So the packaging of Bio-Cultures Complex says that it contains a minimum of a billion viable organisms per capsule.

How has such an estimate been drawn?

Have they counted a smaller amount and then multiplied it?

  • $\begingroup$ Aren't you just describing statistical sampling? $\endgroup$
    – Zhe
    Feb 7, 2018 at 13:46
  • 3
    $\begingroup$ Have you mistaken chemists for chemists? $\endgroup$
    – OrangeDog
    Feb 7, 2018 at 15:53

2 Answers 2


It says a "a billion viable cells". The viable is important because it tells you that they employ a method that determines whether the cells are alive or dead. Most likely they did a simple colony count.

The approach is simple, you take bacteria in suspension, serially dilute them in order to reduce the concentration to numbers that can be counted, and then spread onto a solid media plate (usually containing agar) and leave for 1-3 days (depending on organism) and count the number of colonies produced. You can then multiply up this number by the amount you diluted by to get the number of viable cells present in the original.

A billion is not a lot of bacteria. You can expect 100-1000 billion cells per ml in an overnight culture of these bacteria.

Looking at the product description, it specifically mentions CFUs (colony forming units) so I think we can be pretty sure that this is the approach they are using.

(Note that this is standard microbiological technique rather than something chemists would be doing)

  • $\begingroup$ Welcome to Chemistry! Take the tour to get familiar with this site. Mathematical expressions and equations can be formatted using $\LaTeX$ syntax. $\endgroup$
    – pentavalentcarbon
    Feb 7, 2018 at 14:47

This more a question of Biology than Chemistry, but we have a similar problem when determining particle size distributions.

The principle is always the same: the suspended particles or cells are passed through a capillary tube and than some measurement is done. This can be with light, an electrical field, etc. When a particle passes, there is a change in the variable and a counter is stepped. It can be designed to determine the size also.

The most usual instrument for counting organisms is a Coulter counter. Knowing the volume that has passed through the counter and the number of organisms you know the concentration. Then you can calculate the content in any other volume.


To distinguish between living and dead cells, stains are used. There are several types but they relay on cell membrane integrity. If the membrane is broken, the organism is not viable, but the stains can enter the cell. They than bind to intracellular molecules (DNA, proteins,...) giving an intense color or fluorescence. On the membrane there can also be some of the molecules, but then the color is much less intense.

  • $\begingroup$ Okay so it uses measurement instruments, rather than the human eye. $\endgroup$
    – mavavilj
    Feb 7, 2018 at 14:11
  • $\begingroup$ This method could not distinguish viable and non-viable cells. $\endgroup$ Feb 7, 2018 at 14:36
  • $\begingroup$ @mavavilj In the good old days the sample was put on a slide with a square grid. Then the cells in a predetermined selection of the squares were counted and the total extrapolated. $\endgroup$
    – Raoul Kessels
    Feb 7, 2018 at 14:43
  • $\begingroup$ @Aidley Fluorescent stains are added if the living and dead cells must be distinguished $\endgroup$
    – Raoul Kessels
    Feb 7, 2018 at 14:44
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    $\begingroup$ I have just given you the point of view of a Chemist. For a more Biology based opinion you might have more success at Biology Stack Exchange. Further, I am not going to judge how companies that I do not know are performing their analyses and if they are science oriented or not. $\endgroup$
    – Raoul Kessels
    Feb 7, 2018 at 15:31

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