Filter paper in filtration assembly

Why we used 0.45 micron filter in filtration method?

Actually membrane filters are not having the single uniform sized holes passing from top to bottom , but they are ramification of channels through their whole thickness. So, because of morphology of filters it is not possible for microorganism to pass thorough the filter. Therefore a filter of 0.45 micron size can retain large number of microbial cells smaller than the 0.45 micron.
if we use 0.45 micron filter then microorganisms could be easily retained on the filter paper and the applied vacuum will not have impact on the viability of the microorganism involved in the test but if we use 0.22 micron filter paper stringent pore size and applied vacuum leads to damage of microbial cell which will not further recover during the incubation time. That’s why we use 0.45 micron pore size filter for sterility testing.