COLUMBIA, Mo. » Walmart and health officials awaited tests Thursday on a batch of powdered infant formula that was removed from more than 3,000 stores nationwide after a Missouri newborn who consumed it apparently died from a rare infection.

The source of the bacteria that caused the infection has not been determined, but it occurs naturally in the environment and in plants such as wheat and rice. The most worrisome appearances have been in dried milk and powdered formula, which is why manufacturers routinely test for the germs.

Walmart pulled the Enfamil Newborn formula from shelves as a precaution following the death of Avery Cornett in the southern Missouri town of Lebanon.

The formula has not been recalled, 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," Walmart 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 it for a refund or exchange, Gee said.

The product is not exclusive to Walmart. The manufacturer, Mead Johnson Nutrition, declined to answer questions about whether formula from that batch was distributed to other stores.

"We’re highly confident in the safety and quality of our products," said Christopher Perille, a spokesman for the company, which is based in the Chicago suburb of Glenview.

A second infant fell ill late last month after consuming several different types of powdered baby formula, but that child recovered, 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 tests indicated that Cornett died of a rare infection caused by bacteria known as Cronobacter sakazakii. The infection can be treated with antibiotics, but it’s deemed extremely dangerous to babies younger than 1 month old and those born premature.

The bacteria are "pervasive in the environment," Perille said. "There’s a whole range of potential sources on how this infection may have got started."

A spokeswoman for the Food and Drug Administration said the agency is investigating the death along with the Centers for Disease Control and the Missouri Department of Health.