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Usually bacteria grow by dividing and they can do that very fast. So after a couple of hours or days you have many bacteria in your intestines. And we keep feeding them by eating. In some way you would expect that the growth of bacteria has been so overwhelmed that after a week our intestines would 'explode'.

Of course this doesn't happen, but what factors protects our intestines from bacteria overgrowth?

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  • $\begingroup$ I have a feeling stomach acid is to play here, but I can't seem to find any sources at the moment $\endgroup$ – Malhar Khushu Oct 2 '16 at 10:29
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    $\begingroup$ The intestine actually has specialised immune cells just for itself, much like it has a specialised nervous system. I can't provide you with a detailed overview as it's been a while since I've studied intestinal immunology, but one component that I do remember is the secretion of specialised IgA antibodies which to some extent could regulate the baterial load in the intestine (and certainly regulates the types of bacteria "allowed" to grow there). $\endgroup$ – Armatus Oct 2 '16 at 16:36
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    $\begingroup$ Bacteria also regulate themselves so as to not overwhelm their surroundings. The gut microbiome is an incredibly complex place, with multi-species biofilms that signal amongst themselves to prevent the outgrowth of any one species. The host's immune system is also involved to a significant degree. Remember, these are not laboratory-grade E. coli growing in Luria broth, and they are not pathogenic. They are symbiotes with the host, and both participate to not overwhelm the other. $\endgroup$ – MattDMo Oct 3 '16 at 15:01
  • $\begingroup$ If you take a nutrient agar and do a lawn culture, the bacteria won't grow on forever. There's a limit to the max population. This is without considering the host's immune factors which would play a major role in the gut $\endgroup$ – Polisetty Oct 3 '16 at 21:17
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Bacteria can't create mass, even when they divide. During division, you go from one "big" bacterium to two "little" bacteria, which then have to accumulate nutrients and grow bigger before they can divide again.

When nutrients are plentiful, this can happen rapidly, and you do see exponential growth. But the exponential growth phase is only valid when there is an excess of food in the environment. Bacterial growth curves follow a typical lag/log/stationary/death profile. The log (exponential) phase only applies when nutrients are in excess. Whenever some nutrient becomes limiting (because it's already incorporated into another bacterium, for example) the bacteria switch to stationary phase growth, and reduce replication or stop it completely.

So you would never get an "explosion" of intestinal bacteria - the bacterial mass is strictly limited to the amount of mass (food) input into the system. Once that's all (or mostly) converted, the bacteria will stop growing and start dying.

If you kept accumulating mass in your intestine, you're correct that eventually it would "explode". However, most people regularly (pun intended) eliminate excess intestinal contents. (Which, as bpedit mentions in their answer, contains a high proportion of bacterial cells.) This limits the total mass contained in the intestine, and keeps it from "exploding".

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There are very likely other factors at work as well, I'm no expert. But the following always surprised my Biology students.

About 30% of feces is bacteria. http://bionumbers.hms.harvard.edu/bionumber.aspx?id=108514 Peristalsis continues to move food, food waste and bacteria through your digestive system. So, as new bacteria arise, other bacteria are eliminated.

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