D

Histologic and molecular studies of extraradicular biofilms show that Actinomyces spp. are the most frequent isolates, with Propionibacterium/Propionibacterium propionicum also recovered in a substantial proportion of cases; together they account for the majority of documented extraradicular infections, so all listed organisms are included as common offenders [Wang et al., 2012, PMID 22703660; Del Fabbro et al., 2014, PMID 23766136].