Plaque as a Biofilm

Plaque was first recognized as a biofilm in the early 1990’s and began to be referred to as plaque biofilm rather than plaque alone. This discovery consequently, had an influence on some of the more current strategies used to control and prevent dental diseases.⁷

Previously, bacteria were studied as colonies growing on culture plates in the laboratory. Newer and more sophisticated technologies, 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, permitted examination and understanding of oral biofilms in their natural states.^7,8,9

Microorganisms in biofilm behave differently than planktonic (free-floating) bacteria or those in a culture medium (Table 1).

In general a biofilm is a well-organized, cooperative community of microorganisms.^7,8 The slime layer that forms on rocks in streams is a classic example of a biofilm (Figure 3). Biofilms are everywhere in nature and typically form under fluid conditions. It is estimated over 95 percent of microorganisms existing in nature live in biofilms.⁹ Sometimes biofilms in nature are seen as positive, such as their use for detoxification of waste water and sewage.

One familiar example of a biofilm for dental professionals is the slime layer that forms in dental unit water lines that requires daily flushing. (Figure 4) Biofilms can also be found lining oil pipelines, fish tanks, indwelling catheters, internal implants, contact lenses, and prosthetic devices. Occasionally biofilms are deadly. Legionnaire's disease that killed 29 persons in Philadelphia in 1976 was ultimately traced to bacteria in the biofilm of the air conditioning system. Millions of dollars are spent each year working to control these biofilms.^19,20

Figure 3. Artistic Depiction of Plaque Biofilm.

Figure 3. Artistic Depiction of Plaque Biofilm.

Figure 4. Biofilm found on dental equipment.

Figure 4. Biofilm found on dental equipment.

The microorganisms living with humans also live in biofilms. However, unlike environmental biofilms, humans have a symbiotic relationship with their microbiome in health as it plays a critical role in physiologic, metabolic and immunological functions such as food digestion, energy generation and balances pro-inflammatory and anti-inflammatory processes. Thus, it controls homeostasis and prevents us from disease.¹⁹ However, if a shift in this symbiotic state occurs, the microbiome changes to a dysbiotic state ultimately resulting in disease.

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 (Figures 5 & 6).

Figure 5. Artistic Depiction of Plaque Biofilm.

Figure 5. Artistic Depiction of Plaque Biofilm.

Figure 6. Animation of Biofilm

Figure 6. 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 7). The bacteria in a biofilm use a communication system termed quorum sensing that involves sending out chemical signals (Figure 8). These chemical signals trigger the bacteria to produce potentially harmful proteins and enzymes, virulence factors that help the intraoral biofilm bypass host defense systems.^9,23

Figure 7. Biofilm Fluid Channels.

Figure 7. Biofilm Fluid Channels.

Figure 8. Communication Signals

Figure 8. Communication 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.^9,16 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.^21,22 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 in an attempt to penetrate through the matrix to disrupt the colonies of bacteria inside the biofilm. Biofilms can be removed by mechanical means, however, they immediately begin to reform and are difficult to penetrate, so the search continues for ways to lyse and remove pathogenic biofilms.