Biofilms: the ultimate survival mechanism

 By Debosree Pal, Jawaharlal Nehru Centre for Advanced Scientific Research, India

Communication and decision making often form the keys to survival at the cellular level. Intracellular and intercellular communications have been well established through signaling pathways in multicellular organisms. The often undervalued unicellular denizens of the living world utilize such strategies in rather intelligent fashion to accomplish survival mechanisms in extreme conditions.  Through a simple microscope, Van Leeuwenhoek first observed the formation of microbial biofilms on tooth surfaces. Biofilms are a conglomeration of microbial cells that gain anchorage to a surface through secreted polymeric substances. These intricate architectural biofilms often provide the unicellular organisms with a support system to gain survival advantage in otherwise challenging conditions.

At the molecular level, biofilms act through the mechanism of quorum sensing whereby each microbe in the group secretes chemical molecules into the surrounding environment that act as sensors. Upon reaching a critical level, these molecules initiate expression of specific genes to help them adjust to the environment. Research on biofilms existing in extreme conditions has gained a lot of importance due to their potential varied applications. Miranda et al, for example, characterized a complex biofilm isolated from saline lakes comprising a consortia of cyanobacteria, algae and diatoms. In a simulated aquatic environment consisting of a high concentration of phosphates, ammonia, nitrates and selenium, the biofilm had the capacity to decrease the toxicity of the environment suggesting potential roles for the use of biofilms that are able to survive in high salt conditions for bioremediation purposes.

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Schematic of biofilm formation

In preparation for the EXPOSE-R2 mission, Baque et al conducted a study for the BOSS (Biofilm Organisms Surface Spacing) experiment. The BOSS experiment was designed to test the hypothesis that biofilm form of growth for microorganisms is better suited to life in space. Overall, such studies aim to provide a better understanding about the extreme conditions to which terrestrial life forms can adapt. They have shown that the photosynthetic cyanobacteria Chroococcidiopsis sp. strains CCMEE 057 and CCMEE 029, originally found in the Sinai and Negev deserts, can tolerate high doses of UV radiation or conditions of desiccation and reduced pressure, environments which are of common occurrence in space. Interestingly enough, damage to cellular components by UV radiation was better tolerated in the biofilm form of the species’ as compared to the planktonic form. Although the specific details are unknown, extracellular polysaccharides, a crucial component of biofilms, may act to impart such tolerance to these microorganisms.

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Microbial mat/biofilm at the Grand Prismatic Spring, Wyoming, USA. The innermost layer of yellow mat is formed a cyanobacteria, Synechococcus, that survives at temperatures of 149° Farenheit along with intense UV radiations of the sun. Carotenoids help them assimilate the light for photosynthesis as well as protect cholorophyll from photo-damage. As the temperature cools down slowly in the surrounding regions, a host of different type of thermophilic bacteria thrive, rendering the spring its characteristic rainbow hues.
(Courtesy: By Brocken Inaglory, CC BY-SA 3.0)

Yet another survival advantage that biofilm growth bestows upon unicellular organisms is in chronic infection of hosts. Changes in the environment of the host due to causes like infection by other microbes or drug treatments can lead to opportunistic infections by Candida albicans. In oral, gut or vaginal candidiasis. C. albicans biofilms can be resistant to penetration by the immune factors of the host or by antimicrobial treatments. Furthermore, biofilms of C. albicans can survive on solid surfaces such as silicone, a common material used in medical equipment like intravascular catheters. According to a study by Nobile and Johnson, 15% of hospital sepsis cases are contributed by C. albicans and related species and are the fourth most common cause of bloodstream infections acquired in clinical settings. In chronic diseases like tuberculosis and cystic fibrosis, biofilms also provide a natural mechanism of survival against harsh antibiotics, thereby, contributing to antibiotic resistance.

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Scanning electron microscope image depicts a Proteus mirabilis (ATCC 29906) biofilm growing on polycarbonate coupons using a CDC biofilm reactor. This microbe is responsible for urinary tract infections and displays antmicrobial resistance.
(Content Provider: CDC; Biofilm Laboratory, DHQP; Chelsea Samaniego Meltzer, EID Fellow)

 

Biofilm formation by microorganisms has a greater impact on our daily lives than most people realize. It is intriguing how these simple organisms have developed an elaborate and intricate mode of survival mechanism in extreme conditions. Extensive research still needs to be undergone to either harness the useful properties of biofilms in biotechnological applications like in the production of biofuels, or to address their contributions to human health and disease.

Further reading:

pic4-croppedAbout me:

I am a final year PhD student at the Molecular Biology and Genetics Unit, JNCASR, India. Designing problems and troubleshooting them to understand biological phenomena drives me to the world of molecular biology. I also like travelling and reading in my spare time. You can find me on Twitter @crimsonhues_deb.

This post is the first in our extremes series. If you are interested in reading more on this topic, you can also check out the December issue of The Biochemist magazine on the theme of extremes.

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