First, you need to estimate the volume of the human intestinal tract. To estimate this, we can use this:
pi r^2 h = 3.14 x 0.02^2x7 = 0.009 m^3 =9 x 10^-3 or almost 10^-2 m^3
The volume of a bacterium is approximately 10^-18 m^3 (this is the volumes being cubes of linear dimensions)
if the intestinal tract is 1/10 full, then there are 10^-3 meters available for filling, and there are:
10^-3 m^2/10^-18 m^3/bacterium = 10^15 bacteria
