The bacteria in question occur naturally in the environment and in plants such as wheat and rice. But the most worrisome appearances have been in dried milk and powdered formula, which is why manufacturers routinely test for the pathogens.
Wal-Mart pulled the Enfamil Newborn formula from shelves as a precaution following the death of little Avery Cornett in the southern Missouri town of Lebanon.
The government has not ordered a recall, and the manufacturer said tests showed the batch was negative for the bacteria before it was shipped. Additional tests were under way.
"We decided it was best to remove the product until we learn more," Wal-Mart spokeswoman Dianna Gee said. "It could be returned to the shelves."
Customers who bought formula in 12.5-ounce cans with the lot number ZP1K7G have the option of returning them for a refund or exchange, Gee said.
The product is not exclusive to Wal-Mart. The manufacturer, Mead Johnson Nutrition, did not immediately say how widely distributed the formula was among other stores.
"We're highly confident in the safety and quality of our products," said Christopher Perille, a spokesman for the company based in the Chicago suburb of Glenview.
A second Missouri infant fell ill after consuming powdered baby formula in the last month, but that child recovered, state health officials said.
Powdered infant formula is not sterile, and experts have said there are not adequate methods to completely remove or kill all bacteria that might creep into formula before or during production.
Preliminary hospital test results indicate that Avery died of a rare infection caused by Cronobacter sakazakii. The infection can be treated with antibiotics, but it's deemed extremely dangerous to babies less than one month old and those born premature.
The virus "is pervasive in the environment," Perille said. "There's a whole range of potential sources on how this infection may have got started."
Public health investigators will look at the formula itself, as well as the water used in preparing it and at anything else the baby might have ingested, Perille said.
Such illnesses are rare. Only two to three cases a year are reported in infants worldwide. New Mexico saw two in 2008, including one infant who died and another who suffered severe brain damage. A Tennessee infant died in 2001 after being infected.
The family submitted two types of infant formula for testing - the powdered version and a pre-sterilized, ready-to-eat liquid - as well as the distilled water used to prepare the powdered product.
"We're just trying to test anything that was consumed by the baby," Laclede County Health Director Charla Baker said.
Avery was taken to a pediatrician Dec. 15 - a week after he was born - after showing signs of stomach pain and lethargy. When the pain persisted the next day, his parents took him to an emergency room.
He died Sunday at a hospital in Springfield after being removed from life support.
The Missouri Department of Health advised parents to follow safety guidelines for preparing powdered infant formula, including washing hands, sterilizing all feeding equipment in hot, soapy water and preparing enough formula for only one feeding at a time.
A flood of calls from worried parents prompted state officials to clarify that the formula pulled by Wal-Mart is not being provided to participants in the Women, Infants and Children federal program for low-income parents.
Associated Press Medical Writer Mike Stobbe in Atlanta contributed to this report.
Alan Scher Zagier can be reached at http://twitter.com/azagier .