Biofilms: Bacteria’s Elixer of Survival

In terms of biomass and prevalence in various environments, bacteria are the most successful forms of life (1). Among bacteria’s numerous remarkable survival mechanisms are biofilms. From human teeth to industrial sites, biofilms can form wherever there is water and sufficient nutrients (2). Due to this powerful ability to survive, biofilms significantly affect individual organisms, the environment, industry, and beyond. To humans, they can even pose a severe threat due to their remarkable resistance to antibacterial agents (1).

Biofilms are the slimy layers formed when aggregates of bacteria encase themselves in a hydrated matrix of polysaccharide and protein. (3). They were among the first to be discovered during the initial development of microbiology—the “animalculi” that Anton von Leeuwenhoek observed with a microscope was plaque biofilm from teeth (4). However, it was not until the 1970s that biofilms became of substantial academic interest (4). In the primitive stage of Earth’s formation, biofilms played a crucial role in helping bacteria survive such extreme heat and acidity (1). Even today, in similar environments such as hot springs, bacteria can gradually adapt to their surroundings thanks to the encompassing biofilms. One part of the biofilm that aids survival is special channels in which nutrients circulate (5). But the more fundamental mechanism behind such remarkable vitality is the phenotypic changes in biofilm bacteria: according to the situation, certain genes are selectively expressed in favor of prolonged survival (1).

Electron micrographing at 21,850 x magnification, showing the deatils of the surface of a biofilm, a hydrated matrix of polysaccharide and protein formed by aggregates of bacteria.

Biofilms have a complementary relationship with plankton—dissemination and persistence in the dormant stage are more conducive to the planktonic mode, whereas growth itself favors the biofilm mode (1). After recognizing that the current surroundings can provide sufficient nutrients, the nomadic planktonic bacteria first attach to a compatible surface (6). They then exude extracellular polymeric substances (EPS) that hold the bacteria together as a community (7). Once inside the slimy EPS, the bacteria are free to grow, divide, and even disperse individual cells throughout the water. Within their customized microniches, biofilm bacteria can live in a primitive homeostatic and circulatory system. A coat of biofilm can thus help bacteria thrive in unfavorable environments rather than constantly stay in a dormant state.

Bacteria are able to alternate between the mutually exclusive forms of plankton and biofilm because different genes are activated in each situation (7). Activated genes give rise to amino acids and proteins, which affect the bacteria as a whole; however, not every part of the genetic code is expressed simultaneously (7). This pattern is verified by the sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) technique, which reveals which proteins the cells are producing at a given point in time (7). Proteins run through an electric current in an SDS-PAGE gel are separated according to size and electric charge (8). Comparison of the protein compositions of plankton and biofilm cells indicates which parts of the genome are selectively up-regulated or down-regulated (7). Albeit drawn from the same genetic basis as their plankton counterparts, biofilms exhibit unique protein expression levels that help bacteria live in disadvantageous situations.

To survive, biofilms use a genetic tactic of random mutation as well. A biofilm serves as a safer form for bacteria that are threatened in the plankton stage, but one kind of biofilm alone does not suffice for every case (6). Instead, different nutrient uptake levels within each microniche give rise to random genetic mutations throughout the biofilm (6). As a result, bacteria have even more genotypes to choose from. Such genetic diversity lets bacteria cope with various environments by varying the precise composition of the biofilm.

Although greatly beneficial for bacterial survival, biofilms can have detrimental effects on other organisms. They affect industry and the environment by clogging drains or contaminating drinking water and food (2). More immediately, some forms can even threaten the lives of humans. Biofilms are associated with many human diseases including periodontitis in the teeth and cystic fibrosis in the lungs, as well as various nosocomial infections acquired from hospitals (3). Other kinds of bacteria can also attach to implanted medical devices and persist through the formation of a biofilm (3). One man, for example, suffered from pacemaker endocarditis and recovered only after removal of the pacemaker (3). Endocarditis, the inflammation of the heart valve, occurs when the infecting organism enters the bloodstream and settles in the heart (9). In the United States, it targets approximately 19,000 people each year. The infectious organism is usually “streptococci (‘strep’), staphylococci (‘staph’), or species of bacteria that normally live on body surfaces” (9). This case is notable in that drugs alone could not cure his condition; instead, the specific source of bacteria had to be eliminated. The infected pacemaker did, in fact, turn out to have “localized accretions of coccoid bacteria,” in this case Staphylococcus aureus (3).

Biofilms have been found to preserve the bacteria that form them, but often exacerbate health problems for humans. Above are Staphylococcus aureus biofilms.

This strong resistance to antibiotics makes biofilms difficult to target by medical means. Antibiotics can attack the planktonic cells that are released from the biofilm (4). But the bacterial colony surrounded by biofilm is at least 500 times more resistant to antibiotics (1). Even after the planktonic bacteria dispersed throughout the body are eliminated, the biofilm, the source of the infectious disease, persists. Such a resistance is due to several factors: the failure of the antibacterial agent to fully penetrate into the biofilm, the existence of slow-growing cells in the biofilm that are not susceptible to antibiotics, and the protective phenotype of the biofilm itself (4). Researchers are thus attempting to target these characteristics of the biofilm (3).

These very biofilms, however, can also facilitate water treatment (7). Biofilms naturally form biobarriers to prevent contamination of soil and groundwater, and researchers went a step further to employ them for our everyday needs (2). John Snow found out in the 1850s that sewage water played a significant role in causing cholera and typhoid epidemics—only after he blocked the water supply from sewage regions did the disease dampen (10). Especially when only 0.06% of the wastewater by weight is waste material, efficient water treatment has become an important task for human health and the environment (10). There are now many methods for treating wastewater including the use of chlorine and ultraviolet light (10). Unfortunately, these methods have several drawbacks: chlorine treatment, for example, leaves color and odors. In contrast, biofilms present a more “biologically stable” system for purifying water (7). Although biofilms have threatened many lives, their remarkable ability to withstand environmental disturbances such as antibiotics could be applied for practical use. The only difference is that in order to grow and survive, they will consume the unwanted 0.06% of waste material in wastewater. Even in the 1860s, biofilms were part of sand filter treatment methods—the sand functioned as surfaces onto which the biofilm would attach and grow (7). Based on the same idea, bioremediation utilizes biofilms for clearing oil spills “with natural, non-harmful means” (7).  Perhaps biofilms will soon benefit humans in fields beyond those of the aquatic environment.

As the most successful life forms on earth, bacteria pervade the planet and our daily lives. The source of such survival and longevity is the slimy yet crucial biofilm. Biofilms can be detrimental to human health and the environment because they protect harmful bacteria that would otherwise be killed. Especially with strong resistance to antibacterial agents and the ability to withstand other unfavorable conditions, biofilms provide an additional problem to the targeting of bacteria. These characteristics of biofilms, however, could also be exploited for human advantage, for example by filtering wastewater and clearing oil spills. Future research will determine whether biofilms will prove to be adverse or beneficial to humans.

References

1. J. W. Costerton et al., Annual Review of Microbiology. 49, 711-745 (1995).
2. What is biofilm? Available at http://www.erc.montana.edu/CBEssentials-SW/bf-basics-99/bbasics-01.htm (13 May 2009).
3. P. S. Stewart, J. W. Costerton, The Lancet. 358, 135-138 (2001).
4. J. W. Costerton, P. S. Stewart, E. P. Greenberg, Science. 284, 1318-1322 (1999).
5. D. DeBeer, P. Stoodley, Z. Lweandowski, Biotech. Bioeng. 44, 636 (1994).
6. R. Kolter, E. P. Greenberg, Nature 441, 300-302 (2006).
7. A. B. Cunningham, R. J. Ross, Biofilms: The Hypertextbook. (2001-2005).
8. SDS-PAGE (PolyAcrylamide Gel Electrophoresis). Available at http://www.bio.davidson.edu/
courses/genomics/method/sdspage/sdspage.html (2001).
9. Endocarditis. Available at http://symptomchecker.about.com/od/Diagnoses/endocarditis.htm.
10. Water Environment Federation, Following the Flow: An Inside Look at Wastewater Treatment. (2009).