What is Biofilm?
Biofilm is an association of micro-organisms within a self-generated matrix of extracellular polymeric substance (EPS) in which microbial cells (bacteria, fungi and protist) adhere to each other on biotic or abiotic surfaces. According to National Institutes of Health (NIH) biofilm formation is responsible for about 65% of all microbial infections and 80% of all chronic infections.
Biofilm formation is a multi-step process starting with an initial attachment to a surface followed by formation of micro-colonies resulting into formation of three dimensional structures and finally ending with maturation followed by detachment.
Biofilm formation relies on a very specific mechanism of quorum sensing by which microbes communicate with one another. This microbial biofilm is pretty inaccessible to antibiotics and human immune system thereby posing a big menace to public health because of its engagement to variety of infections. A greater understanding of bacterial biofilm is thus needed for the development of novel and effective control strategies for improvement in patient management and enhancing the clinical decision-making process.
Occurrence of Biofilms
One common example of a biofilm is dental plaque, a slimy build-up of bacteria that forms on the surfaces of teeth. Pond scum is another example. Biofilms have been found growing on minerals and metals. They have been found underwater, underground and above the ground. They can grow on plant tissues and animal tissues, and on implanted medical devices such as catheters and pacemakers.
Composition of biofilms
Microbes inside the biofilm produce an extracellular polymeric substances (EPS) such as proteins (<1-2%) including enzymes), DNA (<1%), polysaccharides (1-2%) and RNA (<1%), and in addition to these components, water (up to 97%) is the major part of biofilm which is responsible for the flow of nutrients inside biofilm matrix.
| Serial No. | Components | Percentage of matrix |
| 1 | Microbial cells | 2-5% |
| 2 | DNA and RNA | <1-2% |
| 3 | Polysaccharides | 1-2% |
| 4 | Proteins | <1-2% (including enzymes) |
| 5 | Water | Up to 97% |
Biofilm formation
Biofilm formation is a complex process. During biofilm formation microbial cells transform from planktonic to sessile mode of growth. Some researchers believe that biofilm formation is dependent on the expression of specific genes that guide the establishment of biofilm. These bacteria switch on some genes that activate the expression of stress genes which in turn switch to resistant phenotypes due certain changes e.g. cell density, nutritional or temperature, cell density, pH and osmolarity.
Biofilm formation has following important steps (a) attachment initially to a surface (b) formation of micro-colony (c) three dimensional structure formation (d) biofilm formation, maturation and detachment (dispersal)
- Initial Attachment
A system of solid– liquid interface can provide an ideal environment for micro-organism to attach and grow (e.g. blood, water) for biofilm formation. In general (although there are exceptions), the rougher and more hydrophobic materials will develop biofilms more rapidly. Presence of locomotor structures on cell surfaces such as flagella, pili, fimbriae, proteins or polysaccharides are also important and may play synergistic effect in biofilm formation. - Formation of micro-colony
Once the microbe adheres to the physical surface/biological tissue, multiplication of bacteria in the biofilm starts as a result of chemical signals. By using such chemical signals, bacterial cell divisions take place within the embedded EPS matrix, finally resulting into micro-colony formation. - Three-dimensional structure formation and maturation
Bacterial attachment triggers formation of EPS matrix. Matrix formation is followed by water-filled channel formation for transport of nutrients and waste materials into/out of the biofilm. - Detachment
After biofilm formation, microbes leave the biofilms on regular basis. By doing so, it can undergo rapid multiplication and dispersal. Dispersed cells from the biofilm have the ability to retain certain properties of biofilm, such as antibiotic in-sensitivity. The cells which are dispersed form biofilm may return quickly to their normal planktonic phenotype.
Quorum Sensing
During biofilm formation microbes communicate with one another through a mechanism called quorum sensing. It is a system of stimulus to co-ordinate gene expression with other cells and response related to the density of their local population. QS system enables communication between intraspecies and interspecies which involves in terms of biofilm formation, food shortages and environmental stress conditions, such as disinfectants, antibiotics, bacterial colonization, the identification of annoying species.
| Serial No. | Common biofilm forming bacterial species |
| 1 | E. coli |
| 2 | P. aeruginosa |
| 3 | S. epidermidis |
| 4 | S. aureus |
| 5 | S. epidermidis |
| 6 | E. cloacae |
| 7 | K. pneumoniae |
| 8 | Actenomyces israelii |
| 9 | Haemophilus influenza |
| 10 | Burkholderia cepacia |
Resistance to Antimicrobial Substances
There are three reasons for the intrinsic antimicrobial resistance of biofilms. First, antimicrobial agents must diffuse through the EPS matrix to make contact and inactivate the organisms within the biofilm. EPSs retard diffusion either by chemically reacting with the antimicrobial molecules or by limiting their rate of transport. Second, biofilm-associated organisms have reduced growth rates, minimizing the rate that antimicrobial agents are taken into the cell and therefore affecting inactivation kinetics. Third, the environment immediately surrounding the cells within a biofilm may provide conditions that further protect the organism.
Conclusion
A thorough study of bacterial biofilm is required to better understand bacterial cells behaviour in biofilm, and also to understand bacterial resistance to multiple drugs. An elaborate molecular study is further needed to understand various gene expression/repression during biofilm formation. These various molecular mechanisms can be targeted to control or inhibit biofilm formation.
References
- Microbes are More Social Than You Think, But Not Always in a Good Way. In: Discover Magazine [2016] August 22.
- Environmental factors that shape biofilm formation. In: Bioscience, Biotechnology, and Biochemistry [2016] 80(1): 7-12.
- Biofilms: Microbial Cities of Scientific Significance. In: Journal of Microbiology & Experimentation [2014] 1(3): 00014.
