Microbes existing harmlessly (even beneficially) in one part of the body, elsewhere painful/deadly?

I remember reading Bill Bryson’s A Short History of Nearly Everything, and there’s one section where he talks about bacteria and infection, noting that some bacteria are harmless if only in the throat, but if they travel elsewhere, they can cause terrible diseases. A part of that chapter is available on the Google books, but some of the pages are removed.

Bryson mentions Necrotizing fasciitis, but curious of other examples of where microbes living happily in say the colon would–perhaps after a car accident–cause serious disease if it moved to the throat or stomach. I’m particularly interested in microbes that are beneficial in one area but detrimental elsewhere.

More or less any of the benign or unharmful bacteria that reside in your colon would give you some serious trouble if they escaped to, e.g. your abdominal cavity. Why do you think a ruptured appendix is a serious condition unless treated promptly? It’s because the intestinal bacteria cause peritonitis.

ETA: And E. coli is found in almost all higher organisms’ colon. Ever heard of the E. coli strain O157:H7?

No I haven’t heard of that strain, but I did think of E. coli because I know that much of the contamination we hear about comes from fecal matter, say, on a leaf of spinach after it had been tracked through by a pig or other animal. In humans, does E. coli exist already in our colon, and when we get sick is it an instance of it being transferred into our mouth/throat/stomach?

Yes, E. coli is the perfect example of what you’re looking for. It exists in all humans old enough to eat solids; I’m not sure exactly when infants are naturally inoculated with it. It’s more than harmless, it’s beneficial, creating much of the Vitamin B our bodies need. As long as it stays in the large intestine, it doesn’t make us sick. But if it gets into the stomach through food, it makes us sick with nausea, vomiting, diarrhea, etc. and may even kill us.

I suspect the other one (okay in the throat, causing necrotizing fasciitis elsewhere), was probably one of the staph or strep bacteria. Both of them can live quite happily on mucus membranes and skin, and they don’t cause illness as long as they stay on top, or in the very top layer of the skin. If they get below that, though, then they cause infection, swelling, pus, all the fun stuff. MRSA is one strain that does this. The ones that are most antibiotic resistant, or that hit people with circulatory or immune problems, like diabetics, can be very difficult to get rid of. If they get down into the layer of the fascia, they can cause necrotizing fasciitis there.

Staph and strep aren’t, as far as I know, directly helpful to humans the way E. Coli is, but their presence can be indirectly helpful - they take up space and food that prevents other more harmful bacteria from taking up residence.

A bit of a hijack: I had a very serious bout of bacterial pneumonia this time last year. One of the offenders was E coli. How the hell did that get in my lungs?:confused:

Bacteria are tiny. Even though only some of them are considered “airborne”, the fact is that any of them can float along in air currents for a short period of time before settling down on countertops and floors. Best guess? Dirty toilet bowl, fresh poop, flush, breathe in the bathroom air… and now there’s an e coli in your lung.

The real/main problem with E coli is getting someone else’s into your digestive system.

Maybe. But, by and large, bacterial infections (aside from those involving the bowel), and especially bacterial pneumonia, comes from one’s own ‘flora’. That is to say, in your case, at some point, your own E. coli took up residence in your mouth (their movement from the bowel to your mouth perhaps as a result of you being run down or debilitated). Once in your mouth (or more accurately in your oropharynx), they were likely inhaled (i.e. aspirated) into your lungs where they subsequently caused the pneumonia.

In the scenario above, I am referring to micro-aspiration which means the aspiration of minute, essentially microscopic amounts of stuff from your oropharynx. This is similar, but quantitatively different from macro-aspiration where one aspirates large, manifest quantities of oropharyngeal or gastric contents (such as a chunk of potato or a few hundred cc’s of vomit). Macro-aspiration can lead to sudden death and is a common cause of such among those with depressed consciousness such as drug users (e.g. Hendrix). OTOH, micro-aspiration, the presumed mechanism for many run-of-the-mill pneumonias (as described above), can kill, but takes some time to do so (to develop the pneumonia).

Regarding micro-aspiration and its putative role in the pathogenesis of bacterial pneumonia, you can see that the particular germ causing someone’s pneumonia will depend on the particular germs that are living in their oropharynx. The latter, in turn, depends on things such as age, “debility”, nutritional status, prior antibiotic use, etc.

Whether it is yours or someone else’s, it is still E. Coli.