In the microbial world, cooperation between multiple species is a novel way to combat the pressures of nutrient limitation, chemical damage, and other struggles that microorganisms constantly face. As such, dynamic and complex communities are frequently formed. An example of this is the biofilm—a thick, highly structured aggregate of microorganisms that often forms in aqueous environments, particularly along surfaces or at water-air interfaces. Biofilms owe their success to the fact that, under the correct conditions, many bacteria can secrete proteins, polysaccharides, and other materials into their immediate environment. When enough organisms accumulate in one location, this effect compounds to produce a dense network of extracellular polymeric substances, often termed EPS. The result is the formation of microscopic labyrinths, containing porous water channels, enzymes to degrade biopolymers into communal nutrients, even reseviors of naked DNA from lysed cells that other bacteria can uptake into their own genome. In this 2010 publication, researchers at the University of Duisburg-Essen performed an in-depth analysis of the structure of biofilm EPS and how they mediate the lives of the organisms within them.

Interestingly, microbial cells only make up 10% of a biofilm’s total mass, with the other 90% contributed by the extracellular matrix, a great deal of which is composed of water. The consistency of biofilms is often described as that of “stiff water,” despite the slimy texture they display when viewed macroscopically. The EPS, particularly exopolysaccharides, account for this rigidity through a variety of bonding forces. Imagine the exopolysaccharides, long chains made of one or more types of sugar molecule, woven together in a dense, mesh-like structure. In this scenario, some functional groups on the exopolysaccharides, such as alcohol (-OH) groups, will have favorable interactions with each other, and bond to provide mechanical strength biofilm. Other groups, such as two negatively charged functional groups, will repel each other—this also provides support, as it keeps sections of the biofilm’s architecture from caving in. Sometimes positively charged particles called cations, such as Ca²⁺, will form supportive crossbridges between polymers, allowing biofilms to grow to thicknesses of almost 300 µm. While this is only about a third of a millimeter, it is an impressive feat considering that bacteria themselves are on average only 2 µm in diameter (.002 mm). Due to the structural integrity afforded by EPS, biofilms can withstand environments of extreme shearing forces, such as on waterfall impact points.

Despite extensive research into the nature of biofilms, much about the EPS remains a mystery. Isolating and identifying EPS compounds are particularly difficult, as each polymer is extensively bound to all others, as described above—removing one component tends to have a house-of-cards type of effect and cause part of the biofilm to collapse. What is known about the overall composition of the matrix is perhaps even more puzzling—even though different types of bacteria secrete different types of polymers, the most populous species does not necessary correspond to the largest component of the matrix (just because species A is dominant does not mean that the polymers created by A will be dominant.) Additionally, structure can vary immensely throughout a biofilm, with different types of EPS found in different locations, and irregular changes in texture, varying from filamentous and fluffy to smooth and flat. Despite this variety, all biofilms seem to aspire to a common goal: holding bacteria immobile in order to amass a large and thriving community.