Can activated charcoal be used to filter bacteria out of air?
Activated charcoal has been shown to be able adsorb certain species of bacteria, but the effectiveness varies between species [1] (which is potentially useful for medical applications), and has not been tested for many, so it is unlikely that activated charcoal could be used to adsorb all the bacteria from the air or a gas. 

Activated charcoal general usage is in the adsorption of toxins. 

[1] Naka, Katsumi, et al. "Adsorption effect of activated charcoal on enterohemorrhagic Escherichia coli." Journal of Veterinary Medical Science 63.3 (2001): 281-285.