Bacterial Biofilm Formation

A biofilm is a structured community of bacteria attached to a surface and embedded in a self-produced matrix of extracellular polymeric substances. Biofilm formation is a developmental process in which free-swimming (planktonic) cells attach to a surface, multiply, build a protective matrix, mature into three-dimensional structures, and eventually disperse to colonise new sites.

Definition

A biofilm is an aggregate of microorganisms attached to a surface or interface and enclosed in a self-produced extracellular matrix; biofilm formation is the regulated developmental sequence of attachment, growth, matrix production, maturation, and dispersal by which such communities arise.

Scope

This topic covers the stages of biofilm development, the composition and role of the extracellular matrix, the cell-to-cell signalling (quorum sensing) that coordinates community behaviour, and the tolerance that biofilms show toward antimicrobials and host defences. It is a reference topic in microbial physiology and provides no clinical guidance.

Core questions

• What are the stages of biofilm development?
• What is the biofilm matrix and what functions does it serve?
• How does quorum sensing coordinate biofilm behaviour?
• Why are biofilms more tolerant of antimicrobials than free-living cells?

Key concepts

• Planktonic versus sessile (biofilm) lifestyle
• Attachment, maturation, and dispersal stages
• Extracellular polymeric substance (EPS) matrix
• Quorum sensing and cell-to-cell signalling
• Biofilm-associated antimicrobial tolerance
• Device-related and chronic biofilm infections

Mechanisms

Biofilm formation proceeds as a developmental programme: planktonic cells make initial reversible contact with a surface, attach irreversibly, and proliferate into microcolonies that secrete an extracellular matrix of polysaccharides, proteins, and extracellular DNA (O'Toole et al., 2000; Flemming & Wingender, 2010). The matrix holds the community together, retains nutrients and enzymes, and forms a barrier; mature biofilms develop characteristic architecture before some cells disperse to seed new surfaces. Quorum sensing, in which cells release and detect diffusible signal molecules to gauge population density, coordinates matrix production and other community behaviours (Miller & Bassler, 2001). Cells within biofilms show reduced susceptibility to antimicrobials and host defences compared with planktonic cells (Costerton et al., 1999).

Clinical relevance

Biofilms are associated with persistent and device-related infections, including those on catheters, implants, and in chronic lung disease, where the protective matrix and altered physiology of biofilm cells contribute to tolerance of antimicrobial agents and host immune clearance (Costerton et al., 1999; Singh et al., 2000). This topic describes biofilm biology for understanding and is not a basis for diagnosis or treatment.

History

Although attached microbial layers were observed long before, the modern concept of the biofilm as a structured, matrix-enclosed community that causes persistent infection was articulated by Costerton and colleagues in the 1990s (Costerton et al., 1999). The framing of biofilm formation as a developmental process and the recognition of quorum sensing as its coordinator established biofilms as a central theme of bacterial physiology (O'Toole et al., 2000; Miller & Bassler, 2001), and direct evidence linked biofilm communities to chronic human infection (Singh et al., 2000).

Key figures

• J. William Costerton
• Roberto Kolter
• Bonnie Bassler
• Hans-Curt Flemming

Seminal works

• costerton-1999
• otoole-2000
• flemming-wingender-2010

Frequently asked questions

What are the main stages of biofilm formation?

Free-living cells attach to a surface, proliferate into microcolonies, produce an extracellular matrix and mature into a three-dimensional structure, and finally disperse cells that can colonise new surfaces.

Why are biofilm bacteria harder to eliminate than free-floating bacteria?

The extracellular matrix acts as a protective barrier and the cells within biofilms are physiologically altered, so the community is more tolerant of antimicrobial agents and host immune defences than planktonic cells; this is associated with persistent and device-related infections.