Building biofilms in vital host tissues: a survival strategy of Actinomyces radicidentis

To investigate the ability of Actinomyces radicidentis to survive and establish in soft connective tissue that grew into subcutaneously implanted tissue cages in Sprague-Dawley rats.

Known concentrations of A. radicidentis suspension, grown on blood agar and broth cultures, were inoculated into tissue cages in rats. The cage contents were retrieved after 7, 14, and 28 days for culturing and correlative light and transmission electron microscopy.

Cell suspensions harvested from both types of cultures showed substantial decline in numbers in tissue cages during the observation period. However, correlative light and transmission electron microscopy revealed numerous aggregates of coccoid bacteria already by 7 days of observation compared with the formation of well established colonies with characteristic actinomycotic features by 14 days after inoculation.

These results suggest that the pathogenicity of A. radicidentis is due to its ability to form large aggregates of cells held together by embedding themselves in an extracellular matrix in vital host tissues. Thus, A. radicidentis, like other pathogenic Actinomyces, existing in the protected biofilm-environment can collectively evade destruction and elimination by host defenses, including phagocytosis.