Plaque as a Biofilm

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 sequencing, 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).

Table 1. Basic Biofilm Properties22,23
  • Cooperating community of various types of microorganisms
  • Microorganisms are arranged in microcolonies with channels between the microcolonies
  • Microcolonies are surrounded by protective matrix
  • Differing environments within the microcolonies in the biofilm
  • Microbial gene expression differs when microorganisms are in a biofilm
  • Microorganisms have primitive communication system
  • Microorganisms in biofilm are resistant to antibiotics, antimicrobials, and host response
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).
Figure 8. Artistic Depiction of Plaque Biofilm.
Image: Artistic depiction of plaque biofilm.
Figure 8a. Animation of Biofilm
Image: Animation of Biofilm
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
Figure 9. Biofilm Fluid Channels
Image: Biofilm fluid channels animation.
Figure 10. Communication Signals
Image: Biofilm bacteria communicate by sending out chemical signals.
Biofilm bacteria communicate by sending out chemical signals.
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. One researcher has 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.