The Final Bacteria filter looks like a spinning top and is the final filter before the oxygen goes to the outlet of the oxygen concentrator. The filter filters up to 0.x micron. Bacteria and other small particles will be filtered out by this. Furthermore, it protects against a sieve blow-out.
At every maintenance inspection, do a visual inspection of the bacterial filter and replace as necessary. It's recommended to replace the bacterial filter when the compressor or sieve gets replaced