Biology Question #1022

Lisa Fondelius, a 40 year old female from Borås, Sweden asks on November 6, 2002,

If pepsin digests proteins in the stomach, how do orally taken vaccines work?

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The answer

Wendy Hutchins answered on July 8, 2003

The first oral vaccines on the market were those based on organisms that infect via the oral route. Organisms such as Salmonella bacteria, and Polio virus naturally enter our bodies via injestion, thus they have to be resistant to the effects of stomach acid as well as all the digestive enzymes. Some have a thick sugar capsule that will protect them. Others use proteins with amino acids that our enzymes don't recognize. The bacteria pass through the stomach and cause disease in either the large or small intestine first, or by invading through the gut lining into the rest of the body. The virus pathogens go the same route, but will multiply in the cells of the gut lining.

Prevention, the goal of a vaccine, is as simple as getting the immune reponse to prevent the bacteria from sticking to the gut lining cells or the virus from getting into the cells. In practice, it isn't so simple. Many of the oral vaccines are live attenuated (not able to cause infection) strains of the original organism. These strains were developed over many years or by genetic modificiation. Some of the bacteria that cause gastric diseases are very closely related to bacteria that we need in our gut, like E.coli, so 1) we wouldn't respond well to a "Walkerton" E.coli (more affectionately called EHEC) vaccine and 2) we wouldn't want to so we can keep the E.coli we need to survive.

The answer is to take the genes from the EHEC strains and move them into the attenuated Salmonella. Other examples of oral vaccines in development are the new ones for cholera. Cholera is caused by a toxin produced by the bacteria, not the bacteria invading the gut like Salmonella and Shigella do. Cholera toxin is a protein that is resistant to acid and digestive enzymes itself. The toxin was able to be de-toxified by making a mutation in it, so now it is being used to try a new type of vaccine called an edible vaccine. Some other oral vaccines in development are not meant to survive the stomach - ones for tooth decay, are the best example of these.

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