Antibiotics and Dentin Biofilm Growth

Researchers in many disciplines of dentistry and medicine are studying the role of biofilms in certain chronic infections that are refractory to conventional pharmacologic therapies. Endodontic biofilms can proliferate beyond the confines of the root canal, and these biofilms may have applicability to biofilms that occur in other parts of the body. Although nonsurgical root canal therapy attempts to debride and sterilize the root canal space with instrumentation and disinfecting irrigants, sterilization is rarely, if ever, achieved; researchers have found that tissue and debris still remain in root canals. In one study, microbial biofilms remained in 14 out of 16 endodontically treated canals. In another study, biofilm formation on dentin and the effects of antimicrobial drug concentrations on these biofilms was observed. This study involved scanning electron microscope (SEM) analysis of 30 recently extracted human teeth that had been diagnosed with necrotic pulps and periapical radiolucent lesions prior to extraction. Wild-strain endodontic bacteria were anaerobically cultured from the pulps of the teeth and used to grow biofilms on sterilized dentin slices. The biofilms were then exposed to the following 5 antibiotics: ampicillin, coxycycline, clindamycin, azithromycin, and metronidazole. Each day for 8 days, specimens were fixed using 2% glutaraldehyde and examined with the SEM to observe the development of wild-strain endodontic bacterial biofilms and the effects of exposing these biofilms to the antibiotic agents at their respective minimum inhibitory concentrations. The SEM analysis found that ma-ture biofilms developed after 8 days of growth, and none of the antibiotics tested was effective in eliminating the biofilm even after 8 days of exposure. The authors note that the inherent resistance of bacterial biofilms to antimicrobial agents has been well documented and seems to occur by multiple mechanisms. Bacterial biofilms are “incredibly recalcitrant” and can reportedly survive antibiotic concentrations up to 1,000 to 1,500 times those needed to kill planktonic cells of the same species. 

This study shows that the 5 antibiotics tested were en-tirely ineffective against endodontic bacterial biofilms, and that higher concentrations are required. The study concludes that bacteria from endodontic infections can form mature biofilms that are resistant to clinically achievable levels of antibiotic therapy.