- The FDA may have traced what caused the salmonella outbreak in millions of eggs on two Iowa farms.

- Salmonella can get into an egg from the inside out or the outside in.

- Experts advise keeping eggs in the refrigerator and cooking them well to avoid getting sick.

On Friday the Food and Drug Administration announced they found salmonella in chicken feed that was used at two Iowa farms where tainted eggs have been traced.

An estimated 2,400 people have been sickened from the eggs and more than 550 million eggs have been recalled since early August.

Even if investigators have indeed found the salmonella source, you may wonder, how can the bacteria get inside the hard shell of an egg? Let us count the ways.

One route is through the insides of a chicken, said Kevin Keener, a food process engineer at Purdue University in West Lafayette, Ind. On average, he said, one out of every 20,000 chicken eggs contains a small amount of salmonella that is deposited into the sac by the hen.

Chickens get doses of salmonella bacteria (of which there are 2,300 kinds) from their environment, which is easily contaminated by rodents, birds and flies. These carriers deliver the bacteria to all types of farms -- regardless of whether they're conventional, organic or free-range.

Once the bacteria get in the chicken, the microorganisms thrive under ideal conditions, with internal temperatures of about 102 degrees Fahrenheit. Yet chickens harbor salmonella without any signs of illness, making it impossible to know which animals are infected.

"Literally," Keener said, "it's a needle in a haystack."

Those few contaminated eggs that come out of a hen usually contain a very low levels of bacteria, Keener said, totaling between two and five microorganisms. It takes a level of at least 100 bacteria to make a person sick.

But multiplication happens fast if the eggs aren't cooled quickly. And if there's a lapse in cleaning practices or an undetected outbreak among the chickens, the percentage of infected animals -- and tainted eggs -- can also increase rapidly.

"Salmonella doubles every 20 minutes under ideal conditions," Keener said. "When sitting there for an hour, two could become 32. At two hours, there would be 1,000 organisms. At eight hours, it would be in the range of millions. In one egg."

Even if chickens remain salmonella-free, their eggs can become contaminated from the outside in.

Flipping your eggs over increases your safety from salmonella poisoning. iStockPhoto

Every egg has about 9,000 pores that salmonella can essentially climb into from say, a bacteria-tainted belt in the processing plant or a vat of egg-cleaning liquid that isn't kept at just the right temperature and pH.

Inspectors are looking into every possibility.

"Right now, FDA investigators are performing environmental assessments of farm conditions and practices including pest and rodent controls, biosecurity plans and controls, environmental monitoring, sanitary controls and feed and laying hen sources," said FDA spokesperson Siobhan DeLancey. "The investigators are also looking at commonalities between Wright County Egg and Hillandale Farms of Iowa, Inc.," the two farms that sourced the tainted eggs.

Even as the investigation continues, experts say that eggs are safe and getting safer. The last recall came from an organic farm in California last year, but the one before that was 16 years ago, said Krista Eberle, director of food safety at the Egg Safety Center, an industry group.

In July, she added, the FDA instituted new egg safety rules that require producers to do things like more carefully clean and disinfect hen houses and get eggs into refrigerators within 36 hours after laying.

And even though 130,000 people get sick each year from salmonella in shelled eggs, Keener said, eggs are responsible for less than 1 percent of all food-borne illnesses.

Meanwhile, scientists are working on new technologies to make eggs even safer. Effective salmonella vaccines for chickens are available and already in use in Europe. And Keener's group is working on a rapid-cooling technique that uses liquid carbon dioxide to bring eggs down to 45 degrees F within five minutes. At that temperature, salmonella can't multiply.

For now, consumers can protect themselves by checking for broken eggs before buying cartons at the store, refrigerating eggs promptly and cooking eggs well. For vulnerable groups, such as the very young, the very old those with immune problems, pasteurized eggs are best.

Salmonella tends to pool in the membrane around an egg's yolk, Keener added. So if you have a sunny-side-up habit, you should probably give it up.

"Flip your eggs over," he said. "That will kill any salmonella present."