Previously, bacteria have been studied as they grew in colonies on culture plates in the laboratory. Newer and more sophisticated technology, such as confocal scanning laser and two-photon excitation technology, as well as molecular analysis methods, such as DNA-DNA hybridization and gene expressions and metabolomics, has permitted examination and understanding of oral biofilms in their natural states.6,20,21
Microorganisms in biofilm behave differently than planktonic (free-floating) bacteria or those in a culture medium (Table 1).
Seen through a microscope, bacteria in a biofilm are not distributed evenly. They are grouped in microcolonies surrounded by an enveloping intermicrobial matrix (Figure 8).
The biofilm matrix is penetrated by fluid channels that conduct the flow of nutrients, waste products, enzymes, metabolites, and oxygen. The microcolonies within the biofilm have micro environments with differing pH's, nutrient availability, and oxygen concentrations (Figure 9). The bacteria in a biofilm use a communication system termed quorum sensing that involves sending out chemical signals (Figure 10). These chemical signals trigger the bacteria to produce potentially harmful proteins and enzymes, virulence factors that help the intraoral biofilm bypass host defense systems.6,7
Our previous attempts to predict and control periodontal diseases have been based on the performance of bacteria cultured under laboratory conditions.1,6 Increased understanding of biofilms have demonstrated great differences between bacterial behavior in laboratory culture and in their natural ecosystems. For example, bacteria in biofilm produce compounds in biofilm that they do not produce when in culture. Also, the biofilm matrix surrounding the microcolonies serves as a protective barrier. This helps explain why systemic and locally delivered antimicrobials have not always proven successful, even when they were targeted at specific microorganisms. Researchers have estimated that it can take 1,000 times the drug to kill a microorganism in a biofilm as it does to kill the same organism in a free floating or planktonic environment.22 The protective matrix of biofilm also helps explain why mechanical plaque control and personal oral hygiene have continued to be an integral part of periodontal therapy. Biofilms can be removed by mechanical means. However, they immediately begin to reform, so the search continues for ways to combat pathogenic biofilms.