Primary (untreated) endodontic infections are predominantly composed of which type of bacteria?

Primary endodontic infections (in a necrotic, untreated pulp space) are predominantly composed of obligate anaerobic bacteria, reflecting the low-oxygen environment of the necrotic pulp space — this differs somewhat from secondary/persistent endodontic infections, which can show a different, sometimes more facultative bacterial profile related to prior treatment and changed canal conditions.