In untreated water, the main cause of apprehension in drinking would be bacteria, cysts and viruses when it comes to microorganisms. The sizes of these microorganisms are also measured in microns. The pore size of the ceramic does not directly relate to the removal properties with respect to the size of the microorganism. In the case of a sieve, there would be typically only one layer to filter the required particles. But in the case of the ceramic cartridge, there are millions of layers that the microorganism needs to pass through before it could leave the filter.  

Using a sophisticated ceramic filter, cysts and bacteria are effectively removed. It is not possible to remove viruses using ceramic water filters due to their infinitesimal micron size.

In most filters, the microorganisms filtered out will start cultivating in the outer layer. A premium ceramic cartridge would have colloidal Silver impregnated in it. As Silver is known for its sterilizing properties, this would inhibit the growth of microorganisms in the ceramic structure.