Writer: Evra Haspolat
Globally, more than 90% of all microorganisms are organized in biofilms and found in ecosystems such as lakes, surface water, groundwater, and terrestrial ecosystems. Biofilms are complex communities of microorganisms (bacteria, protozoa, algae, fungi) that attach to living or non-living surfaces and are embedded in a matrix of an extracellular polymeric substance they produce. These organisms are protected from effects such as host cell response, antimicrobial treatment, and adverse environmental conditions since they protect from desiccation, shear stress, toxic compounds, and protozoan grazing in nature (Herrling et al., 2019), (Ali et al., 2023), (Aslan, 2020).
Formation Process
We can study the formation process of biofilms in 5 steps. These steps include initial attachment, irreversible attachment, microcolony formation, maturation, and dispersion.
Initial Attachment
Biofilm formation begins with the initial attachment of free-floating (planktonic) microbial cells to a surface. Attachment is often initially reversible, allowing cells to detach and reattach depending on environmental conditions as adhesion between cells and the surface weakens. These weak interactions are generally hydrophobic, electrostatic forces, and Van der Waals interactions (weak intermolecular forces that contribute to the reversible attachment of cells). (Ankara Üniversitesi, n.d.), (Aslan, 2020), (Toyofuku et al., 2016).
Irreversible Attachment
When bacterial cells find a suitable surface, they start searching for nutrients that will enable them to survive on the surface before forming a biofilm structure. Then, unlike initial attachment, they bind to the surface with strong bonds and achieve irreversible attachment to the surface. Next, bacterial cells adhere by the exopolysaccharides they produce (EPS) while some bacteria adhere by their extracellular extensions such as flagella and pili. Microorganisms are tightly attached to the surface and each other in this step. (Aslan, 2020), (Ankara Üniversitesi, n.d.), (Toyofuku et al., 2016)
EPS
EPSs (exopolysaccharides) are unique polymers generated by living organisms such as algae, fungi, and bacteria to protect them from environmental factors. After a fermentative process, these polymers are extracted from the medium culture. In the biofilm, EPS is responsible for adhesion to surfaces, scaffolding cells together, and maintaining the three-dimensional structure of the biofilm. EPS matrix consists mainly of polysaccharides, proteins, nucleic acids, lipids, and other biopolymers. (Paul et al., 2023) (Ankara Üniversitesi, n.d.)
Microcolony Formation
After irreversible attachment, bacterial cells develop microcolonies by assembling previously attached cells and undergoing cell division. These microcolonies grow via cell proliferation and produce EPS. Production of EPS is crucial in this step because the reproduction of bacteria and the increase in the number of these microcolonies in the biofilm structure depend on the organization of EPS. In most mature biofilms, EPS represents more than 90% of the dry mass. (Sharma et al., 2023), (Ankara Üniversitesi, n.d.), (Toyofuku et al., 2016), (Aslan, 2020), (Kostakioti et al., 2013)
Maturation
The system necessary for microorganisms to survive in EPS is provided through capillary channels. The structure gains its 3D shape through these channels. Nutrients circulate dissolved in water in these capillary tubes, and toxic wastes of bacteria are released into these channels. While the circulation within these channels is provided by passive diffusion, the intake of nutrients from the outside is achieved by facilitated diffusion. However, due to certain limits of diffusion, nutrient and oxygen exchange in the lower layers is quite low in mature biofilm structures. This situation affects the metabolic rate of the bacteria in the bottom layer. The decrease in metabolic rate reduces the production of exopolysaccharides and cells begin to tend to migrate from the lower layer to the upper layer. This migration tendency of microcolonies causes the formation of column-like structures in the form of microscopic mushrooms in the biofilm layer. This can be observed in Pseudomonas aeruginosa, the model biofilm-forming bacterium, which forms a mushroom-shaped structure with channels between microcolonies consisting of rod-shaped cells under oxic conditions. (Aslan, 2020), (Sharma et al., 2023), (Ankara Üniversitesi, n.d.), (Toyofuku et al., 2016)
Dispersion
Cells in the biofilm structure provide nutrients and oxygen through capillary channels. However, after the cell density within the structure reaches saturation, nutrients, and oxygen cannot reach cells. This saturation state is detected by the signal molecules or peptide-derived substances secreted by the cells by the Quorum-sensing (the way some microorganisms communicate with each other by signaling molecules) mechanism. Some of the biofilm cells detach from the structure and become free (planktonic phase), or they detach from the structure and attach to nearby surfaces and form new biofilms. Thus, dispersal is not only the final stage of the biofilm life cycle but also the start of another lifecycle. (Aslan, 2020), (Sharma et al., 2023), (Ankara Üniversitesi, n.d.), (Toyofuku et al., 2016)
Why Biofilm Formation is a Serious Issue
Understanding the formation process of biofilms is crucial, as these resilient structures pose significant challenges in various fields. Although microorganisms are thought to tend to live solitary, they tend to live collectively by forming biofilms. This tendency of microorganisms causes many problems such as contamination, sterilization, and drug resistance in many areas like water systems, the food industry, medicine, and dentistry. Several studies on infectious diseases have pointed to an association with biofilms. Conservative estimates suggest that roughly 70% of bacterial infections are linked to biofilms and are both device-related and non-device-related. They are dangerous since they are resistant to antibiotics. Biofilms protect microorganisms against antibiotics. Bacteria in biofilms have a much higher rate of antibiotic resistance than free bacteria. This makes treating chronic infections especially difficult. (Sharma et al., 2023), (Aslan, 2020), (Ngehdzeka & Elizabeth, 2023), (Topçu, 2018)
They also make it difficult to treat chronic illnesses. Biofilms play a critical role in developing various chronic infections in the human body. They can be found in the lungs of patients with cystic fibrosis, cases of endocarditis, and conditions such as dental plaque. These infections may be resistant to treatment as biofilms have a protective nature. (Sharma et al., 2023), (Aslan, 2020)
One of the ways biofilms cause infection is by water contamination. Organic molecules in water group together and stick to the surface as the water enters the pipe systems. This adhesion occurs primarily due to environmental effects in the initial attachment phase. Biofilm formation in pipe systems not only causes water contamination but also causes corrosion in pipe systems and wear of surfaces. (Aslan, 2020)
Conclusion
In conclusion, biofilms appear in food, industry, and medicine, causing significant financial and health problems. The presence of biofilms in medical devices, water systems, and other environments can lead to difficulties such as chronic infections, water contamination, and increased maintenance costs. Understanding the formation of biofilms is important for developing effective strategies to prevent and control their impact on human health and infrastructure.
References
Ali, A., Zahra, A., Kanthan, M., Husain, F. M., Albalawi, T., Zubair, M., Alatawy, R., Abid, M., & Noorani, M. S. (2023, July 29). Microbial Biofilms: Applications, clinical consequences, and alternative therapies. MDPI. https://www.mdpi.com/2076-2607/11/8/1934
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Herrling, M. P., Lackner, S., Nirschl, H., Horn, H., & Guthausen, G. (2019). Chapter Four - Recent NMR/MRI studies of biofilm structures and dynamics. Science Direct. https://www.sciencedirect.com/topics/materials-science/biofilms
Kostakioti, M., Hadjifrangiskou, H., & Hultgren, S. J. (2013, April). Bacterial Biofilms: Development, dispersal, and therapeutic strategies in the dawn of the Postantibiotic era. PubMed Central (PMC). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3683961/
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Paul, P., Nair, R., Mahajan, S., Gupta, U., Aalhate, M., Maji, I., & Singh, P. K. (2023, July 15). Traversing the diverse avenues of exopolysaccharides-based nanocarriers in the management of cancer. ScienceDirect. https://www.sciencedirect.com/science/article/abs/pii/S0144861723002862
Sharma, S., Mohler, J., Mahajan, S. D., Schwartz, S. A., Bruggemann, L., & Aalinkeel, R. (2023, June 19). Microbial Biofilm: A review on formation, infection, antibiotic resistance, control measures, and innovative treatment. MDPI. https://www.mdpi.com/2076-2607/11/6/1614
Topçu, A. W. (2018). Biyofilm Nedir? | Makale | Türkiye Klinikleri. Türkiye Klinikleri. https://www.turkiyeklinikleri.com/article/tr-biyofilm-nedir--83796.html
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