When collecting the supernatant after packaging virus in a cell line such as 293T, why do most protocols ask for a 0.45um filter? I understand that filtering is done to eliminate cells and cellular debris so a 0.45um is all that is needed. Also, it should also be done with a filter that has low protein binding. However, what is the harm of using a 0.2um filter? It seems like a 0.2um filter would provide the added benefit of filtering out any bacteria if contamination did occur during the virus production. Any thoughts on this?