Isnt your stomach acid basically around pH2 and it would kill all the bacteria?
In: Biology
There are species of bacteria that can survive extreme pH levels, e.g. acidophilic bacteria that produces vinegar can thrive in a pH below 4.0 and even survive down to pH 0.
Bacteria in yoghurt, ([lactobacilli](https://www.sciencedirect.com/topics/food-science/lactic-acid-bacteria) has a optimum pH of ~4), depending on which species of lactobacillus can survive the pH 2. Here’s a [study](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1151822/) done on lactobacillus’ survival in gastric acid.
Certain bacterial species may also be pathogens, e.g. Helicobacter pylori, which causes 90% of duodenal ulcers and 80% of stomach ulcers([sauce](https://www.cdc.gov/ulcer/keytocure.htm)).
That level of acidity, at that temperature, is enough to kill a great many bacteria. For sure.
First, there’s no single, simple answer. Here are a few of the common or interesting ones.
One strategy is sheer luck. To get an idea, think of every few years when a horrible catastrophe, like a fireworks factory blows up and someone standing right in the middle walks out without a scratch. This happens in the stomach, and in biology in general. It’s one reason the idea of “survival of the fittest” is incomplete.
Some cells like to cling to the side of the stomach and try to minimize the contact with the acid, in part to minimize the damage.
Another strategy is essentially turtling up. A lot of cells can make a protective outer coating when the environment looks bad and stop their normal cell stuff. When the outside of the cells senses things are better, the cell can go back to doing cell things. An example of this is endospore formation, which is usually associated lack of food, but has come to be appreciated as a way that cells handle lots of environmental extremes.
The last one, which is maybe the coolest, is an example of how ingenious microbial survival tricks can be. pH stands for percent hydrogen, and at low pH (high acidity), you have a lot of hydrogen ions floating around. E. coli actually uses this free floating hydrogen to “turn on” special proteins inside itself that protect against the damage that the acid would do! The acid actually causes the proteins to change shape, so that they can help repair other parts of the cell. When there’s no acid around, they don’t really do much of anything.
That may not seem super cool right off the bat, but consider a rough analogy. A lot of people know that metal expands when it gets hot. Imagine you lived in a fire prone area, and your house had a fire extinguisher in a locked cabinet. When you moved in, you got a key from the agent, but it didn’t work. Then, one day, the house is on fire and everything is hot as hell. The lock and key are both metal, and now that it’s hot, they fit perfectly. Extinguisher comes out, house is safe, and and all is well.
Now imagine there’s no locksmith to plan it that way. That’s how fucking cool that is.
This is part of the reason why E. coli is always found in the mammalian gut… it’s great at living in there, and it’s great at getting there!
I also wanted to sneak in one of my all time favorite bit of scientific cheekiness. There’s [a very good paper on this topic](https://www.pnas.org/content/110/14/E1254), published in a very well regarded scientific journal. It’s pretty hard to understand, so the editors wrote an introduction for it… this is where humor is sometimes allowed to sneak through. I present you: *[How bacteria survive and acid trip](https://www.pnas.org/content/110/14/5279)*.
On the more extreme side, there are organisms that thrive in ultra-acid and ultra-basic pH and temperature regimes that make the stomach look like a Club Med. Nearly boiling water and pH’s so high and low that animals that get too close either die or turn into stone ([no joke](https://www.smithsonianmag.com/science-nature/this-alkaline-african-lake-turns-animals-into-stone-445359/)). It violates our intuition about biology, but many of these organisms actually can’t live outside of these extremes. Nature be scary.
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