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Name: Lon H.
Status: Student
Age: 40s
Location: N/A
Country: N/A
Date: N/A 

What is the reason that certain bacteria that enters the body through the digestive tract are not killed by the acidic conditions in the stomach yet survive to go on to cause illness, disease or death. If possible, any examples of bacteria that are and are not affected by the stomach acids would be much appreciated.

Bacteria that are not killed by acid, that in fact love to live in an acidic environment, is Helicobacter pylori. As you can imagine they live in the stomach which is their natural niche. However, most bacteria do not like acidity and will suffer. So how can so many infectious organisms pass the stomach? first of all, you should know that the acidity of the stomach is not constant. A full stomach, just after a big meal, finished with a huge glass of milk may be hardly acidic, and is nothing compared to an empty, hungry stomach, ready for food. So bacteria 'sneek in' with the food. Then the question is, how many bacteria do you swallow, and how many of those are killed in the stomach. An egg with Salmonella in it could easily harbor a million times a million organisms. By cooking (but not through) you may have reduced that to 100,000 organisms. The stomach may kill 99%, still leaving 1000 organisms. That is sufficient for an enteritis.

It all depends on the infectious dose of a particular organism. Which organisms are succesful to survive the stomach? exactly those that cause gastrointestinal diseases. Salmonella, E.coli, Shigella, Campylobacter, Listeria, Bacillus species, Vibrio cholerae, to name a few.

Finally, do not forget that many 'good' bacteria live in the intestine. We are born 'sterile' so those bacteria must have entered our body at one time and then multiplied. The bacterial content of our intestines is not static, bacteria come and go. Also the good bacteria have to survive the stomach first. Just as well that they do, because nobody can live without bacteria.

Trudy Wassenaar
Curator of the Virtual Museum of Bacteria

I am not an expert in this field, but some general rules are apparent: Certain strains of e-coli survive the stomach, as does typhoid fever, any of the flora found in the small/large intestine must fall into this class, because most likely that is how they got there.

Think a Web search on: "water borne diseases" would provide more specific examples of acid resistant pathogens.

Vince Calder

H.pylori is not killed by stomach acid. It depends on the nature of the bacterial cell wall usually.

Peter Faletra Ph.D.
Office of Science
Department of Energy

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