Gastric acid as a chemical barrier
Stomach acid, at a pH of around 2, is a chemical barrier: it makes the stomach hostile to most swallowed bacteria without recognising any particular one.
Remember
Stomach acid at a pH of around 2 kills most swallowed bacteria. When acidity is reduced, for example by proton pump inhibitors (medicines that reduce the amount of acid the stomach makes), the stomach is less effective as a chemical barrier and the risk of infection of the gut (the long tube from the mouth to the anus) may rise.
Memory aid
less acid, less killing
Use it to link a rise in stomach pH to weaker killing of swallowed microbes.
The stomach makes stomach acid, which is composed mainly of hydrochloric acid and holds the stomach contents at a pH of around 2. (pH is a measure of how acidic or alkaline something is: 7 is neutral, anything lower is acidic and anything higher is alkaline.) The scale is logarithmic, meaning that each whole pH unit is a tenfold change in acidity, so pH 2 is very acidic.
At that level the acid de-activates proteins and damages and kills most of the bacteria that are swallowed with food and drink. This makes the stomach a chemical barrier: a defence that works through the chemical conditions it creates and not by recognising a particular microbe. It belongs to innate immunity, the defence present from birth that acts on any microbe without earlier exposure.
The barrier depends on the acid. When acidity is reduced, more microbes survive and the risk of infection of the gut (the long tube from the mouth to the anus) rises. A proton pump inhibitor is a medicine that reduces the amount of acid the stomach makes, so it raises the stomach pH and weakens the barrier.
Other defences work differently. Mucus in the stomach is a gel layer that protects the stomach wall from its own acid. Of these stomach features, only the acid directly lowers microbial survival. Vomiting and diarrhoea expel microbes. Antibodies recognise one specific antigen, so they belong to adaptive immunity.
Key facts
- Acid
- stomach acid is composed mainly of hydrochloric acid and holds the stomach at a pH of around 2.
- pH scale
- it is logarithmic, so pH 2 is about a hundred thousand times more acidic than neutral pH 7.
- Effect
- the acid kills most of the bacteria that are swallowed with food and drink.
- Type of defence
- a chemical barrier of innate immunity, active without earlier exposure to the microbe.
- Sorting rule
- ask what the barrier does to the microbe. Physical barriers block it or sweep it away, physiological barriers expel it, biological barriers use the normal flora (the harmless bacteria that normally live on us) to crowd it out, and a chemical barrier makes the surroundings hostile.
- Weak point
- when acidity is reduced, for example by acid-suppressing medicines, the stomach is less effective as a chemical barrier.
Exam traps
- Mucus in the stomach is not the barrier lost when acid is reduced: its job is to protect the stomach wall from acid and digestive enzymes, and it does not kill swallowed microbes.
- The normal stomach is not held at a pH of around 1, which is ten times more acidic than pH 2, even though it is strongly acidic.
- The mildly acidic pH of the skin, about 5.5, is a separate chemical barrier of the skin surface, and a proton pump inhibitor acts on stomach acid, not on the skin.
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