Biofilm formation and dynamic behavior

Biofilms are populations of microorganisms that form following the adhesion of bacteria , algae, yeast , or fungi to a surface. These surface growths can be found in natural settings, such as on rocks in streams, and in infections, such as on catheters. Both living and inert surfaces, natural and artificial, can be colonized by microorganisms.

Up until the 1980s, the biofilm mode of growth was regarded as more of a scientific curiosity than an area for serious study. Then, evidence accumulated to demonstrate that biofilm formation is the preferred mode of growth for microbes. Virtually every surface that is in contact with microorganisms has been found to be capable of sustaining biofilm formation.

The best-studied biofilms are those formed by bacteria. Much of the current knowledge of bacterial biofilm comes from laboratory studies of pure cultures of bacteria. However, biofilm can also be comprised of a variety of bacteria. Dental plaque is a good example. Many species of bacteria can be present in the exceedingly complex biofilm that form on the surface of the teeth and gums.

The formation of a biofilm begins with a clean, bacteria-free surface. Bacteria that are growing in solution (planktonic bacteria ) encounter the surface. Attachment to the surface can occur specifically, via the recognition of a surface receptor by a component of the bacterial surface, or nonspecifically. The attachment can be mediated by bacterial appendages , such as flagella, cilia, or the holdfast of Caulobacter crescentus.

If the attachment is not transient, the bacterium can undergo a change in its character. Genes are stimulated to become expressed by some as yet unclear aspect of the surface association. This process is referred to as auto-induction. A common manifestation of the genetic change is the production and excretion of a large amount of a sugary material. This material covers the bacterium and, as more bacteria accumulate from the fluid layer and from division of the surface-adherent bacteria, the entire mass can become buried in the sugary network. This mass represents the biofilm. The sugar constituent is known as glycocalyx , exopolysaccharide, or slime.

As the biofilm thickens and multiple layers of bacteria build up, the behavior of the bacteria becomes even more complex. Studies using instruments such as the confocal microscope combined with specific fluorescent probes of various bacterial structures and functional activities have demonstrated that the bacteria located deeper in the biofilm cease production of the slime and adopt an almost dormant state. In contrast, bacteria at the biofilm's periphery are faster-growing and still produce large quantities of the slime. These activities are coordinated. The bacteria can communicate with one another by virtue of released chemical compounds. This socalled quorum sensing enables a biofilm to grow and sense when bacteria should be released so as to colonize more distant surfaces.

The technique of confocal microscopy allows biofilms to be examined without disrupting them. Prior to the use of the technique, biofilms were regarded as being a homogeneous distribution of bacteria. Now it is known that this view is incorrect. In fact, bacteria are clustered together in "microcolonies" inside the biofilm, with surrounding regions of bacteria-free slime or even channels of water snaking through the entire structure. The visual effect is of clouds of bacteria rising up through the biofilm. The water channels allow nutrients and waste to pass in and out of the biofilm, while the bacteria still remain protected within the slime coat.

Bacterial biofilms have become important clinically because of the marked resistance to antimicrobial agents that the biofilm bacteria display, relative to both their planktonic counterparts and from bacteria released from the confines of the biofilm. Antibiotics that swiftly kill the naked bacteria do not arm the biofilm bacteria, and may even promote the development of antibiotic resistance . Contributors to this resistance are likely the bacteria and the cocooning slime network.

Antibiotic resistant biofilms occur on artificial heart valves, urinary catheters, gallstones, and in the lungs of those afflicted with cystic fibrosis, as only a few examples. In the example of cystic fibrosis, the biofilm also acts to shield the Pseudomonas aeruginosa bacteria from the antibacterial responses of the host's immune system . The immune response may remain in place for a long time, which irritates and damages the lung tissue. This damage and the resulting loss of function can be lethal.

See also Anti-adhesion methods; Antibiotic resistance, tests for; Bacterial adaptation