chapka

A random question for the biologists here:

I recently read Bill Bryson's excellent science/history of science book A Short History of Nearly Everything, which everyone here should read if they haven't already. In it, he discussed the microorganisms which live in the human digestive system, and which are required for it to function efficiently. (And I'd assume the same goes for other mammals).

My question is, how do these microorganisms get there? I assume they can't be passed in utero, since from what I understand the reproductive system is mostly designed to keep microorganisms away from the developing fetus. Are they just present in the environment in enough numbers to be able to fortuitously colonize human infants? Are they passed in mother's milk? Are they somehow selected for in the intestine, to insure that the environment is suitable for good bacteria but not for bad? Does everyone have the same ones, or are there a lot of different mixes of bacteria that will do the trick?

We're used to thinking that humans come out of the womb with everything they need to survive. But it seems like this is an important component that needs to be added later. How does this work?

IamMoose

Interesting question .. I would have thought food but I don't know. I love Bryson though so had to respond!

Roland98

Pretty much the latter. You might be amazed at the level of bacteria which surround us. They're everywhere!

They shouldn't be present in the milk, exactly, but some certainly live on the skin, and on pretty much everything the infant touches.

It's not really known. There's an entire field of study in this area, which really is (fairly) new. Many species may be transient colonizers; it takes awhile to establish a more set intestinal flora.

The latter. Although pretty much everyone has types of E. coli, for example, there are a lot of other bugs which we just don't know their prevalence in the population. It complicates things because many species are unculturable by most techniques. There is at least one project I know of looking at the 16sRNA of all bacteria in the gut flora; what they've found, so far, is pretty much a mess. Some diseases (such as Chron's and irritable bowel syndrome) have been suggested to have a microbial cause; that is, a species may be either lacking or present in a higher amount in the intestinal flora of cases than in controls. Nothing is definitive in that area, however.

This abstract may be interesting to you. Basically, it's a layer effect; there are early colonizers, which are generally transient, and replaced by a more stable and long-term bacterial flora.


Edited to add: though bacteria flora "stabilizes" later in life, it still is in a state of flux pretty much every day. The most radical disruptions occur when one takes antibiotics, but any illness (especially intestinal ones, for obvious reasons) can lead to a rapid change in bacteria flora, and even many of the more "stable" species of bacteria can have a fairly quick turnover if you look at carriage of particular strains by various molecular typing methods.

epepke

I'm not a biologist by trade; it's just a hobby.

They generally have to be passed by eating adults' shit. Human children are discouraged from doing so, but fortunately there is enough contamination on the fingers of adults that this generally happens anyway.

There are different mixes. The guts of vegans and omnivores will have different colonies. This is why vegan farts smell so terrible. It's also why after becoming a vegan, if the person eats meat, he and/or she will feel sick. The gut flora have died off. But this isn't an indication that meat is bad; you'll get the same thing if you just eat meat for a month or so and then try some vegetables. It's also the most common reason that people who visit Mexico get the green apple quickstep. Not Montezuma's Revenge, which everybody in Mexico knows about so they use boiled or purified water, and in any event you can get a remedy at any drug store. It's because they're not used to the kinds of fruits and tubers used often in Mexico. If you all of a sudden start eating a lot of fruit, the relative amounts of flora are going to have to change.

There is a sort of selection going on. The flora that cause so much damage that they kill the host don't spread as well, because their primary vector is just in the ground somewhere or got cremated. So we're left with stuff that doesn't cause a lot of damage. Similarly, we've evolved to live off the stuff the flora excrete.

Godless Wonder

Hmm, well, just and observation, but babies' digestive systems don't appear to work particularly well, right out of the chute...

chapka

Thanks for the responses--very interesting. One question:

Is this what happened in the case of peptic ulcers a few years back, when they found out there was a microbiological cause? Or is that a different phenomenon? If so, I've heard that some people think that some cancers can be caused in the same way. Is there any real evidence for this?

Roland98

Similar. We're finding that a lot of "chronic" diseases are actually caused by microbes. What you're thinking of is Helicobacter pylori, which is the cause of most peptic ulcers. Link to story on the guy (Barry Marshall) who pushed that hypothesis--one of the great stories in recent scientific history.

Absolutely. In fact, Helicobacter has been implicated in stomach cancer. One reason it's thought to be a factor is simply because it causes a chronic inflammation. The other factor involved is diet. For example, the rate of stomach cancer is very high in Japan. One thing they eat that we don't (in such high rates, anyway) is smoked food, which is high in carcinogens. If you have chronic inflammation and open wounds in your stomach due to Helicobacter, this allows for even more damage due to the carcinogens present in the food you eat. After years of this, the end result can be cancer. So Helicobacter isn't exactly the cause by itself, but it's one factor in the development.

Infectious agents have been implicated in a lot of chronic conditions, actually; Streptococcus pyogenes is known to cause rheumatic fever and heart disease, for instance, but is also thought to be involved in some types of tic disorders, and possibly Tourette's syndrome as well; Chlamydia pneumoniae has been linked to heart disease; multiple sclerosis has been linked to several viral and bacterial infections, just for starters. And of course, viruses have been known to cause cancers for over a century now.

A good book on the chronic/infectious disease overlap is Paul Ewald's "Plague Time," by the way. I have a few quibbles with a few things in there, but overall, a good introduction.

Prester John

Getting born is a pretty good way to start off your collection of bacteria, the vaginal tract is not sterile, it contains a very nice collection of bacteria, including some anaerobic flora. Its also situated very close to the anus, another site with a large collection of bacteria.
As previously stated it takes a little while for your flora to settle down and you can be susceptable to opportunist infections (eg Group B streptococci).

Even if you are born by section, the environment to literally loaded with bacteria, as is every person you come into contact with so you will soon acquire the neccessary bacteria.

Heres my 2 fave factoids, firstly you have approx 100 Trillion bacteria in and on you, thats a magnitude greater than the number of cells that you have. Secondly your gut flora is predominantly anaerobic flora (which is quite logical really) and E.coli is a relativly minor player, although it is easy to isolate.

premjan

could we say that all multicellular organisms are really bacterial symbionts?

Amit Misra

Though the gut is quite large, we must also remember that there are several other non-sterile areas of the body where microorganisms congregate in large numbers: the mouth, nose, lungs, and the epithelia of the reproductive organs.
Which brings me to confess that I'm quite intrigued by the microbial flora in the vagina, and how they never seem to get past the cervix and into the uterus despite such large numbers just inches away. What's holding them back? Spermatozoa, much larger than most bacteria and molds, get past the cervix, don't they?
And, Premjan, the good word isn't 'symbiont': its 'commensal'