Biofilm

Biofilms are structured communities of microorganisms that adhere to surfaces and are embedded within a self-produced matrix of extracellular polymeric substances. These complex microbial systems form in a vast range of natural, industrial, and clinical environments, and their adaptive capabilities allow them to survive in conditions that would be challenging for free-floating planktonic cells. Biofilms exhibit distinctive physiological and genetic characteristics, contributing to their resilience, ecological significance, and medical relevance.

Characteristics and Structure

A biofilm consists of microbial cells enclosed in a hydrated matrix composed primarily of extracellular polysaccharides, proteins, lipids, and extracellular DNA. This matrix forms a three-dimensional structure that provides physical and chemical protection to the cells within. Biofilms have often been metaphorically compared to microbial cities due to their organised, cooperative, and highly interactive communities.
The matrix functions as:

• A protective barrier, guarding against desiccation, antibiotics, disinfectants, and host immune responses.
• A structural scaffold, maintaining the integrity of the biofilm and supporting communication channels.
• A nutrient reservoir, enabling metabolic interactions such as syntrophy.

Cells in biofilms differ from planktonic microorganisms in gene expression, metabolic activity, and resistance characteristics. Subpopulations within biofilms may vary widely in activity, including metabolically active cells, dormant or persister cells, and viable-but-non-culturable cells. These differences contribute to their ability to withstand environmental stresses.
Biofilms may form on both biotic surfaces—such as animal tissues, dental enamel, or plant roots—and abiotic surfaces, including medical devices, industrial equipment, and natural substrates like rocks or sediment.

Origin and Early Evolution

Biofilms are believed to have originated very early in Earth’s history. Evidence from fossil records suggests the presence of microbial communities over 3.25 billion years ago. In primitive environments marked by extreme temperatures, ultraviolet radiation, and limited resources, biofilm formation likely provided protection and homeostasis for early prokaryotic life. By enabling cooperative behaviour and stable microenvironments, biofilms may have contributed to the evolution of complex interactions between early microbial cells.

Formation and Development

Biofilm formation is a multi-stage developmental process influenced by surface properties, environmental cues, and intercellular communication. The typical stages include attachment, colonisation, maturation, and dispersal.

Initial Attachment

Free-swimming planktonic cells first encounter a surface and attach through weak interactions such as van der Waals forces and hydrophobic attraction. If not removed, they strengthen this attachment using specialised structures including pili. Certain archaea possess unique adhesion structures known as hami, long filamentous appendages with hook-like ends used for binding to surfaces or other cells. Hyperthermophilic archaea also produce filaments homologous to bacterial TasA proteins, underscoring evolutionary parallels between bacterial and archaeal biofilms.
Surface hydrophobicity plays a significant role in adhesion: increased hydrophobicity reduces electrostatic repulsion, facilitating attachment. Motile bacteria attach more efficiently than non-motile species, which often rely on joining existing microcolonies.

Colonisation and Growth

During colonisation, attached cells undergo a phenotypic shift resulting in differential gene regulation. This includes activation of genes needed for adhesion, matrix production, and communal survival. Communication between cells occurs through quorum sensing, involving signalling molecules such as N-acyl homoserine lactones. These signals coordinate communal behaviours, including virulence factor production and matrix synthesis.
The extracellular matrix—predominantly composed of polysaccharides—encloses the growing community. The matrix can also integrate environmental materials such as soil particles, minerals, or biological components like fibrin or erythrocytes.

Maturation and Structure

As a biofilm matures, it develops into a highly structured system with distinctive architecture. Channels within the matrix allow the distribution of nutrients, oxygen, and signalling molecules. The dense structure and close cell-to-cell contact foster increased rates of mutation and horizontal gene transfer, enhancing adaptability.
Biofilm maturation leads to:

• Increased resistance to antimicrobials and host defences.
• Functional differentiation, with subpopulations specialising in motility, matrix production, or sporulation.
• Stable microenvironments, providing gradients of oxygen, nutrients, and metabolic by-products.

Dispersal

Dispersal marks the final stage of biofilm development. Cells exit the biofilm to colonise new surfaces, a process essential for microbial propagation and ecological success. Dispersal can be triggered by enzymatic degradation of the matrix—mediated by enzymes such as dispersin B or DNase—and by signalling molecules. Fatty acid messengers like cis-2-decenoic acid, secreted by Pseudomonas aeruginosa, can induce dispersal across multiple microbial species, including yeasts such as Candida albicans. Nitric oxide at subtoxic levels is another cue known to initiate biofilm dispersal.
Research indicates that dispersed cells represent a unique physiological state: they differ significantly from both planktonic and biofilm cells. In Pseudomonas aeruginosa, dispersed cells display heightened virulence and altered stress sensitivities, including increased susceptibility to iron stress. Distinct spatiotemporal patterns in dispersal influence recolonisation dynamics and disease progression.

Ecological and Medical Significance

Biofilms are integral components of many natural ecosystems. They constitute essential microbiomes in aquatic environments, soils, and host organisms. Their role in nutrient cycling, pollutant degradation, and microbial succession contributes to ecosystem stability.
In clinical settings, biofilms are of particular concern due to their association with persistent infections. Biofilms commonly form on medical devices such as catheters, prosthetic materials, and implants. Dental plaque, a well-known biofilm, contributes to tooth decay and periodontal disease. Their inherent resistance to antibiotics and immune responses complicates treatment, often requiring combined mechanical, chemical, and pharmacological interventions.
In industrial systems, biofilms may cause biofouling, leading to reduced efficiency in pipelines, water treatment facilities, and heat exchangers. Conversely, controlled biofilms are employed beneficially in wastewater treatment, bioremediation, and certain fermentation processes.

Adaptive Advantages

The biofilm mode of life offers microorganisms several advantages:

• Enhanced survival under hostile conditions through collective protection.
• Shared metabolic activities, allowing syntrophic interactions and resource partitioning.
• Increased gene exchange, promoting rapid adaptation.
• Improved resistance to antimicrobial agents and environmental stresses.