What Are the Main Components of a Biofilm?

A biofilm is a collective of microorganisms, often from many different species, attached to a surface and to each other. These communities are encased in a self-produced, slimy substance that forms a protective matrix around the cells. Biofilms are found nearly everywhere in nature, from plaque on teeth to the slippery coating on river stones. They also form in industrial settings like water pipes and on medical devices, where they can have significant impacts.

Microbial Cells: The Living Core

The foundational components of any biofilm are the microbial cells. These organisms are the architects and residents of the structure, initiating its formation and carrying out metabolic activities. While many types of microbes can form biofilms, bacteria are the most common and widely studied builders. Species such as Pseudomonas aeruginosa and Staphylococcus aureus are well-known for creating robust biofilm communities in medical and industrial settings.

The composition of these cellular communities can be diverse. While some biofilms consist of a single species, most are complex, mixed-species assemblies that can include bacteria, fungi like Candida species, archaea, and even algae. In these communities, diverse organisms work together, performing specialized functions. The specific mix of microbial residents varies greatly depending on environmental conditions, such as nutrient availability and the surface they colonize.

The EPS Matrix: Scaffolding and Protection

Surrounding the living cells is the extracellular polymeric substance (EPS) matrix, a complex, self-produced material that acts as a structural scaffold. This matrix is the “slime” that gives a biofilm its distinct texture and three-dimensional form, accounting for 50% to 90% of the total organic matter. The EPS is not a uniform substance; its composition varies between species and changes based on environmental cues, allowing biofilms to adapt their properties to suit their surroundings.

The primary structural components of the EPS are polysaccharides, which are long chains of sugar molecules. These exopolysaccharides form a dense, gel-like network that holds the cells together. Different bacteria produce different types of polysaccharides; for instance, Pseudomonas aeruginosa produces Pel, Psl, and alginate. These molecules can be neutral or charged, which influences how they interact with each other and with surfaces.

Embedded within the polysaccharide network are various proteins. Some are structural, such as amyloid fibers, which add strength and integrity to the matrix. Other proteins are enzymes that can modify the matrix, break down external food sources, or help regulate development. Fragments of cellular appendages like pili and flagella can also become integrated into the matrix.

Another component of the EPS is extracellular DNA (eDNA). This DNA is released from cells through active secretion or when cells lyse and becomes an integral part of the matrix. The eDNA acts as a structural element and can serve as a reservoir of genetic information, facilitating gene transfer between neighboring cells.

Finally, the matrix contains lipids and other biomolecules. Lipids can influence the hydrophobic or hydrophilic nature of the biofilm, which affects how it interacts with water and surfaces. The matrix may also trap non-cellular materials from the environment, such as mineral crystals or clay particles.

Water: The Essential Medium

Water is the most abundant component of a biofilm by mass, often constituting over 90% of the entire structure. It is an active medium that sustains the community, filling the space within the EPS matrix to create a hydrated environment. This high water content is necessary for metabolic activity and protects the cells from desiccation.

The water within a biofilm exists in different states. Some is “bound water,” physically adsorbed to the surfaces of cells and the EPS matrix polymers. This bound water has different physical properties than “free water,” which flows more readily through the biofilm’s open spaces. The interaction between water and the EPS polymers gives the biofilm its characteristic viscoelastic properties.

This aqueous medium is also the primary system for transport. Biofilms are permeated by a network of microscopic water channels that connect the community to the external environment. Nutrients and oxygen diffuse through these channels to reach cells deep within the structure, while waste products are carried away.

Functional Roles of Biofilm Components

The components of a biofilm work together to create a highly successful living arrangement. A primary function is adhesion, where microbial cells attach to a surface. This attachment is mediated by adhesion proteins and the sticky polysaccharides in the EPS, which also promote cohesion by binding cells to one another to build the three-dimensional structure.

The dense EPS matrix acts as a physical shield, defending the embedded cells from a wide range of environmental threats. This includes protection from antimicrobial agents, disinfectants, UV radiation, and the host immune system. The matrix can prevent these harmful substances from penetrating deep into the biofilm or can bind to them, neutralizing their effects.

The biofilm structure is also effective for managing resources. The matrix can trap and concentrate nutrients from the surrounding environment. The water-filled channels then facilitate the distribution of these captured nutrients throughout the community and carry waste away.

This combination of components provides remarkable structural integrity. The cross-linked network of polysaccharides, proteins, and eDNA creates a stable scaffold that withstands physical forces. The close proximity of cells also facilitates communication and the exchange of genetic material, allowing the community to adapt to changing conditions.