Bacterial Biofilms – Cities of Slime
Microbiologists Must Rethink Bacterial Growth and Bacterial Ecology
"Microbiologists have traditionally focused on free-floating bacteria growing in laboratory cultures; yet they have recently come to realize that in the natural world most bacteria aggregate as biofilms, a form in which they behave very differently. As a result, biofilms are now one of the hottest topics in microbiology." (Potera 1996)
Simply put, biofilms are a collection of microorganisms surrounded by the slime they secrete, attached to either an inert or living surface. The most familiar biofilms form in the human mouth on the teeth. A trip to the dentist or a thorough brushing leaves the teeth feeling slick and clean but in just a few hours, certainly overnight a rough coating (plaque) can be felt forming on the teeth.
In reality, biofilms exist wherever surfaces contact liquids from the slime on river rocks to the plaque on human teeth and from the inside of a vase that has held flowers for a week to contact lenses. They can be found clogging the insides of pipes and fouling the bottom of seagoing vessels. They are also found at times inside the human body coating objects ranging from catheters to implants.
Rethinking Bacterial Growth and Ecology
Biofilms were first discovered in 1978 by microbiologist William Costerton. Costerton and his team wondered why there were so few bacteria floating in the clear waters of a frigid stream in British Columbia, while there were many billions of them nestled in the nooks and crannies of the rocks on the bottom of the stream.
Upon closer examination, Costerton found that the bacteria were forming very complicated structures (bacterial “cities”) which were encased in a slippery gel-like substance. Costerton’s published research coined the term biofilm and necessitated a paradigm shift in the way microbiologists view the growth and ecology of bacteria.
Single-celled organisms generally exhibit two distinct modes of behavior. The first is the familiar free floating, or planktonic form in which single cells float or swim independently in some liquid medium. The second is an attached state in which cells are closely packed and firmly attached to each other and usually form a solid surface.
Bacteria living in a biofilm usually have significantly different properties from free-floating bacteria of the same species, as the dense and protected environment of the film allows them to cooperate and interact in various ways. One benefit of this environment is increased resistance to detergents and antibiotics, as the dense extracellular matrix and the outer layer of cells protect the interior of the community.
Biofilms are usually found on solid substrates submerged in or exposed to some liquid although they can form as floating mats on water and also on the surface of leaves, particularly in high humidity climates. Given sufficient resources for growth, a biofilm will quickly grow to be macroscopic (visible to the unaided eye). Biofilms can contain many different types of microorganism, e.g. bacteria, archaea, protozoa, fungi and algae; each group performing specialized metabolic functions. However, some organisms will form monospecies (single species) films under certain conditions.
Not only do the bacteria in biofilms live together, they chemically communicate with each other. Each bacterium releases autoinducers (chemical signaling molecules) into the environment. When the local concentration of autoinducers gets high enough, the bacteria flip from solitary mode to group behavior. Known as quorum sensing, this chemical “counting of heads” is how bacteria coordinate their actions in biofilms.
Biofilms and Infectious Human Diseases
Biofilms have been found to be involved in a wide variety of microbial infections in the body. One estimate puts the rate at 80% of all infections. Biofilms have been implicated in such common problems as urinary tract infections, catheter infections, middle-ear infections, formation of dental plaque and gingivitis, and coating contact lenses.
Biofilms can also be formed on the inert surfaces of implanted devices such as catheters, prosthetic cardiac valves, joint prostheses, and intrauterine devices. Biofilms on prostheses (implants) are now recognized a serious and growing health problem. It is estimated that bacterial infections hit 2 percent to 4 percent of all implants. Of the 2 million hip and knee replacements performed worldwide each year, 40,000 become infected. More than a third of these infections lead first to amputation and then usually to death.
Biofilms may be harnessed for constructive purposes. Many sewage treatment plants include a treatment stage in which waste water passes over biofilms grown on filters. The bacterial biofilms on these filters extract and digest organic compounds from the sewage.
Biofilms can also help eliminate petroleum oil from contaminated marine systems. The oil is eliminated by the hydrocarbon-degrading activities of microbial communities, in particular by a remarkable recently discovered group of bacterial specialists, the so-called hydrocarbonoclastic bacteria (HCB).
As microbiologists continue to unlock the mysteries of the cities of slime called biofilms, the biological community is forced to shift its paradigms regarding bacterial growth and ecology.
Source:
Potera, C. “Biofilms Invade Microbiology”, Science 273, pp. 1795-1797 (September 27, 1996).
Biofilms are Bacterial Communities
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