We hear all the time about how bacteria are becoming resistant to various antibiotics, threatening to evolve into superbacteria against which we’ll have no defense.
Do bacteria also evolve to counter the human immune system? Why don’t we have other superbugs just by virtue of our immune systems not killing every last one of them? Didn’t this sort of this happen even before the invention of antibiotics?
I believe most of the action of the human immune system is physical – the (large) white blood cell surrounds the bacterial cell and crushes it, or breaks it into pieces. It’s really hard to evolve a valid resistance method to that.
You could reasonably argue that some of those bugs that cause us significant problems are already outwitting our immune system to some extent. But it is messier.
Our immune system is significantly different to antibiotics. Antibiotics typically disrupt a specific chemical pathway in the bacterium. If the bacteria can find a backup pathway, it becomes unaffected by the antibiotic. It is a highly specific game. A mutation to bridge such a pathway is not too hard to get, and many bacteria will have a variety of pathways. The immune system essentially blasts the bacterium where it stands. The only ways a bacterial infection succeeds against this is to either outrun the immune system by growing faster than the immune system can kill it, or by hiding from it. Infections that secrete themselves in abscesses or other little hideaways, or colonise parts of the body that are not well supplied with blood.
However there is a counter evolutionary force that also shapes the bacteria. A highly aggressive infection that slays the host too quickly is selected against - as it can’t spread as well as infections that allow the host to remain mobile and infectious for longer. The steady state is for the bacteria to evolve to be sub-lethal, or at least not quickly lethal.
Poster child for such a bacteria would be tuberculosis. It kills, but slowly, and it is able to secrete itself in the body in all manner of protective enclaves where it proceeds to pursue its destructive ways.
One of the main ways the immune system works is by using antibodies, which attach to bacteria (viruses and other thighs too) and mark them for destruction by other immune cells. In order to attach to the bacteria, the antibody has to have a complementary shape to some small part on the outside of the bacteria. So the immune system is constantly churning out antibodies with random shapes. If one of those shapes “hits” (attaches), there’s a feedback to the cell that made that shape telling it to make more, much more, antibodies of that shape, and that shape gets added to the permanent repertoire. Undoubtedly (I would think) it must sometimes happen that a species of bacteria becomes resistant to one of the antibodies against it, by, for example, a mutation that changes the shape of whatever small part of the bacteria the antibody was attachi
(continued) …ng to. But there are many antibodies that would be directed against that bacteria, each one complementary to a different small part, and as said new antibodies are always being made, so as long as the immune system keeps working sufficiently, the bacteria never becomes effectively resistant. Infection happen due to the reasons mentioned upthread.
Nah, it’s not physical. And bacteria can get around it.
Different cells in the immune system can attack bacteria differently. They can either release different chemicals or send different signals to destroy the bacteria. But, bacteria can still find a way around it, and different types of bacteria have different ways of doing it.
I can give you the classic example of tuberculosis (Mycobacterium tuberculosis, TB). There is a white blood cell called a monocyte/macrophage. These cells destroy bacteria (and everything else) by “eating it”, then digesting it with enzymes. Seems great, and it is an awesome way to kill bacteria… Except that TB found a way to, once it is eaten, disrupt the digestion process. So the bacteria remains cocooned inside the living macrophage, replicating happily, but the macrophage is unable to finish them off because they’ve stopped the digestion.
Other bacteria hide inside other type of cells, and that way they avoid being identified by the immune system surveillance.
Our bodies are, of course, overrun with bacteria, trillions of the little buggers, and without it we wouldn’t survive. Bacteria in the digestive system also supply us with needed vitamins like biotin and vitamin K, and are our primary source for some of these nutrients. On the outside it is our first line of defence against harmful invaders.
The widespread advertising, and consequent use, of antibac products, is probably doing more harm than good.
Viruses (sort of) do this all the time. That’s why we keep having to update the annual flue vaccine mix each year. Bacteria are more complex and do a better job at this.
But there is no “purpose” or any such thing to mutate to overcome our defenses. Evolution doesn’t work like that.
They just keep mutating because that is what life does. Some mutations don’t overcome our resistance, some do. So those that mutations that do help to overcome our resistance are just quirks of nature which end up helping the bugs spread so they are kept.
And of course, at the same time our immune systems are evolving to better deal with the invaders.
So can viruses, such as the herpes simplex, which hides inside a ganglium.
Or rabies, which travels through neurons. But I was thinking of the specific question in the OP, against bacteria specifically.