# Bacteria: how many colony-forming units is worrisome? **URL:** \ **Category:** Factual Questions\ **Created:** [November 29, 2016, 6:23pm UTC](https://boards.straightdope.com/t/bacteria-how-many-colony-forming-units-is-worrisome/773393 "2016-11-29T18:23:36Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [November 29, 2016, 6:23pm UTC](https://boards.straightdope.com/t/bacteria-how-many-colony-forming-units-is-worrisome/773393/1 "2016-11-29T18:23:36Z")
[This article](https://www.washingtonpost.com/news/wonk/wp/2015/09/08/the-dirtiest-places-on-an-airplane-ranked/) ranks the filth of various surfaces a passenger is likely to touch on a commercial airliner in terms of colony forming units per square inch. Predictably, they give no context as to how many CFUs is enough to think “I probably shouldn’t put my sandwich on that,” and how many CFUs is enough to think “I should take a bath in Betadine after touching that.” So what’s the scoop? Without considering the types of bacteria iinvolved, how many CFUs should be cause for concern on the surfaces I interact with? What should I never eat food off of, what should I wash my hands after touching, and what should I just never ever touch? ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [November 29, 2016, 7:08pm UTC](https://boards.straightdope.com/t/bacteria-how-many-colony-forming-units-is-worrisome/773393/2 "2016-11-29T19:08:00Z")
> [@Machine\_Elf](#): > > Without considering the types of bacteria iinvolved, how many CFUs should be cause for concern on the surfaces I interact with? What should I never eat food off of, what should I wash my hands after touching, and what should I just never ever touch? But you can’t say anything useful without considering the types of bacteria. You should never touch anything that has any amount at all of MRSA on it. On the other hand, it’s probably a net positive healthwise to not only touch but actually eat something with lactobacillus bacteria at 1 × 10^6 CFU per gram per milliliter [[cite](https://en.wikipedia.org/wiki/Yogurt) in second full paragraph] ---
**Author:** ![Kelevra](https://avatars.discourse-cdn.com/v4/letter/k/97f17d/32.png) [@Kelevra](https://boards.straightdope.com/u/Kelevra)\ **Post date:** [November 29, 2016, 7:10pm UTC](https://boards.straightdope.com/t/bacteria-how-many-colony-forming-units-is-worrisome/773393/3 "2016-11-29T19:10:08Z")
It’s not “surface” bacteria, but the standard for E. Coli for contact recreation on the Ohio River states “Content shall not exceed 130 CFU’s/100mL as a monthly geometric mean, based on not less than five samples per month, nor exceed 240 CFU’s/100mL in any sample.” The numbers for Fecal Coliform bacteria are 200 and 400. ---
**Author:** ![lazybratsche](https://avatars.discourse-cdn.com/v4/letter/l/ba8739/32.png) [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)\ **Post date:** [November 29, 2016, 8:29pm UTC](https://boards.straightdope.com/t/bacteria-how-many-colony-forming-units-is-worrisome/773393/4 "2016-11-29T20:29:04Z")
To reconcile the previous two comments, you need to realize that health standards for fecal coliform CFU are meant to be indicators of general contamination. The vast majority of those bacteria are completely harmless – you’re filled with trillions of fecal coliform bacteria _right now_. But there’s a small chance that one of those bacteria is pathogenic, and that chance is proportional to the total fecal coliform CFU on a surface or in a body of water. If a sewage treatment plant isn’t getting rid of the pedestrian bacteria present in all sewage, it’s also not getting rid of the _C. difficile_ that’s coming down the pipes from every hospital and nursing home… ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [November 29, 2016, 9:44pm UTC](https://boards.straightdope.com/t/bacteria-how-many-colony-forming-units-is-worrisome/773393/5 "2016-11-29T21:44:55Z")
I just clicked through to the article. Key line :“Thankfully, all the samples were negative for fecal coliforms like E. coli, which can make people fatally ill.” Now, that’s already exaggerating the risk, in that the overwhelming vast majority of \*E. coli \* strains won’t make anybody sick. And of those that do, it’s very rarely fatal. Again, as **lazybratsche** said, we only look for _E. coli_ because that indicates that something worse might be there. But the key point is that even _E. coli_ wasn’t found. So the ‘bacteria’ numbers don’t mean much of anything as far as disease risk goes. I mean, it’s probably not in general a good idea to go around licking every surface on an airplane-- you could well pick up a cold that way-- but it doesn’t mean that the tray table is ten times more disease-ridden than the toilet flush handle (which is maybe the only thing on that list that I’d want to wash after touching). ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [November 30, 2016, 1:40am UTC](https://boards.straightdope.com/t/bacteria-how-many-colony-forming-units-is-worrisome/773393/6 "2016-11-30T01:40:25Z")
Not to defend airplane cabin cleanliness, but without running the very same tests on the table at your local bar and grill, your favorite restaurant, the seat of a city bus, your home dining table, and the arms of your armchair we haven’t learned anything. People sometimes use some sort of bacterial detection system / spray / whatever to show how “scary dirty” your home is. I take exactly the opposite lesson from exactly the same data. The fact nobody in decades has gotten ill eating food from my kitchen is proof that however much bacteria there is, it’s less than the harmful amount. That’s the evidence that matters. Approach the problem this way and examine the data again. ---
**Author:** ![USCDiver](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/uscdiver/32/354_2.png) [@USCDiver](https://boards.straightdope.com/u/USCDiver)\ **Post date:** [November 30, 2016, 1:55am UTC](https://boards.straightdope.com/t/bacteria-how-many-colony-forming-units-is-worrisome/773393/7 "2016-11-30T01:55:29Z")
For a urine culture, the standard CFU to consider a sample to represent a true infection (in a Petri dish, ~10 square inches) is 100,000 ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [November 30, 2016, 3:05am UTC](https://boards.straightdope.com/t/bacteria-how-many-colony-forming-units-is-worrisome/773393/8 "2016-11-30T03:05:53Z")
> [@LSLGuy](#): > > …Approach the problem this way and examine the data again. Yes, well put. Another way to underline the point that it’s the _type_ of contamination that matters, not the quantity. This kind of nonsense is positively dangerous if people take the opposite interpretation that they need be _less_ careful about (say) washing their hands after pooping. Natural selection has been carrying out far more rigorous experiments for millions of years. We can’t trust our evolved instincts to be completely reliable in the modern environment, but I think we can trust our aversion to fecal contamination and close contact with people who seem sick. Assays like the one **USCDiver** and **Kelevra** have noted make sense because they are comparing apples to apples.