Biofilms & Dead Zones: The Microbe-Environment Connection

Biofilms as Water Purifiers

What's a Biofilm

No, The Life of Emile Zola is not an example of a biofilm. A biofilm is a special type of microbial community. We have mentioned on several occasions that most microorganisms are single-celled. That immediately raises the impression that an individual bacterium may spend its life suspended in the water of a lake or in the intestine of an animal essentially isolated from the other organisms around it, carrying out its independent existence like some microbial Flying Dutchman. While this may be true in some instances, it is believed that in nature most microorganisms form aggregates or clumps. The aggregates may consist of millions of cells of a single species or a mixture of two or more species. There are a number of advantages for a microorganism to become a member of an aggregate. An aggregate tends to trap nutrients, and the members are protected from predators and toxic chemicals. Most significantly, members of such an aggregate may improve their survivability through various symbiotic relationships that involve cross feeding, exchange of genetic information and communication.

Cross-feeding means a bacterial cell may overproduce a certain nutrient (an amino acid or a vitamin, for example) that leaks into the surrounding medium and is used by a second bacterium. In exchange the second organism may release a different nutrient that the first organism can use. The end result is that both species of bacteria can survive while neither might alone.

Microbial aggregates can be freely suspended in a liquid habitat such as lake or river water or more often, attached to a surface. When attached to a surface the accumulation of aggregates is referred to as a biofilm. Biofilms are frequently found on a wide variety of natural and artificial surfaces that are continually wet, such as boat hulls, the inner surfaces of pipes and cooling towers, food manufacturing equipment, submerged rocks, teeth, contact lenses, the lung tissues of cystic fibrosis patients and implanted medical devices like catheters and artificial joints. In each of these instances, individual bacteria initially attach themselves to the surface through various adhesins, molecules that have adhesive properties. Contact with the surface initiates rapid reproduction, resulting in the formation of a microcolony. Chemical signaling among members of the microcolony, known as quorum sensing, then triggers the establishment of a mature biofilm consisting of scattered colonies each containing millions of members (Figure 3.2).

Members of the biofilm release large amounts of sticky polysaccharides which form a semi-rigid matrix that stabilizes the biofilm. Water channels between the colonies in the matrix function as conduits for nutrients and waste materials. In many natural settings, other bacteria, algae (if light is available), protozoa, worms, insects and other higher forms may join the bacteria that began the biofilm. A biofilm may become several millimeters thick, becoming what is referred to as a microbial mat. These mats sometimes can build up to nearly a meter or more thick to become stromatolites (Figure 3.3). Fossilized stromatolites have been found in several regions of the world and some appear to be over 3 billion years old, leading to the conclusion that life on Earth learned to socialize very early.

In the case of a boat hull, barnacles, mussels and other marine organisms also may become part of the biofilm. Biofilms can frequently lead to serious problems. The buildup of organisms on the boat hull eventually adds weight and increases resistance to movement through water, impairing the operation of the boat and requiring periodic removal. A similar situation often occurs with water intakes for power plants and other industrial facilities. The accumulation of organisms in the pipe can significantly reduce its carrying capacity. To prevent the formation of biofilms, so-called anti-fouling paints can be applied to boat hulls, and strong disinfectants may be flushed periodically through water intakes to discourage microbes from starting a biofilm. The application of anti-fouling paints and disinfectants must be carefully monitored, for they usually contain toxic chemicals which when released into the surrounding water can adversely affect plants and animals in adjacent habitats.

Biofilms also are involved in certain aspects of human health. As mentioned above, biofilms can occur in the body on implanted medical devices such as artificial joints or catheters as well as natural surfaces like teeth. As much as 65 percent of human bacterial infections may involve biofilms. Biofilms afford pathogens extra protection against the body's normal defense mechanisms such as antibodies and phagocytosis, as well as against any introduced antimicrobial drugs, allowing the organisms to grow unhindered. The treatment of infections associated with biofilms is clearly a difficult medical challenge.

However, bofilms can be applied to solving a number of environmental problems. A bed of gravel or plastic pieces, for example, on which a biofilm of appropriate organisms has formed, can remove pollutants from water that flows through the bed. The microorganisms in the biofilm use the pollutants as sources of energy and material, and the polysaccharide matrix traps heavy metals. This principle is used in applications such as aquarium filters and in the treatment of waste water and urban runoff.