Microbiology, 2021

9.1 • How Microbes Grow 343
microorganisms embedded in a matrix interspersed with open water channels. The extracellular matrix
consists of extracellular polymeric substances (EPS) secreted by the organisms in the biofilm. The
extracellular matrix represents a large fraction of the biofilm, accounting for 50%–90% of the total dry mass.
The properties of the EPS vary according to the resident organisms and environmental conditions.
EPS is a hydrated gel composed primarily of polysaccharides and containing other macromolecules such as
proteins, nucleic acids, and lipids. It plays a key role in maintaining the integrity and function of the biofilm.
Channels in the EPS allow movement of nutrients, waste, and gases throughout the biofilm. This keeps the cells
hydrated, preventing desiccation. EPS also shelters organisms in the biofilm from predation by other microbes
or cells (e.g., protozoans, white blood cells in the human body).
Biofilm Formation
Free-floating microbial cells that live in an aquatic environment are called planktonic cells. The formation of a
biofilm essentially involves the attachment of planktonic cells to a substrate, where they become sessile
(attached to a surface). This occurs in stages, as depicted in Figure 9.17. The first stage involves the attachment
of planktonic cells to a surface coated with a conditioning film of organic material. At this point, attachment to
the substrate is reversible, but as cells express new phenotypes that facilitate the formation of EPS, they
transition from a planktonic to a sessile lifestyle. The biofilm develops characteristic structures, including an
extensive matrix and water channels. Appendages such as fimbriae, pili, and flagella interact with the EPS, and
microscopy and genetic analysis suggest that such structures are required for the establishment of a mature
biofilm. In the last stage of the biofilm life cycle, cells on the periphery of the biofilm revert to a planktonic
lifestyle, sloughing off the mature biofilm to colonize new sites. This stage is referred to as dispersal.
Figure 9.17
Society for Microbiology)
Stages in the formation and life cycle of a biofilm. (credit: modification of work by Public Library of Science and American
Within a biofilm, different species of microorganisms establish metabolic collaborations in which the waste
product of one organism becomes the nutrient for another. For example, aerobic microorganisms consume
oxygen, creating anaerobic regions that promote the growth of anaerobes. This occurs in many polymicrobial
infections that involve both aerobic and anaerobic pathogens.
The mechanism by which cells in a biofilm coordinate their activities in response to environmental stimuli is
called quorum sensing. Quorum sensing—which can occur between cells of different species within a
biofilm—enables microorganisms to detect their cell density through the release and binding of small,
diffusible molecules called autoinducers. When the cell population reaches a critical threshold (a quorum),
these autoinducers initiate a cascade of reactions that activate genes associated with cellular functions that are