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I am a computer science student and I'm interested in algorithmic aspects of cancer! Once I heard that there exist more bacteria in human body than our own cells, I wondered that why bacteria, which divide faster than cell bodies, do not cause something like cancer in body? Isn't it evolutionary more probable for bacteria to gain aggressive properties and invade human tissues? Or maybe is it the case that bacteria really produce some lethal colonies like cancers in the body?

One difficulty with cancers is that cancer is a very complex disease because of it's intra-tumor and inter-tumor heterogeneity. Isn't it more probable for bacteria to cause extremely heterogenous infections with regard to existing simple evolutionary models which are used to describe cancer?

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  • $\begingroup$ Most of the bacteria you refer to live in the gut, and have evolved to coexist, not invade. Pathogenic bacteria are aggressive, and have evolved to invade human tissues, but our immune systems keep them in check. You would not call them "cancers" though. $\endgroup$
    – Karl Kjer
    Commented Dec 13, 2017 at 13:19
  • $\begingroup$ @KarlKjer shouldn't it be that the pathogenic bacteria have not yet evolved to coexist with humans? $\endgroup$
    – Polisetty
    Commented Dec 13, 2017 at 14:39
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    $\begingroup$ @Polisetty it's a common misconception that pathogens evolve toward "coexistence". Pathogens are selected for transmission, and while low virulence can sometimes help transmission, so can high virulence. So saying "not yet" implies an inevitability that isn't real. $\endgroup$
    – iayork
    Commented Dec 13, 2017 at 16:10
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    $\begingroup$ It's meaningless to suggest a bacterial tumor; tumors are by definition from the same organism as the host. If you have an overgrowth of bacteria you would just call that an infection, there's no need to reuse the term tumor in that context because an entire separate vocabulary exists to describe infections. $\endgroup$
    – Bryan Krause ♦
    Commented Dec 13, 2017 at 19:17
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    $\begingroup$ @CharlesE.Grant You are right but the question is really that what is inherently different? Why something like anti-bacterial drugs do not exist for cancer! As far as I know one of the popular answers to this question is tumor heterogeneity and that cancer is not a single disease! The question is why it is not the case in the bacteria scenario! $\endgroup$
    – Dandelion
    Commented Dec 13, 2017 at 20:33

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Cancer is not inherently more lethal than bacterial infections. That's simply an artifact of the time we live in. Broad spectrum anti-bacterial drugs we invented in the 1st half of the twentieth century. Before that, most folks died of infections. Cancer was relatively rare because most folks didn't live long enough for cancers to become apparent.

Why do we have generic drugs for bacteria and not for cancer? Largely because bacteria are not eukaryotes and we are. The biology of bacteria and human cells are so different that it's relatively easy to find drugs that dramatically affect bacteria, but not people. I say relatively easy, but still took several thousand years of human history to figure it out.

Because cancer cells are just variations on our normal cells, it's very hard to find something that kills the cancer cells, but not the rest of us. That's why chemotherapy makes folks so sick: the chemotherapy agent is killing, or significantly impairing, fast growing healthy cells along with the cancer cells. The most successful cancer drugs have identified rare metabolic pathways that are important to specific types of cancer, but not so important to healthy cells.

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  • $\begingroup$ Thank you but I think that extremely huge number of bacteria living in our body still seems unbelievably harmless! Almost all infections -as far as I know and of course I know just very little molecular biology and almost nothing else in the field of biology and bacteriology- have external sources!!! $\endgroup$
    – Dandelion
    Commented Dec 14, 2017 at 3:05
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    $\begingroup$ All the bacteria in our bodies, not just the infections, come from external sources. You start collecting them as you path through the birth canal and never stop. As to being "unbelievably harmless": there are lots of bacteria that are perfectly harmless when they are living in your gut, but potentially lethal if they start growing in your blood, your lungs, or your urinary tract. It's worth keeping in mind though that cooperation (symbiosis) can be a powerful evolutionary strategy. $\endgroup$
    – Charles E. Grant
    Commented Dec 14, 2017 at 4:04
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Charle's E. Grant's answer, that bacteria are typically different from us and hence easier to kill, is most of the answer. Remember that this applies not only to drugs but to our immune system, which can have difficulty detecting cancers (while catching most of them) but is excellent at detecting foreign organisms like viruses or bacteria.

You do still ask a valid question however - shouldn't the enormous number of replicating cells in our gut mean that some bacteria occasionally arise which break the co-operation between bacterium and host, and cause problems (albeit problems mainly limited to the gut) ?

The answer seems to be - yes. We're only beginning to understand how people's microbiomes work, but it seems likely that they can be dysfunctional and this causes health problems. Sometimes this involves a single strain of bacterium overwhelming all others - this is in some ways analogous to a tumor.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4315779/

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Cancer cells are technically human cells and programming the immune system to recognize them can cause autoimmune problems. Some cancer mutations also fail to manifest on cell surfaces, making the cancer cells near impossible to medicate (without widespread friendly-fire on healthy cells) or immune-target.

Bacteria are plenty different enough that the immune system can target them with abandon and almost no risk of autoimmune problems. If they aren't resistant, you can shut them down with antibiotics (specifically designed against prokaryotes). A common approach includes interfering with bacterial ribosomes (which are fortunately different enough that the treatment doesn't significantly interfere with human ribosomes). If the antibiotic ever stops working, throw in some bacteriophages.

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