I have personally never heard of using the geometric mean for such a calculation.

It is generally accepted to use a regular (Arithmetic) mean.

It makes no biological sense to use the geometric, you are enumerating bacterial load, the numbers are in a simple scale.

However, there are a couple of notes worth making:

1. you may want to present your result on a logorithmic scale if the difference is in the 10^X fold

2. It is very common to show not just the mean but also the range, especially as a visual figure.

For those, like me, who were not sure what a geometric mean is:

Arithmetic mean of 11, 13, 17 and 1,000 = (11 + 13 + 17 + 1,000) / 4 = 260.25 Geometric mean of 11, 13, 17 and 1,000 = 4th root of (11 x 13 x 17 x 1,000) = 39.5

https://sciencing.com/differences-arithmetic-geometric-mean-6009565.html

Thanks a lot. You have been very helpful Amir Nakar

For MPN regular mean is used!