From watching many documentaries on micro-organisms, I can tell water typically contains quite a lot of them.

But what about rainwater? (before it hits the ground). I know nothing about any micro-organisms that live in the sky or clouds. Does rainwater contain fewer, and less dangerous, micro-organisms?

  • $\begingroup$ If my response answers your question, please accept it. Otherwise, please leave a comment about how I could further answer your question. Thanks. $\endgroup$ Mar 6, 2017 at 23:36
  • $\begingroup$ @theforestecologist It's almost a complete answer. Your last link says 80,000 bacteria per milliliter of riverwater. But I could not find a number per mL for rainwater (rain before it hits the ground). $\endgroup$
    – DrZ214
    Mar 7, 2017 at 4:20
  • $\begingroup$ Found a study for you and have updated my answer. Please accept if all is ok. $\endgroup$ Mar 15, 2017 at 18:59
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    $\begingroup$ Who voted to close this as "homework??" 1. This question was not easy to answer, so it is not trivial (i.e., even after an initial search, very little reputable sources can be found). 2. it's already well-received with a well-received answer. I vote to keep open. $\endgroup$ May 11, 2017 at 19:16
  • $\begingroup$ There seems to be a basic problem with the question, which is that not all rivers are alike. Indeed, I would think that the number of microorganisms varies greatly within a single river, e.g. the Mississippi/Missouri near its headwaters, and just downstream of New Orleans. $\endgroup$
    – jamesqf
    Oct 15, 2017 at 5:40

1 Answer 1


According to a number of citations listed on Kenyon College's MicrobeWiki, rain can contain microorganisms via a process called "bioprecipitation."


  • Essentially, microorganisms, dust and other small particles get swept up into the atmosphere, and cold temperatures cause atmospheric water vapor to freeze around the organism/particle. Once the ice-covered particle picks up enough mass, it falls from the sky and reaches earth as precipitation (likely rain if temperature is warm enough).

Here is a schematic from Wright et al. (2104) of how an increase in relative humidity, due to a cold-frontal passage, can trigger biological ice nuclei release followed by seeding of the frontal cloud band with the lofted ice nuclei:

Wright et al. (2104) schemaic

So How Many (and Which) Bacteria Are in Rain?

Which Bacteria?

According to Wikipedia:

The ice-nucleating bacteria currently known are mostly plant pathogens

  • According to MicrobeWiki, these include:

    Exserohilum turcicum, Pseudomonas viridiflava, Pseudomonas fluorescens, Pantoea agglomerans and Xanthomonas campestris.

    The most well described organism that demonstrates ice nucleation is Pseudomonas syringae, which was determined to specifically supply a source of ice nucleators by Leroy Maki in the 1970’s.

According to work by Natasha DeLeon-Rodriguez, Athanasios Nenes, et al. (full paper here and NPR story here), 100s of species of bacteria exist in the clouds and it's not uncommon to find almost 20 species in any given sample (60% of which on average were still living).

  • Here's a graph showing the relative amounts of various taxa found in their samples:

    DeLeon-Rodriguez et al's (2013) relative taxa abundances

How Many Bacteria?

DeLeon-Rodriguez et al's (2013) results revealed an average bacterial concentration of somewhere between $5.1 × 10^3 cells / m^3$ to $1.5 × 10^5 cells / m^3$ of cloud (depending on method of counting).

  • Quickly doing some math (1 m$^3$ = 1000000 mL), their results suggest that, on average across a cubic meter of cloud, there is <1 bacteria / mL of moisture.

According to research by Joly et al (2014) that specifically examined ice nuclei (or "IN"):

there were 0 to ~220 biological IN/mL of cloud water (i.e. 0-22 per m$^3$ of cloudy air based on cloud liquid water content estimates) and these represented 65% to 100% of the total IN.

Comparatively, river water (and drinking water) can contain a lot more bacteria (i.e., 80,000 bacteria / mL) and other microorganisms.

However, this disparity is not always so great, especially if we focus at comparing counts of only one or a few species (and not total bacterial counts).

  • Based on research of a tropical reservoir by Kaushik et al (2014):

    Levels of E. coli were found to be in the range of 0 CFU/100 mL – 75 CFU/100 mL for the rainwater, and were 10–94 CFU/100 mL for the reservoir water.

    • Note: CFU = colony forming units. (estimation of microbial numbers by CFU will, in most cases, undercount the number of living cells. [wikipedia]).


  • DeLeon-Rodriguez, N., Lathem, T. L., Rodriguez-R, L. M., Barazesh, J. M., Anderson, B. E., Beyersdorf, A. J., ... & Konstantinidis, K. T. (2013). Microbiome of the upper troposphere: species composition and prevalence, effects of tropical storms, and atmospheric implications. Proceedings of the National Academy of Sciences 110(7), 2575-2580.

  • Joly, M., Amato, P., Deguillaume, L., Monier, M., Hoose, C., and Delort, A. (2014). Direct Quantification of Total and Biological Ice Nuclei in Cloud Water. Atmos. Chem. Phys. Discuss 14(3):3707–3731.

  • Kaushik R, Balasubramanian R, Dunstan H (2014) Microbial Quality and Phylogenetic Diversity of Fresh Rainwater and Tropical Freshwater Reservoir. PLoS ONE 9(6): e100737. doi:10.1371/journal.pone.0100737

  • Wright, T. P., Hader, J. D., McMeeking, G. R., & Petters, M. D. (2014). High relative humidity as a trigger for widespread release of ice nuclei. Aerosol Science and Technology 48(11), i-v.

  • For further reading, also see the associated citations from MicrobeWiki.

  • I think you would also benefit by perusing the works of Pierre Amato.

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    $\begingroup$ A fairly recent study examined this as well: Microbiome of the upper troposphere: Species composition and prevalence, effects of tropical storms, and atmospheric implications $\endgroup$
    – jzx
    Mar 1, 2017 at 8:50
  • $\begingroup$ You cite two wildly disparate figures for bacteria in river/drinking water, the latter of which seems to be relatively close to the number in rain water. Is one more trustworthy than the other? $\endgroup$
    – canadianer
    Mar 15, 2017 at 20:02
  • $\begingroup$ Two things to note: 1. One source presents "bacteria / mL" while the other is CFU (colony forming units) -- estimation of microbial numbers by CFU will, in most cases, undercount the number of living cells [wikipedia]. 2. Regardless, the Kaushik study looks at ONLY E coli in a reservoir (where enterobacteria such as E coli are typically managed to stay very low). I guess I could edit the answer to better contextualize all of this... $\endgroup$ Mar 15, 2017 at 20:29
  • $\begingroup$ One source is talking about only E. Coli, while another source says 80k bacteria (any bacteria). They are not really comparing apples to apples. BTW in looking at the new link, I saw this: "Both rainwater and reservoir water samples were collected from the sampling locations at regular weekly intervals and within 24 h of rainfall events." This brings up a good question, namely how far apart rainfall events should be to avoid contaminating riverwater with rainwater. $\endgroup$
    – DrZ214
    Mar 15, 2017 at 20:35
  • $\begingroup$ @DrZ214 I've already addressed that in the above comment and in my answer edits. Your "This brings up a good question..." statement should be asked as a new question and is not relevant here. $\endgroup$ Mar 15, 2017 at 21:18

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