Biofilms: The EPS matrix

Has ever the sight of slime on showerheads been revolting? Does the slipperiness encountered when walking into a shallow stream full of slurry-coated rocks cause aversion? In my case, I would answer 'yes' for both the accounts. Since young, I had always strongly disliked the formation of biofilm (a word that I learnt subsequently) on household items and even on the pebbles in my tiny fish tank. I was always scrubbing away the slimy surfaces as hard as I could and once my mother absent-mindedly questioned if I knew what was actually causing the build-up of mucous on pipes and sinks. All the while, it had never occurred to me to look up on biofilm before that day and I thank my mother for pointing a pathway for my academic future.

Biofilm are caused by adherence of bacterial cells to a surface through van der Waals forces (a weak attractive force) initially. Then, they go through irreversible attachment stage where cell adhesion structures are employed to anchor themselves onto the surface permanently. After the maturation step, this group of living cells is then dispersed to colonize new surfaces. Biofilms are involved in nearly all of microbial infections on humans. Microorganisms such as Pseudomonas aeruginosa, Staphylococcus aureus, Streptococcus pneumoniae, Salmonella enterica, Haemophilus influenzae and even the notorious Mycobacterium tuberculosis are known biofilm producers and all these bacteria cause chronic infections in humans. Malaysia, being a tropical country, provides an optimal growth condition for these biofilm-forming bacteria to continuously thrive especially in the clinical settings .

This condition is often difficult to treat as the biofilm formation renders antibiotics from penetrating the extracellular polymeric substance (EPS) matrix as shown in Figure 1, which is the biofilm wall of defense. So, what is so special about this EPS matrix? How does this feature make biofilm stronger? EPS is actually polymers synthesized by the bacteria and it is mainly made up of proteins, polysaccharides, lipids and extracellular DNA. Harsh environmental conditions such as rise in temperature, altered pH levels, drought and even salinity can trigger bacteria to produce EPS. Once EPS is synthesized, it allows bacteria to adhere to the surface, interact with one another and also acts as a glue between individual cells. So, this results in a 3D formation of a gel-like substance around the bacterial cells (that’s the slime that we often see.!!).

Figure 1. A schematic representation of a biofilm where antibiotic molecules are not able to penetrate the biofilm layer. 

This EPS matrix acts as carbon reserve, traps nutrient from the environment, has high water-holding capacity and it also keeps bacteria from desiccation, shelters bacteria against extreme temperatures and most importantly EPS protects bacteria against antibiotics. There are several ways EPS inhibit antibiotics from penetrating the matrix; (1) EPS is negatively charged, so it has the ability to bind positively charged compounds and also repels negatively charged molecules from entering the matrix layer, (2) buildup of antibiotic-degrading enzymes in the EPS matrix to breakdown antibiotic molecules (3) presence of extracellular DNA which offers resistance to antibiotics and also (4) EPS causes nutrient gradient in the matrix which results in groups of bacteria at different stages of growth that may not respond to the antibiotic. Therefore, researchers are mainly focused on targeting EPS as one of the crucial ways of removing biofilms.

References

Costa, O. Y., Raaijmakers, J. M., & Kuramae, E. E. (2018). Microbial extracellular polymeric substances: ecological function and impact on soil aggregation. Frontiers in microbiology, 9, 1636.

Sharma, D., Misba, L., & Khan, A. U. (2019). Antibiotics versus biofilm: an emerging battleground in microbial communities. Antimicrobial Resistance & Infection Control, 8(1), 1-10.

Koo, H., Allan, R. N., Howlin, R. P., Stoodley, P., & Hall-Stoodley, L. (2017). Targeting microbial biofilms: current and prospective therapeutic strategies. Nature Reviews Microbiology, 15(12), 740.