Subgingival Bacterial Resistance in a Biofilm

Bacteria in the traditional in vitro culture environment are in a planktonic state. The result of this state is that these bacteria are immediately susceptible to factors such as antibiotic therapy. A protective measure that is utilized by dental bacteria is that of a biofilm formation. The biofilm community is initially formed through bacterial interactions with the tooth and then through physical and physiologic interactions among different species found within the microbial mass. It has been known for some time that bacteria growing in microbial communities are more resistant to antimicrobial therapy (Allison 1995).

Bacteria adopt a different phenotype when they adhere to a surface or interface and initiate biofilm formation. The resistance of biofilms to antibiotics can be seen as the expression of different sets of genes. The genes expressed in a biofilm differ from those expressed in the corresponding planktonic cell. A biofilm will not express a single phenotype, but their gene expression goes through a whole spectrum of changes as the community matures. As a result, the planktonic phenotype begins to emerge as the biofilm begins to shed mobile cells. By shedding these planktonic cells, the biofilm community can rid themselves of susceptible bacteria and thus become more resistant to antibiotics (Costerton 1999).

Biofilm studies also show a significance of structural and physiologic interactions between bacterial species. Bramanti studied F. Nucleatum in experimental mixed biofilm communities and found that the existence of this bacteria is beneficial to the high numbers of anaerobic species (P. Nigrescens and P. Gingivalis). The ability of F. Nucleatum to reduce the reduction-oxidation potential of its environment may provide a protective niche for other anaerobic species. By positive interactions between species of bacteria in the biofilm, the bacteria are able reduce the effect of antimicrobials by overpopulation.

A third method which could explain the resistance of bacteria in a biofilm has to do with gene transfer. Wang and associates examined S. Gordonii and T. Denticola in dental biofilms and noted that S. Gordonii was capable of transferring a gene responsible for resistance to erythromycin to T. Denticola. As a result, both bacteria were now resistant to erythromycin, an antibiotic commonly used to treat periodontal disease. By use of gene transfer among bacteria that reside in the biofilm, the community can become more resistant to antibiotic therapy.

Due to the resistance developed by bacteria in a biofilm, it is imperative to remove the biofilm daily. Techniques such as brushing, flossing, and regular trips to the dentist can reduce the accumulation of bacteria and formation of a biofilm. The results of a well developed biofilm are cavities and periodontal disease, both of which can ultimately lead to tooth loss.