000 to 1,500 cases of listeriosis, the severe flu-like symptoms caused by the bacteria, are reported each year. About 30 percent of patients die. The disease can be picked up from water, mud and other sources, and clusters of cases from the same source occur only once or twice a year, Fleming said.

According to the Associated Press, 43 deaths among 129 cases of the disease have been reported during the current outbreak, brought to light last week by Los Angeles health officials after the Atlanta centers confirmed that samples of Jalisco-brand cheese contained the bacteria. Officials have declined to say how many of the cases can be directly blamed on the cheese.

A few deaths have been reported in Arizona and Texas, where Jalisco products also circulated, and some illnesses have been reported in Colorado and Oregon, but most of the deaths have occurred in California. The Associated Press today reported 40 apparent listeriosis deaths in the state, including 24 in Los Angeles County, during the outbreak.

Most of the dead have been Hispanics. Some critics have charged local health officials with moving too slowly to identify the outbreak's source. Los Angeles Herald Examiner columnist Joe Morgenstern wondered today if one official would "have done more . . . if the cheese had been Brie, rather than some Mexican varieties, and the center of the afflicted area had been Beverly Hills rather than the Latino neighborhoods of Los Angeles?"

Local health officials said they moved as fast as possible to identify the source of the outbreak but could not ethically blame any company until samples were checked. The Los Angeles County Board of Supervisors today requested a grand jury probe to determine if state or county inspections of the Artesia plant were faulty. State authorities have said one inspector failed to submit reports on the plant but found no major violations.