Understanding How the Immune System Recognizes Bacteria

[Phobos]

How does the immune system distinguish between beneficial bacteria (e.g., those that aid in digestion) vs. harmful bacteria, particularly when they can be one in the same (e.g., E. coli)? Or perhaps our bodies fight all bacteria regardless of their status and it's just that the sheer numbers of bacteria in our digestive tract ensures their continued presence?

 

[matthyaouw]

Very good question. The immune system identifies organisms by their antigens (proteins or glycoproteins sticking out of their membrane, or in the case of a virus, their outer capsule). I forget whether this is true of all cells and antibodies, but many posess the receptors for only one type of antigen, which will usually be unique to a specific pathogen/strain. I would assume that natural selection would remove any individual with an immune system that destroys it's own benificial bacteria from the gene pool, leaving a population that simply lack immune cells able to attach to them.

I'm not sure what active/selective immune responses take place outside the blood, tissue, and lymphatic systems though, so I don't know how relevant this response is. It may be simply that in most situations, these bacteria don't come into contact with immune cells.

 

[Phobos]

Thanks.

So perhaps we have a weak response against something like E. coli, which allows that species to do well in our gut but also makes it all the more dangerous when it enters elsewhere into our body? The weak response may still be sufficient in something like the blood stream where there would be fewer bacteria.

(again, I'm just speculating here...)

 

[Intuitive]

How does the immune system distinguish between beneficial bacteria (e.g., those that aid in digestion) vs. harmful bacteria, particularly when they can be one in the same (e.g., E. coli)? Or perhaps our bodies fight all bacteria regardless of their status and it's just that the sheer numbers of bacteria in our digestive tract ensures their continued presence?

I believe that Antibiotics actually cut off the food supply of the Bacteria by shutting down certain protiens in the body temperarely until the bacteria die of starvation.

I don't think the body itself actually has any immunity to any Bacteria at all.

Good bacteria can become bad bacteria if it gets deep into the blood stream.

 

[Monique]

First line of defense in the immune system: physical barrier. The digestive tract is technically the outside of your body, so the bacteria are tolerated there. Once the bacteria start invading your body, they fully encounter the immune system. Bacteria contain certain 'danger' signals that the body recognizes and which are needed to elicit an immune response (toll-like receptors are responsible for recognizing pathogen-associated molecular patterns).

There are many different strains of E.coli, the one that resides in your gut is non-pathogenic. There are also pathogenic strains of E.coli that you better not encounter.

 

[matthyaouw]

How does the body fight against pathogenitic e-coli in the gut, if it is outside of our immune system?

 

[Monique]

Some strains are invasive, so the immune system does encounter them and starts an inflammation reaction. If the strain is non-invase, you don't get an inflammation reaction. You then still get sick because of the toxins produced by the bacteria. I'm not sure how the body fights such an infection (it could sense the toxins and set up a response).
Some different E.coli strains and their characteristics:
ETEC
fimbrial adhesins e.g. CFA I, CFAII, K88. K99
non invasive
produce LT and/or ST toxin
watery diarrhea in infants and travelers; no inflammation, no fever
EIEC
nonfimbrial adhesins, possibly outer membrane protein
invasive (penetrate and multiply within epithelial cells)
does not produce shiga toxin
dysentery-like diarrhea (mucous, blood), severe inflammation, fever
EPEC
non fimbrial adhesin (intimin)
moderately invasive (not as invasive as Shigella or EIEC)
does not produce LT or ST; some reports of shiga-like toxin
usually infantile diarrhea; watery diarrhea similar to ETEC, some inflammation, no fever; symptoms probably result mainly from invasion rather than toxigenesis
EAggEC
adhesins not characterized
non invasive
produce ST-like toxin (EAST) and a hemolysin
persistent diarrhea in young children without inflammation, no fever
EHEC
adhesins not characterized, probably fimbriae
moderately invasive
does not produce LT or ST but does produce shiga toxin
pediatric diarrhea, copious bloody discharge (hemorrhagic colitis), intense inflammatory response, may be complicated by hemolytic uremia.
from http://textbookofbacteriology.net/e.coli.html

 

[Moonbear]

Just to help clarify or elaborate on what Monique is saying, if they stay in your gut, your body won't really do anything...they'd either be digested, hang out and do whatever they do, or get passed out the other end. If they are invasive, that means they start attacking the cells lining your gut; they don't just hang out feasting on the food you eat, but instead start invading your body. So, it is their behavior of beginning to invade your cells and penetrate your body beyond your gut that triggers the immune response.

 

[Phobos]

Just to help clarify or elaborate on what Monique is saying, if they stay in your gut, your body won't really do anything...they'd either be digested, hang out and do whatever they do, or get passed out the other end. If they are invasive, that means they start attacking the cells lining your gut; they don't just hang out feasting on the food you eat, but instead start invading your body. So, it is their behavior of beginning to invade your cells and penetrate your body beyond your gut that triggers the immune response.

Is that the case for all "stomach bugs"? (including bacteria in spoiled food?)

In other words, do bacteria have complete free reign in our digestive tract up to the point where their wastes become toxic?

 

[Phobos]

The digestive tract is technically the outside of your body,

Interesting...I've never heard it described that way before.

There are many different strains of E.coli, the one that resides in your gut is non-pathogenic.

But fecal coliforms are hazardous outside the digestive tract (which is why "employees must wash hands" signs abound everywhere). Or perhaps the pathogenic bacteria stay dormant in the digestive tract until they find another way into the body?

 

[Moonbear]

Is that the case for all "stomach bugs"? (including bacteria in spoiled food?)
In other words, do bacteria have complete free reign in our digestive tract up to the point where their wastes become toxic?

A lot of them simply succumb to the low pH in the stomach. It's not just when their wastes become toxic, but when they begin to invade cells. If they get past the epithelial cells lining the digestive tract, then the immune/inflammatory response is triggered by the antigens on their cell surface. It's akin to bacteria on your skin. They don't do anything unless they break through that barrier. Triggering an immune response to every bacteria on our skin or just passing through our gut would be more detrimental than helpful to us.

 

[Monique]

Interesting...I've never heard it described that way before.
But fecal coliforms are hazardous outside the digestive tract (which is why "employees must wash hands" signs abound everywhere). Or perhaps the pathogenic bacteria stay dormant in the digestive tract until they find another way into the body?

You don't want to start spreading bacteria that are adapted to living inside your body to food items where they can multiply and reach high numbers, think for instance the ice-cream man. In your gut you have a combination of 'good and bad' bacteria. In general the 'bad' bacteria are kept in check by the massive amount of 'good' bacteria, so everything is in equilibrium. Now if you start ingesting contaminated food, this balance is thrown off and the 'bad' bacteria start to overgrow and cause illness. I describe them as 'good' and 'bad' since I'm not a bacteriologist, but you get the point.

Also, viral Hepatitis A is common in the US, which is spread by feces. Hepatitis E is also spread in this way, but is not common in the US (but is in other countries). So wash your hands

 

[quetzalcoatl9]

I believe that Antibiotics actually cut off the food supply of the Bacteria by shutting down certain protiens in the body temperarely until the bacteria die of starvation.

that depends. most antibiotics actually work by ribosomal inhibition, thus preventing or disrupting protein synthesis. some, such as penicillin, inhibit enzymes responsible for synthesizing the cell wall.

I don't think the body itself actually has any immunity to any Bacteria at all.

this is clearly not true