This represents a huge number of organisms, the most important of which are Bacteroides spp and Prevotella, but not limited to these organisms. They are the predominant bacteria of the gi and gu tract.
As a general rule they are part of mixed infections and/or in dead tissue.
The problem with anaerobes is that resistance to clindamycin is increasing rapidly, 30% as I write this. My order for reliable killing of anaerobes is cold steel, oxygen, metronidazole, carbapenems, penicillin/beta-lactamase inhibitors, and cefoxitin, cefmetazole.