Microbial Biofilms: Structure, Function, and Environmental Impact

Keywords: Microbial biofilms, Extracellular Polymeric Substances (EPS), biofilm architecture, water treatment, environmental microbiology, IUPAC definition.

A microbial biofilm is a structured community of microorganisms adhering to a surface and encased within a self-produced protective matrix. This matrix is primarily composed of extracellular polymeric substances (EPS), which form a complex, three-dimensional architecture. The EPS include a mixture of polysaccharides, proteins, lipids, and extracellular DNA secreted by the resident microbes into their immediate environment. Functionally, these EPS act as a biological "glue," providing the structural integrity that holds the entire biofilm consortium together. Beyond the organic and microbial components, biofilms often incorporate inorganic elements such as mineral particles, or in medical contexts, tooth or bone structures, which provide physical support and can sometimes serve as nutrient sources.

The presence of microbial biofilms is ubiquitous in nature, covering a vast array of surfaces from soil particles to industrial piping. Their metabolic activity can significantly alter their immediate microenvironment. For instance, biofilms can change the surface charge of mineral particles, influence the aggregation of soil and sediment, and modify local chemical conditions. These modifications include shifts in pH, redox potential (Eh), and the concentrations of dissolved metals and other constituents in the surrounding solution, thereby playing a crucial role in biogeochemical cycling.

Biofilms thrive in a remarkably wide spectrum of environments, from medical devices to extreme natural habitats, often exhibiting complex and colorful morphologies. A prime example is shown in Figure 1, which depicts a vibrant biofilm community in an extreme geothermal environment at Yellowstone National Park. The biofilms associated with Yellowstone's various hot springs display distinct characteristics that vary systematically with factors like water temperature, chemical composition, underlying geology, and hydrological flow conditions. These unique ecosystems have been the subject of intensive scientific research for many decades, serving as model systems for understanding microbial life in extreme environments.

Figure 1. A photograph of a microbial biofilm (in the form of an extensive microbial mat) in Yellowstone. Shown in this image are conical, columnar, and lilypad stromatolites shaped by cyanobacteria (Leptolyngbya) that form in some of the quiet hydrothermal pools [1]. Silicate precipitates are present, in association with the biofilm. This image represents some of the complex 3-dimensional architecture that can be associated with a biofilm. Source: Photo by P. Maurice

Biofilms and Interdisciplinary Research. Biofilms represent a quintessential example of the confluence between fundamental science, applied technology, and societal challenges, particularly in the context of water. On one hand, they can create significant and expensive problems, such as when they clog membranes in water purification systems or contribute to biofouling. On the other hand, their metabolic capabilities can be harnessed and purposely cultivated for beneficial use in various water and wastewater treatment technologies, including bioreactors. Consequently, biofilm research often demands interdisciplinary collaboration, bringing together experts from fields such as microbiology, chemistry, geology, biomedical engineering, and even social sciences to address the full scope of their impacts and applications.

The IUPAC Definition of a Biofilm.To standardize scientific communication, the International Union of Pure and Applied Chemistry (IUPAC) provides a formal definition of a biofilm. IUPAC defines it as an “aggregate of microorganisms in which cells adhere to each other and/or to a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance (EPS)." This definition is supplemented by two clarifying notes that provide further depth and context to the understanding of these complex structures, emphasizing their dynamic nature and composition.

The first note states that a biofilm is a fixed system that can be adapted internally to environmental conditions by its inhabitants, highlighting their resilience. The second note specifies that the self-produced matrix of EPS, also commonly referred to as slime, is a polymeric conglomeration generally composed of extracellular DNA, proteins, and polysaccharides in various configurations. This precise terminology helps distinguish true biofilms from other forms of microbial aggregates and underscores the critical role of the EPS matrix in biofilm physiology and ecology.