Bacteria, mostly harmless…

Not Guilty credited

I was doing a little research about the 5 second rule recently (more on that soon) and I came across a number that surprised me. According to this site, only 5% of bacteria are pathogenic. This number struck me as surprising for a few reasons.

First, I would have thought that figuring this out would have been very difficult because of the unculturable bacteria phenomena. If the going guesstimates are to be believed, more than 99% of bacteria can not be cultured in the laboratory. This is based on the number of bacteria that we can “see” in water or soil samples compared to the number that we can grow in the laboratory. We simply don’t know what many of these organisms require and it is likely that what they require is a mix of chemicals that they manage to get from one another. If we don’t put them in a vial with the right complicated compliment of other microbes they will simply not have what they need to sustain them. This is fascinating and if you want to learn more about the great uncultivable bacteria phenomenon you can start with a review article (here).

The second reason is related to the first: the Koch’s postulate problem. in 1884 Robert Koch invented a set of criteria in order to determine whether or not a microorganism can be said to be responsible for a disease:

1) The microorganism or other pathogen must be present in all cases of the disease.

2) The pathogen can be isolated from the diseased host and grown in pure culture.

3) The pathogen from the pure culture must cause the disease when inoculated into a healthy, susceptible laboratory animal.

4) The pathogen must be reisolated from the new host and shown to be the same as the originally inoculated pathogen. 

The non-culturable problem certainly puts a damper on that second criteria. To learn more about Koch and his postulates try wikipedia.


The three primary mechanisms of Horizontal Gene Transfer (HGT). Transformation (naked DNA taken up from environment), plasmid transfer and accidental bacteriophage mediated transfer.

The third reason is the Horizontal Gene Transfer issue (HGT). This is the exchange of genetic material between bacteria, even those that are not closely related. HGT takes place in nature at a rate that we can’t easily quantify and the genes that are transferred can make harmless bacteria more pathogenic.  So for this and other reasons, the number of organisms that are “pathogens” is very likely not fixed. There are bacteria that have clearly gone from being “mostly safe” to “mostly dangerous” after receiving genes from closely related bacteria that are pathogens.  Maybe this is pretty stable on the whole, but this all does seem to complicate coming up with a percentage of bacteria that are pathogens.I would actually guess that the percentage is a lot smaller than 5%. My guess would be more like 0.0005%.

In any case, what can we do with this information? Most bacteria, regardless of the percentage that you settle on are actually not pathogens. Many of them will look (as in my line up at the top of this post) like they ARE pathogens because particular cell shapes are associated with “bad guys”. Bacterium number 1 may be a nasty version of Streptococcus pneumoniae but it may also be Lactococcus lactis cremoris, used for cheese fermentation in some countries (like New Zealand).

You simply can’t judge a book by it’s cover. Perhaps one larger lesson to take away then is that if most bugs are good and we can’t tell the difference on first inspection maybe we should re-think our germophobic tendencies. Embrace the microbial world! You are 90% microbial, after all and that makes you mostly harmless too.

About drhhnz

Microbiology, Evolution and Bacteriophages. Lecturer, Institute of Natural and Mathematical Sciences. Massey University. Superhero name: Microbiology Girl. Auckland, New Zealand · Twitter: drhhnz
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1 Response to Bacteria, mostly harmless…

  1. Pingback: Five Lies You’ve Been Told About Germs

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