Most common organism responsible for acute apical abscess:
Trepenoma denticola
Dilaster invisus
E. faecalis
All of the above
The bacteria most often recovered from infected vital pulps are streptococci and staphylococci, but many other microorganisms including anaerobes have also been isolated.
With the introduction of improved methods of molecular identification of pathogens, many new organisms have been identified by researchers. 
Species that have been found significantly include Porphyromonas gingivalis, Porphyromonas endodontalis, Fusobacterium nucleatum and others. 
Polymerase chain reaction (PCR) methods show higher prevalence of Treponema denticola, a recognized periodontal pathogen, in association with Tannerella forsythia and P. gingivalis.