Bacteria and Stomach Acid
Name: Lon H.
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
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
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
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.
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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Update: June 2012