Most people think of bacteria as solitary cells swimming around in search of nutrients. Now scientists are realizing that this viewpoint misses the true nature of these microorganisms. The vast majority actually live in highly organized communities called biofilms, which can contain many different species. These groups of bacteria can have far-reaching consequences, from slowing down ships to aiding water purification. Scientists are now taking advantage of new technologies to understand the behavior of these bacterial societies, and to attempt to control them.
What are biofilms?
Bacterial films can form on nearly any surface that is in contact with water: the insides of pipes, the outsides of boats, and even the surfaces of your teeth and the lining of your intestines. Microbes floating in water often come into contact with a surface without sticking, but sometimes these cells grow structures that allow them to permanently adhere. They excrete “slime,” technically known as glycocalx, which traps nutrients. This initial colony grows by reproducing to form daughter cells and trapping other microbes that become part of the biofilm. The result is not a uniform blob of identical cells. Instead, the cells take on different roles, including the initial free-swimming bacteria, the scaffolding-producing cells, and, eventually, a third type of cell that send out spores to start new biofilms.
Why would a lone bacteria swimming through a liquid want to switch to a sedentary life? There are numerous advantages to being part of a microbial community, especially improved access to food and protection from antibacterials. By forming a wide network with the scaffolding, known as the extracellular matrix, the bacteria can collect nutrients from a larger area. Moreover, in mixed-species biofilms, the waste products of one type of bacteria can be used by another species. The extracellular matrix also offers a protective layer against antimicrobial chemicals. Like a school of fish being attacked by a shark, even if the outer cells in the colony are killed, the inner cells can survive or even develop resistance. In addition, the diversity of bacteria in a colony can make it difficult for a single treatment to destroy the biofilm.
The consequences from these bacterial societies are wide-ranging. The attachment of these films to the outsides of ships, known as biofouling, is a major problem for the international shipping industry. Biofilms on ships slow the ships down by causing drag, causing increased fuel consumption. These unwanted travelers can only be removed through heavy-duty scraping or harsh chemicals. Biofilms are also a major source of corrosion in the oil industry. In many older oil wells, much of the liquid extracted is water, which makes the insides of pipelines an ideal growth medium for biofilms. On the surfaces of water pipes, these biofilms can be a serious obstacle to producing clean drinking water. Hospital-acquired infections are the fourth leading cause of death in the United States, behind heart disease, stroke, and cancer. Biofilms are a major suspect in such cases, especially since they are resilient enough to live for months on gloves, floors, surgical instruments, and other hospital surfaces. People can become infected through open wounds or implanted devices, like heart valves and artificial hips. On the other hand, biofilms have proven to be very useful in water purification, so these seemingly troublesome communities can have positive applications as well.
How do scientists study biofilms?
Why did it take so long for scientists to recognize the true nature of these bacterial communities? For many decades, biologists followed the standard technique of isolating and purifying an organism to study it. This technique favored organisms that could grow well floating in liquid, but could not detect the diverse congregations of bacteria that would typically form on surfaces. More recent studies have found that the vast majority of bacteria in streams and water pipes, for instance, live in these colonies on surfaces, rather than in the water itself. New advances in microscopy and other scientific tools are only now allowing scientists to understand how these bacteria can band together at a detailed level. For instance, Professor Kolter’s lab at Harvard is studying how the cells switch between their various roles in different parts of the biofilm. Three different fluorescent tags cause the bacteria in to glow when viewed in a microscope, depending on whether they are in their free-swimming, scaffold-producing, or spore-producing stage. The lab can also create mutants of the bacteria that lack a crucial protein, so that they are unable to produce the scaffold material, for instance. These mutants can lead to biofilms that look completely different (such as smooth instead of wrinkly), which provides evidence about the function of these proteins.
In addition to gathering new information about how biofilms form and grow, scientists are developing technologies to control the spread of unwanted biofilms. One method consists of making surfaces to which bacteria don’t stick. For instance, Krystyn van Vliet and Michael Rubner, two materials science professors at MIT, recently found a way to make surfaces “pillowy,” instead of rigid, so that the bacteria can’t adhere. Prof. Whitesides’ lab at Harvard is also examining how to modify surfaces at a microscopic level, so that bacteria are unable to adhere. Researchers in his group are trying to understand whether bacteria prefer to attach to smooth or rough surfaces.
On the other hand, biofilms can also be useful in many situations. For instance, in organic wastewater treatment plants, they can remove many types of pollutants from the water. Dr Bruce Rittman at the Biodesign Institute Center for Environmental Biotechnology has developed a biofilm reactor that replaces the chlorine in toxic chemical pollutants with hydrogen, making them harmless. The bacteria in your intestine can also be a crucial part of the digestive process (see the November 2007 SITN Flash for more information).
Even with the new advances in microscopy techniques, scientists are only beginning to understand the behavior of colonies of biofilms that contain just a single species. The interactions between different species, which often occurs in the wild, is still an area filled with the potential for new discoveries. The potential rewards are enormous: improved efficiency in a wide range of industrial process: reduced drag on ships, prevention of hospital-acquired infections, and increased access to clean water, among others.
–Naveen Sinha, Harvard School of Engineering and Applied Sciences