What causes anaerobic bacteria in the nose?

Entirely normal: Anaerobic bacteria are normal in the nose and all body openings, i.e. the mouth, genital tract, and intestines, and in the pores of our skin. In fact, anaerobic bacteria outnumber other bacteria. They tend to cause infection when they are trapped by blockage -- for example, a cold or allergy blocking sinus drainage: backed up bacteria, including anaerobes, grow to larger numbers.

Anaerobic bacteria: anaerobic bacteria - or bacteria that don't require oxygen to live and multiple are generally found in closed off spaces with low oxygen such as abcesses. The nose normal has airflow so anaerobic bacteria are rare. However, if swelling reduces or closes off passageways or sinuses - these types of bacteria start to flourish https://search.yahoo.com/yhs/search?p=sinus+passage+diagram&ei=UTF-8&hspart=mozilla&hsimp=yhs-004