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[00:00.42] Somewhere on a farm in Iowa in 2010, a hen lays an egg.

[00:05.73] In just a few short weeks, this egg will be part of a massive infection event: thousands of people will fall ill, millions of eggs will be recalled, and several egg industry titans will ultimately land in jail, all thanks to a microscopic but mighty bacterium.

[00:19.81] Salmonella infects millions worldwide each year, causing fever, stomach cramps, and diarrhea.

[00:25.92] And these effects can be extreme: Salmonella is the leading cause of hospitalizations and deaths from food poisoning.

[00:33.36] So, let’s follow this microbe to find out how it makes so many people sick.

[00:38.14] We begin in the chicken's digestive tract, a major source of all Salmonella infections.

[00:43.19] In chickens, Salmonella bacteria often go undetected, allowing them to spread to eggs either through the developing yolk or by passing through feces, which can then contaminate shells.

[00:54.34] Under unhygienic farming conditions, this Salmonella-laden feces may also infect or contaminate other animals and crops, causing various food-linked outbreaks.

[01:04.44] Meanwhile, chicken meat can be exposed to intestinal Salmonella during processing.

[01:09.48] On its journey from farm to plate, the microbe can survive extreme cold, wet, and dry conditions.

[01:15.59] However, once it moves into a human body, Salmonella reveals its true talents for survival.

[01:21.44] The first hurdle is the stomach.

[01:24.36] Here, most bacterial invaders are killed off by stomach acid.

[01:28.08] But Salmonella cells can detect acidic conditions, which triggers the production of acid shock proteins.

[01:34.72] These molecules shield the bacteria from damage just long enough for it to pass into the intestines.

[01:40.83] Salmonella then faces the next gauntlet, as intestinal cells swiftly unleash microbe-destroying immune cells.

[01:47.73] But once again, the bacteria detect these changes with inbuilt sensors.

[01:52.52] And embedded within Salmonella’s genome are pathogenicity islands, clusters of adaptive genes that launch the next phase of attack. They signal the construction of a specialized system that resembles a needle and syringe.

[02:06.06] Within seconds, it injects molecules called effector proteins into the intestinal cells, causing them to change their structure and swallow up the Salmonella.

[02:15.36] Once inside, Salmonella can then exploit the cell machinery to replicate and spread.

[02:21.47] But these invaded intestinal cells don’t go down without a fight— as soon as this breach begins, they release cytokines, chemical messengers that prompt the immune system to launch into action.

[02:33.16] Fleets of white blood cells seek out and destroy Salmonella microbes and infected cells.

[02:38.74] This inflammatory response is also what leads to symptoms like abdominal pains and fever.

[02:44.85] And it further damages the breached intestinal cells, limiting their usual ability to absorb water.

[02:52.55] So, whatever’s in the digestive tract gets released in watery diarrhea.

[02:55.21] While this inflammatory response may feel unpleasant, it effectively purges Salmonella from the body within 2 to 7 days for most people, without the need for antibiotics.

[03:05.03] But there are times when a Salmonella infection may require more treatment.

[03:08.75] It can lead to severe dehydration, particularly in children and older patients.

[03:13.80] And in some unusual cases, Salmonella can continue to spread through the body, hiding inside immune cells, invading other organs and tissues, and even poisoning the blood.

[03:24.16] These cases occur if a person is infected by a rare but powerful type of Salmonella called S. typhi.

[03:30.27] Unlike other strains, S. typhi doesn’t infect chickens— It spreads from person to person, mainly via poor sanitation and untreated drinking water.

[03:43.55] Although it’s uncommon in many parts of the world, typhoid fever, the disease that S. typhi causes, still kills over 100,000 people yearly.

[03:48.60] Thankfully, there are vaccines to prevent infection by S. typhi.

[03:52.05] To avoid milder variants, there are steps that everyone can take, like washing your hands, avoiding unpasteurized milk, and cooking meat and eggs thoroughly.

[04:01.35] Cookie dough should be off limits: raw eggs and flour both carry a Salmonella risk.

[04:07.19] And there are ways to stop Salmonella at its source.

[04:10.38] Investigations into the outbreak in 2010 revealed one company's dark history of unhygienic farming conditions, bribery of health officials, and mislabeled eggs.

[04:19.41] Since then, the United States has taken steps to put stricter regulations in place.

[04:24.46] In Europe, many countries have successfully reduced Salmonella by requiring testing on farms and before products reach shelves.

[04:31.90] But there's still work to be done if we want to stop the spread of this incredibly crafty pathogen.