By Dr. Kelly Reynolds, MSPH, PhD
The water industry has long been plagued by the effects of biofilm formation in water supplies, pipes, fittings and filters. Such biomasses lead to adverse taste, odor and possible health effects in the water and decrease the life of treatment equipment.
Scientists are just beginning to solve the mystery of how and why biofilms form. New evidence indicates that microorganisms behave in a highly ordered way, effectively communicating with one another to enhance their survival and proliferation.
Understanding the complex interactions of bacteria in the environment can provide new insights into prevention, control and treatment of bacterial infections.
Biofilm in nature
Biofilms form all around us; consider the layer of slime brushed from our teeth every morning! In our mouths and in water holding and distribution systems, these bacterial mats form rapidly and persist in just about any moist environment.
Typically biofilms form from a complex mixture of microbes, organic and inorganic matter and are considered a natural component of water distribution systems. Although expected, biofilms are not always welcomed.
Bacteria that form into biofilms may persist even in very low nutrient environments, such as distilled or ultrapure water. It is hypoththesized that as part of routine biofilm maintenance, bacterial cells will release from the mass and disperse to new locations.
This continual slow release of organisms can result in a persistent source of water contamination. Sheering forces related to water flow may result in increased biofilm sloughing from surfaces into the water supply, sometimes in high concentrations.
Large deposits of biofilms can impart bad tastes and odors to drinking water, increase the chlorine demand of water (decreasing chlorine residual during distribution) and cause increased pipe corrosion, introducing unwanted color to water. They can also impact the hydraulic flow of water supplies.
Adverse health concerns
Although corrosion, taste and odor issues are aesthetically unacceptable, the larger question remains: Do biofilm bacteria cause disease in humans? For years, researchers debated over the health risks related to general bacteria (aka, HPC or heterotrophic plate count bacteria) in drinking water.
General bacteria are known to grow to levels several orders of magnitude higher in POU drinking water treatment devices, relative to source water values. Consensus remains that in the absence of fecal contamination, there is no established relationship between general bacteria ingested in water and human health effects in the general population.
More research is needed, however, given increasing evidence of potentially harmful bacteria associated with biofilm growth in distribution systems and new surveillance data regarding Legionella occurrence and outbreaks linked to drinking water.1 The information is available from the Centers for Disease Control and Prevention.
This information is not yet established for biofilms. There is a growing body of evidence suggesting that Legionella in cooling towers and distribution systems may account for sickness, even in healthy individuals. However, given the low rate of reported diseases that have been traced back to a biofilm, the may be less concern that bacterial biofilms are a risk to the general population.
Immunocompromised individuals, however, are at increased risk. Pseudomonas aeruginosa, for example, causes infections in persons with compromised immunity and is a problem in hospital-acquired infection. It is also a well-known biofilm organism.
Pseudomonas bacteria produce a slimy material, known as polysaccharide, that covers and protects them from the detrimental effects of drying, UV light and other chemical disinfectants. The polysaccharide slime layer begins forming almost immediately, trapping nutrients and possibly sustaining hundreds of millions of bacteria.
Although most biofilm bacteria are considered harmless to healthy populations, human viruses and pathogenic Cryptosporidium have been found trapped within biofilms after contamination events. In addition, microbes damaged during conventional water treatment that collect in biofilms, can recover, repair and regrow in the system.
Organisms including Legionella (Legionnaire’s disease), Helicobacter pylori (primary cause of stomach ulcers), Mycobacterium (causing life-threatening infections in AIDS patients), Naegleria (free-living amoeba causing rare but fatal infections via recreational water exposures) and various fungi have all been associated with drinking water biofilms and adverse human health outcomes.
Cellular communications
Methods to eradicate biofilm in water treatment and distribution systems generally involve the use of high-pressure forces, heat shocks or the uses of disinfectants (chlorine, monochloramine, etc). However, these methods are temporary fixes and can result in a major sloughing of the biofilm and a potentially greater adverse effect. In the end, the biofilm will likely recur. New methods are needed to eliminate biofilm without creating a greater exposure potential.
Recently, scientists have discovered the ability of biofilm-associated bacteria to ’talk‘ to one another. These communications lead to increased biofilm formation in a cooperative effort among bacterial populations.
Specifically, researchers at Texas A&M University have published a series of papers describing cell-to-cell conversations between E. coli bacteria in biofilm communities.2 The formation of biofilms appears to be a very controlled process where bacteria signal neighboring cells of their presence, staking claim to their designated location.
The signaling mechanism is actually a chemical compound released into the cell’s local environment. This chemical material dictates how and when bacteria build biofilms. Interestingly, the bacteria appear to produce varying chemical signals relative to temperature, creating different communications to promote activity inside the human body or outside in the environment.
Understanding how these cellular communications change the behavior of bacterial populations and the subsequent formation of biofilms, could help to prevent and treat diseases in humans as well as improve drinking water quality.
Quorum sensing
Quorum sensing is the ability of bacteria to direct the behavior of other bacteria via signaling molecules. Bacteria use quorum sensing to coordinate efforts of survival and specific activity via hormone-like chemicals (known as autoinducers) and receptors, enabling cell-to-cell signaling. Autoinducers have been identified from E. coli, Salmonella, Pseudomonas, and other bacteria.
Autoinducers effect gene expression and thus promote cells to change their behavior. Such changes often fulfill a more cooperative role for the overall population- increasing virulence or promoting biofilm growth.3
Bioluminescent Vibrio bacteria (indigenous glow-in-the-dark marine water organisms) were among the first studied relative to quorum sensing where cell-to-cell communications initiate cooperative luminescence in the population. This cooperative expression of luminescence is necessary whereas a single cell expression would not be visible.
Cooperative interactions lead to actions that improve nutrient acquisition, survival and the development of environmental niches.4 DNA exchange, virulence factors, sporulation and biofilm production have all been shown to be promoted by quorum sensing.
Signals may be exchanged via intraspecies or interspecies communications. Understanding bacterial signals may provide a mechanism to interrupt communications and control biofilm formations. In fact, researchers at Texas A&M recently discovered a way to alter the chemical signal between bacteria, tricking them into changing the course towards biofilm production.2
Interventions are being developed, particularly for clinical applications, to interfere with the formation of biofilms and have been tested against Staphylococcus bacteria, which have been problematic in hospitals.5 Interventions are primarily focused on the addition of hydroxyl groups to known biochemical signaling mechanisms of the bacteria.
Conclusion
It is difficult to think of single celled organisms as highly evolved but recent research shows bacteria have the ability to behave cooperatively, as a multi-celled organism via quorum sensing.
Chemicals released by cells signal neighboring cells and other bacterial species when and how to grow in both human hosts and environmental reservoirs. An increased understanding of communication skills among bacteria provides knowledge of potential mechanisms to manipulate or interrupt these behavior patterns, including the mechanisms that trigger biofilm formation.
Such interventions may help to prevent or treat microbial infections and control the formation of biofilms in water treatment equipment or distribution systems.
References:
- NRC. 2006. “Drinking Water Distribution Systems: Assessing and Reducing Risks. Committee on Public Water Supply Distribution Systems: Assessing and Reducing Risks”, National Research Council, National Academy of Sciences. Available at: http://www.nap.edu/catalog/11728.html
- Jintae, L., Jayaraman, A., Wood, T.K. 2007. “Indole is an inter-species biofilm signal mediated by SdiA”. BioMed Central, 7(42):1-15.
- Williams, P, Winzer, K, Chan, W. C., Cámara, M. 2007. “Look who’s talking: communication and quorum sensing in the bacterial world.” Philosophical Transactions of the Royal Society of London B Biological Sciences. 29:362(1483):1119-34.
- Miller, M.B., Bassier, B. L. 2001. “Quorum sensing in bacteria.” Annual Reviews in Microbiology. 55:155-69.
- Abraham, W. R. 2006. “Controlling biofilms of gram-positive pathogenic bacteria.” Current Medicinal Chemistry. 13(13):1509-24.
About the author
Dr. Kelly A. Reynolds is an associate professor at the University of Arizona College of Public Health. She holds a Master of Science degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds has been a member of the WC&P Technical Review Committee since 1997. She can be reached via email at [email protected].