Can you please add details to your question? Is it a specific microbe that you are looking?If you are  not interested in specific organism, you can measure OD of microorganisms at 600 nm in spectrophotometer. Centrifuge some ml of your beer or more and then dissolve it in saline and measure the Optical density of the suspension. You can standardize OD and no. of cells.

Thank you for your answer, microorganism (mayor lactic bacteria) are contaminant we could find them in 10exp4 or nothing in 1 ml, and together with yeast that can be 10exp4 or more, the amount of cell yeast is different from sample to sample so I think that this approach is not reliable because yeast can interfere in readings.

Would it be possible to filtrate two times? For example, filtration of sample through filter pores that are not small enough to contain bacteria but not large enough to allow excess yeast through -- followed by bacterial filtration...

It might also be possible to centrifuge your sample in four test tubes of 25 mL volume at high speed (discarding supernatant) followed by suspension of the four obtained pellets into one test tube of small volume to obtain a highly concentrated, low volume sample for plating on solid media at various dilutions.

If you had/have access to a real-time thermal cycler PCR system, you might even be able to run an bacterial-specific assay with a mixture of yeast and bacterial DNA without yeast-interference in your results.

Dear Michael, thank you. I think its hard to perform two sterile filtration for the risk of adding other contamination, but I have to evaluate it with my technician. The second possibility is not reliable because yeast interfere when it is too much, and I don't have access to PCR, but only with traditional method (Petri dishes and culture medium). Have a good day.