The term “slime wall” may evoke images from fiction, but it accurately describes a widespread biological phenomenon known as a biofilm. A biofilm is a structured community of microorganisms that adhere to each other and to a surface, all encased in a self-produced slimy matrix. These microbial cities thrive in nearly any environment where moisture is present, from the inside of a water pipe to the surface of a human tooth.
The Building Blocks of Slime
A biofilm’s structure consists of two primary components: the living microorganisms and the substance they create, called the Extracellular Polymeric Substance (EPS). The living residents are most often bacteria, but can also include a diverse mix of fungi, algae, and other microbes. In fact, these microbial cells typically only make up 10-25% of the biofilm’s total mass.
This EPS is a complex and glue-like material that acts as the scaffolding for the microbial city. It is primarily composed of long-chain sugars called polysaccharides, but also contains a mixture of proteins, lipids, and even extracellular DNA (eDNA). The EPS is not just structural; it also functions as a protective barrier, shielding the inhabitants from environmental threats.
The specific composition of the EPS can vary significantly depending on the types of microbes present and the local environmental conditions. For example, the EPS in a biofilm found in acidic mine drainage will have a different chemical makeup than the EPS in dental plaque. This adaptability allows biofilms to thrive in a wide array of environments, from industrial settings to the human body.
How a Slime Wall Is Formed
The formation of a biofilm begins when free-floating, or planktonic, microorganisms encounter a suitable surface in a moist environment. This initial attachment is often weak and reversible, representing the first tentative step toward colonization.
If conditions are favorable, the microbes transition to a permanent, irreversible attachment. They begin to multiply and secrete the EPS matrix. During this stage, microbes communicate using chemical signals, a process known as quorum sensing, to coordinate their activities and attract more microbes to the growing community.
As more cells join and contribute to the EPS, the biofilm matures. It develops into a complex, three-dimensional structure that can resemble mushrooms or towers, with channels for water and nutrients to flow through. The final stage of the lifecycle is dispersal, where some microbes detach from the main colony to seek out new surfaces and begin the process again.
Real-World Examples of Slime Walls
In the home, the plaque that forms on teeth is a classic example of a bacterial biofilm, a community of hundreds of bacterial species. The pink or black gunk that can accumulate in shower drains or on grout is another common household biofilm. Even the slimy interior of a dishwasher or washing machine can harbor these microbial communities.
In the natural world, the slippery coating on rocks in a stream is a biofilm, often composed of a mix of algae and bacteria that play a role in nutrient cycling. Fossilized biofilms, known as stromatolites, provide some of the earliest evidence of life on Earth, dating back over three billion years.
Biofilms have significant implications for medicine and industry. They can form on medical implants like catheters, prosthetic joints, and heart valves, leading to persistent infections. It is estimated that biofilms are involved in up to 80% of all microbial infections in the body. In industrial settings, they can clog water pipes, foul ship hulls, and contaminate food processing surfaces.