Microbial Attachments and Biofilm Formation ◾ 75
electrostatic forces. In the second stage, the cell produces EPS molecules to attach its outer surface
to the surface of a food (biotic) or equipment (abiotic), and the process is irreversible. The adherent
cells in the biofilm are referred to as sessile cells. Besides attachment, EPS contributes to multiple
tasks within the biofilm, which are summarized in Table 7.1. A three-step process for attachment
that includes adsorption, consolidation, and colonization has also been suggested. In the reversible
adsorption stage, which can occur in 20 minutes, the cells attach loosely to the surface. During
the consolidation stage, the microorganisms produce fimbriae, curli, flagella, adhesion protein,
and capsules and firmly attach to the surface. Cells grow in close proximity and exhibit intense
interaction, including cell-to-cell communication (quorum sensing) and formation of synergistic
microconsortia. Autoinducers, such as N-acyl homoserine lactone AI-2, help in cell-to-cell communication and regulate gene expression for survival, growth, cell density, resistance to antimicrobials, and tolerance to desiccation (see Chapter 6). At this stage, the cells cannot be removed by
rinsing. In the colonization stage, which is also irreversible, the EPS or complex polysaccharides
may bind to metal ions on equipment surfaces, and the cells may metabolize products that can
damage the surfaces. 1–5 As a microcolony continues to grow, cells accumulate, forming a matured
biofilm with three-dimensional scaffolds. Loose cells are then sloughed off from the matured biofilm and convert into planktonic cells, which again attach to a new surface preconditioned by food
particles or substrates completing the life cycle of a biofilm.
influencing Factors
The level of attachment of microorganisms to food-processing equipment surfaces is found to
be directly related to contact time. As the contact time is prolonged, more cells attach to the
table 7.1 Role of extracellular Polymeric Substances (ePS) in a Biofilm
Function
Relevance for Biofilms
Adhesion
Initial attachment and colonization to solid surface, long-term
attachment of biofilm to surfaces
Aggregation of cells
Bridging of cells, high cell density, cell-to-cell communication
Cohesion of biofilms
Forms hydrated polymer network, mechanical stability, threedimensional configuration, biofilm architecture
Retention of water
Maintains hydrated microenvironment, resistance to desiccation
Protective barrier
Resistant to antimicrobial agents, antibiotics, biocides, sanitizers,
and host defense (during infection)
Sorption of organic and
inorganic compounds
Accumulation of nutrients and toxic metal ions
Enzymatic activity
Digestion of exogenous macromolecules and EPS for nutrients
Nutrient source
Source of carbon, nitrogen, and phosphorous-containing
compounds
Genetic exchange
Horizontal gene transfer between biofilm cells
Source: Adapted from Flemming, H.-C. and Wingender, J., Nat. Rev. Microbiol., 8, 623–633, 2010.
electrostatic forces. In the second stage, the cell produces EPS molecules to attach its outer surface
to the surface of a food (biotic) or equipment (abiotic), and the process is irreversible. The adherent
cells in the biofilm are referred to as sessile cells. Besides attachment, EPS contributes to multiple
tasks within the biofilm, which are summarized in Table 7.1. A three-step process for attachment
that includes adsorption, consolidation, and colonization has also been suggested. In the reversible
adsorption stage, which can occur in 20 minutes, the cells attach loosely to the surface. During
the consolidation stage, the microorganisms produce fimbriae, curli, flagella, adhesion protein,
and capsules and firmly attach to the surface. Cells grow in close proximity and exhibit intense
interaction, including cell-to-cell communication (quorum sensing) and formation of synergistic
microconsortia. Autoinducers, such as N-acyl homoserine lactone AI-2, help in cell-to-cell communication and regulate gene expression for survival, growth, cell density, resistance to antimicrobials, and tolerance to desiccation (see Chapter 6). At this stage, the cells cannot be removed by
rinsing. In the colonization stage, which is also irreversible, the EPS or complex polysaccharides
may bind to metal ions on equipment surfaces, and the cells may metabolize products that can
damage the surfaces. 1–5 As a microcolony continues to grow, cells accumulate, forming a matured
biofilm with three-dimensional scaffolds. Loose cells are then sloughed off from the matured biofilm and convert into planktonic cells, which again attach to a new surface preconditioned by food
particles or substrates completing the life cycle of a biofilm.
influencing Factors
The level of attachment of microorganisms to food-processing equipment surfaces is found to
be directly related to contact time. As the contact time is prolonged, more cells attach to the
table 7.1 Role of extracellular Polymeric Substances (ePS) in a Biofilm
Function
Relevance for Biofilms
Adhesion
Initial attachment and colonization to solid surface, long-term
attachment of biofilm to surfaces
Aggregation of cells
Bridging of cells, high cell density, cell-to-cell communication
Cohesion of biofilms
Forms hydrated polymer network, mechanical stability, threedimensional configuration, biofilm architecture
Retention of water
Maintains hydrated microenvironment, resistance to desiccation
Protective barrier
Resistant to antimicrobial agents, antibiotics, biocides, sanitizers,
and host defense (during infection)
Sorption of organic and
inorganic compounds
Accumulation of nutrients and toxic metal ions
Enzymatic activity
Digestion of exogenous macromolecules and EPS for nutrients
Nutrient source
Source of carbon, nitrogen, and phosphorous-containing
compounds
Genetic exchange
Horizontal gene transfer between biofilm cells
Source: Adapted from Flemming, H.-C. and Wingender, J., Nat. Rev. Microbiol., 8, 623–633, 2010.