Salmonella is a type of bacteria that was discovered in 1887 by an American veterinarian named Daniel Elmer Salmon (hence the name), and there are over 2,000 strains that we know of. Salmonella is actually quite common in the environment and even in our own gut, but luckily only some of those thousands of strains are harmful to humans. Many of those harmful strains happen to be carried by birds and other animals, including chickens, as a small part of their normal gut flora. All chickens can have varying amounts of Salmonella in their bodies, no matter how clean their surroundings are kept.
So how does Salmonella get into eggs?
It usually doesn’t. Nature has worked very hard to ensure that the inside of an egg is protected from Salmonella and other bacteria in the environment. When a hen lays an egg, she also deposits a natural antibacterial coating over the surface, often called the “bloom” or the “cuticle”. After all, eggs are meant to be incubated by the hen for three weeks, and if bacteria get inside during that time, the developing embryo would most likely die. There are lots of tiny pores in the eggshell that allow oxygen to get in, and the antibacterial bloom helps to keep bacteria from entering as well.
Since chickens carry Salmonella within their bodies, occasionally a hen might lay an egg with a small amount of Salmonella inside. This happens very rarely. On average 1 in every 20,000 eggs laid contains Salmonella bacteria, and even then it’s a tiny amount, most often five or less bacteria. It usually takes at least 100 Salmonella bacteria to make someone sick. Bacteria can sometimes get in through the shell, especially if it’s damaged or cracked.
Factory farm eggs increase risk…
In factory farms, thousands upon thousands of chickens are crowded together, making it very easy for diseases to spread. Baby chicks are raised in isolation from adults and also fed antibiotics, which means they never acquire the natural gut microbes that normally protect adult birds from being colonized by large numbers of potential human pathogens like Salmonella.
In this environment, the bacteria that do colonize poultry tend to be the ones that have become resistant to the antibiotics the hens are fed. Chickens under high stress are also more likely to have large amounts of Salmonella, since stress hormones have been shown to increase the bacteria’s growth rate.
To wash or not to wash…
Washing removes most of the protective bloom from the surface of the shell, but In the U.S., producers are required to wash their eggs, at a temperature at least twenty degrees warmer than the inside of the egg. The logic behind the higher temperature is that cold water could cause the contents of the egg to contract, drawing contaminants inside through the pores. After washing, commercially produced eggs are then rinsed with a chemical sanitizer (which can also enter the egg) and dried, because bacteria cannot go through the pores of an egg without moisture as a vehicle to carry them across.
In Europe, there are very different thoughts on egg safety. European laws prevent producers from washing eggs at all, so that the natural antibacterial “bloom” remains intact. In European supermarkets, eggs are not refrigerated, because if they begin to warm up as you bring them home, condensation can form on the surface, and since bacteria needs a moist surface in order to enter an egg, this increases that chance of bacteria getting inside.
And buying eggs from your local farmer?
In contrast to large factory farms, happy, free-roaming hens are less likely to be stressed and more likely to have a normal, diverse gut flora that are in competition with each other and preventing a prevalence of pathogens like Salmonella. In uncrowded conditions, there is less likely to be a large build-up of bacteria in the environment, and in the absence of antibiotics, diverse communities of bacteria vie with each other for resources, not allowing any one type to become dominant.
So are eggs from small farms safer? I know which ones I’d rather eat…