Periapical granuloma is
A direct lift from Cohen.
Bacteriological features of periapical granuloma:

Majority of the studies have been based upon bacteriological cultures taken after the extraction of tooth
Pre extraction cultures have been made in a few instances through the root canal or the alveolar plate and these have been relatively free of actual contamination.
The microorganisms that have been isolated by such techniques were, generally found in oral cavity such as  s.viridans, s.albus, e.coli and pneumococci.
Seldom can microorganisms actually be demonstrated histologically in the periapical granuloma.
 Some investigators have suggested that dental granuloma is usually a sterile lesion.
Anaerobic infection
Aerobic infection
Mixed infection
Sterile