What Is Subgingival Biofilm and How Does It Form?

Subgingival biofilm is a structured community of microorganisms existing below the gumline, in the small space, or sulcus, between the teeth and gingival tissue. Unlike the more commonly known supragingival plaque on visible tooth surfaces, this biofilm is largely hidden from view. This location makes it particularly difficult to manage with routine oral hygiene. The environment below the gums is also distinct, providing different nutrients that influence which types of bacteria can thrive there.

This biofilm is not a random collection of germs but a highly organized and cooperative community. Consequently, the presence of this subgingival community is a primary factor in the development of inflammatory gum conditions.

Formation of Subgingival Biofilm

The development of subgingival biofilm is a systematic process that occurs in distinct stages. It begins on a clean tooth surface with the formation of the acquired pellicle, a thin, saliva-derived layer of glycoproteins and proteins. This initial layer forms within minutes and acts as the foundation for the first bacteria, known as pioneer colonizers, to attach. These initial inhabitants are often gram-positive bacteria, such as species of Streptococcus and Actinomyces.

Once these early colonizers have established a foothold, they multiply and form small microcolonies. Their metabolic activities alter the local environment, making it more suitable for other bacteria to join the community through a process called coaggregation. This succession transforms the simple layer of bacteria into a more complex and diverse structure.

As the biofilm continues to grow, it expands downwards into the gingival sulcus. This progression marks the transition from a supragingival to a subgingival biofilm. The structure becomes a three-dimensional community with intricate channels for nutrient transport and waste removal, where different bacterial species perform specialized roles.

Composition of the Microbial Community

As the biofilm establishes itself deep within the gingival sulcus, the environment changes. Oxygen levels become very low, creating an anaerobic setting that favors the growth of specific microorganisms. This leads to a shift from the predominantly gram-positive bacteria of early plaque to a community dominated by gram-negative anaerobic species. These bacteria are adept at surviving in the low-oxygen conditions found in deep periodontal pockets.

This mature subgingival community is often described in terms of bacterial complexes, which are groups of species frequently found together. The “red complex,” for instance, includes three species—Porphyromonas gingivalis, Tannerella forsythia, and Treponema denticola—that are strongly implicated in the tissue destruction seen in periodontitis. The presence of these pathogenic bacteria can indicate a shift to a state of dysbiosis, where harmful species proliferate.

A defining feature of the biofilm is the extracellular polymeric substance (EPS), a gel-like matrix produced by the bacteria. This matrix, composed of polysaccharides, proteins, and extracellular DNA, encases the bacterial cells and holds the community together. The EPS serves as a protective barrier, shielding the embedded bacteria from the host’s immune cells and reducing the penetration of antimicrobial agents, making the biofilm exceptionally resilient.

Impact on Periodontal Health

Subgingival biofilm causes disease not by direct bacterial invasion, but by triggering a persistent inflammatory response from the host’s immune system. Gram-negative bacteria within the biofilm release substances like endotoxins, which the immune system recognizes as a threat. The resulting immune response, while intended to be protective, becomes the primary driver of tissue damage in periodontal disease.

This chronic inflammation first manifests as gingivitis, characterized by red, swollen, and bleeding gums. At this stage, the damage is reversible if the biofilm is controlled. However, if the inflammation continues, it progresses to periodontitis, leading to the destruction of the connective tissues and alveolar bone that support the teeth.

A direct consequence of this tissue breakdown is the formation of periodontal pockets. As gum tissue detaches from the tooth, the shallow gingival sulcus deepens into a pocket. These pockets create an even more protected, anaerobic environment ideal for pathogenic bacteria. This establishes a destructive cycle where the biofilm triggers inflammation that deepens the pockets, and the deeper pockets in turn harbor more biofilm.

Management and Disruption of Biofilm

Controlling subgingival biofilm requires combining professional dental care with diligent at-home maintenance. Because established biofilm below the gumline is inaccessible to toothbrushes and floss, its physical removal by a dental professional is necessary. The primary procedures for this are scaling and root planing. Scaling uses specialized instruments to scrape away hardened biofilm (calculus) and soft deposits from tooth surfaces below the gumline.

Following scaling, root planing is performed to smooth the root surfaces. Rough spots on the root are more likely to harbor bacteria, so creating a smooth surface makes it more difficult for bacteria to re-colonize. These procedures physically disrupt the pathogenic community, reducing the bacterial load and allowing inflamed gum tissues to heal.

Daily oral hygiene is focused on consistently disrupting the biofilm to prevent it from maturing. Brushing removes plaque from visible tooth surfaces, but cleaning between the teeth is equally important. Interdental cleaners, such as floss, water flossers, or small brushes, access these tight spaces and disturb bacterial colonies. While antimicrobial mouthwashes can help, they cannot eliminate a mature subgingival biofilm on their own; physical disruption remains the primary method of control.