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I came across a Wikipedia article titled "Urine diversion". It explains different ways, including using special diversionary toilets, and treating the collection of human feces and urine separately as opposed to just flushing everything down the toilet, maybe something most people in well developed countries are used to. These sentences caught my attention:

Separate treatment of the two types of waste is justified since urine is nearly sterile and low in pathogens, provided an individual is healthy.[3] This means that urine can be readily utilized as a fertilizer or discharged with less risk to community.[4]

Human feces, on the other hand are high in pathogens, including up to 120 viruses and need to be treated well before it can be safely used in agriculture.
Urine diversion - principle (Wikipedia article)

So urine is nearly sterile and low in pathogens, however it qualifies that statement with the condition that the person producing that urine is healthy. I assume when they compare this to feces they do so with all other things being equal (ie., provided an individual is healthy).

It claims that there may be up to 120 viruses in feces, as opposed to urine which it characterizes as being relatively low risk. I assume that feces are riskier in transmission of diseases because it naturally contains more pathogens, but it doesn't explain what these viruses or pathogens are. I was wondering if someone could elaborate on the risks of feces over urine, and also, notwithstanding that it says that urine is relatively low-risk, what health issues can be caused by reusing urine.

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  • $\begingroup$ I like your question, but it seems a bit unfocused — I've made a suggested edit to the title that I hope better reflects the core of what you were asking. If you approve of that change, I suggest editing the body to focus on that particular question. I would then separate out the other question (about risks) into a new question. $\endgroup$ – tyersome Jul 28 at 18:50
  • $\begingroup$ @tyersome Thanks for the suggested edit. I edited the title and made the question about 50 words shorter. Hopefully it's more focused and clearer. Feel free to make any improvements/edits. $\endgroup$ – Zebrafish Jul 29 at 7:35
  • $\begingroup$ You're welcome — that is much better! 😊 $\endgroup$ – tyersome Jul 29 at 23:18
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Many gut microbes are pathobionts or opportunistic pathogens — these organisms can be part of the normal gut flora (microbiome) of healthy individuals, but under the right (for them) circumstances (e.g.s: when introduced into the bloodstream by a cut, or due to immunosuppression) they can cause disease.1,2

As for urine, there are many fewer microbes present (>400 species and 109 to 1011 bacteria/g of contents in the large intestine vs. <100 species and on the order of 105 bacteria per mL of urine).3-5 Note, however, it is still far from sterile!5-7

You might also find the following of interest: https://www.who.int/water_sanitation_health/dwq/iwachap5.pdf


References:

1: Hornef, M. (2015). Pathogens, commensal symbionts, and pathobionts: discovery and functional effects on the host. ILAR journal, 56(2), 159-162.

2: Price, L. B., Hungate, B. A., Koch, B. J., Davis, G. S., & Liu, C. M. (2017). Colonizing opportunistic pathogens (COPs): the beasts in all of us. PLoS pathogens, 13(8), e1006369.

3: Davis, C. P. (1996). Normal flora. In Medical Microbiology. 4th edition. University of Texas Medical Branch at Galveston.

4: Barnett, B. J., & Stephens, D. S. (1997). Urinary tract infection: an overview. The American journal of the medical sciences, 314(4), 245-249.

5: Lewis, D. A., Brown, R., Williams, J., White, P., Jacobson, S. K., Marchesi, J., & Drake, M. J. (2013). The human urinary microbiome; bacterial DNA in voided urine of asymptomatic adults. Frontiers in cellular and infection microbiology, 3, 41.

6: Hilt, E. E., McKinley, K., Pearce, M. M., Rosenfeld, A. B., Zilliox, M. J., Mueller, E. R., ... & Schreckenberger, P. C. (2014). Urine is not sterile: use of enhanced urine culture techniques to detect resident bacterial flora in the adult female bladder. Journal of clinical microbiology, 52(3), 871-876.

7: Whiteside, S. A., Razvi, H., Dave, S., Reid, G., & Burton, J. P. (2015). The microbiome of the urinary tract—a role beyond infection. Nature Reviews Urology, 12(2), 81.

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  • $\begingroup$ It's not sterile, but it has far fewer microorganisms than nearly any other bodily fluid. $\endgroup$ – forest Aug 22 at 0:54
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Urine is contaminated with skin bacteria and possibly something excreted from your blood. A very tiny fraction of urine is microbial.

Feces is basically a solid mass of undigested food, gut bacteria, and their byproducts. Many gut bacteria in particular have evolved in the conditions the human body produces so many can survive well in human tissue. about 30% of the solid weight of feces is bacteria or bacterial byproducts.

All things being equal, feces contains orders of magnitude more bacteria than urine by unit mass.

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The kidney is essentially a filter which extracts dissolved waste products & water from the blood. It basically allows only molecular-sized things to pass, otherwise it would continually leak blood cells.

The digestive tract OTOH is basically a tube. Stuff gets mashed up at one end and ejected at the other. Even fairly large objects can pass through unaltered. So any bacteria that enter the tract can (if they can survive stomach acids &c) pass through the tract. Moreover, many find the intestines a warm & comfortable place to live and reproduce. Once the population has grown, some of it will be ejected with each bowel movement. While most of the bacteria are harmless or even beneficial (see "gut microbiome" for more info), some, like some varieties of E. coli or the cholera bacterium, can cause serious illness.

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  • $\begingroup$ Thanks, this addresses my query about the difference between feces and urine. I was just wondering, since you said that E. Coli or the cholera bacterium can be found in stool, does this mean that the person who passed the stool is infected with these bacteria? If that's the case, are we still talking about a "healthy individual"? $\endgroup$ – Zebrafish Jul 30 at 7:12
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I can only speak to what I am familiar with, but I would assume that the large majority of those viruses are bacteriophages. These bacteriophages, or "phages" for short, are viruses that infect bacteria. I should mention that these phages are not human pathogens though. When we go out into the field to discover novel phages, we often go to wastewater treatment facilities to get a sample of raw sewage. These samples contain billions of phages, and after some treatment, you can use the sample and a stock culture of bacteria to propagate that phage. The pathogens are also most likely opportunistic pathogens, like staphylococcus epidermidis. These are bacteria that are a component of the normal human microbiome but may become pathogens in the right circumstances. You could also find clostridium bacteria in human feces and salmonella among many others. Also, if you are infected with an infectious agent, bodily fluids and stool will be a easy place to find them.

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