An array of bacterial biofilms. Clockwise from top left: Cyanobacterial bloom in a lake; Bacillus subtilis biofilm repels red water droplets; Timelapse of a growing Pseudomonas aeruginosa colony; Iridescent layer of Leptothrix discophora metabolizes heavy metals on the surface of water; Collection of microbial colonies on a petri dish. (National Oceanic and Atmospheric Administration, National Biofilms Innovation Centre, Scott Chimileski)
Bacteria are the most abundant form of life on Earth, outnumbering the billions of stars in the Milky Way galaxy. Let that sink in: when you look up at the night sky, the distant, glowing specks you see are a tiny fraction of the amount of bacteria that exist on this planet. In order to become so incredibly abundant, bacteria had to get very creative. They have evolved to thrive in pretty much any environment you can imagine— in the soil, on rocky mountain tops, in the very depths of the ocean, in extreme heat and freezing cold, within other organisms such as ourselves— the list goes on. One important adaptation bacteria have that allows them to be so successful is their ability to form biofilms.
A biofilm is an aggregation of many bacterial cells on a surface. To keep themselves protected and stuck together, they secrete extracellular polysaccharides, or EPS. Biofilms are advantageous for a number of reasons. They concentrate digestive enzymes, provide a defensive barrier against antibiotics, give bacteria strength in numbers against predators and immune cells, facilitate the transfer of genetic information, and allow bacteria to communicate with one another. Bacteria are also capable of arranging their biofilms into complex 3D structures, which can be seen at the start of this post in images of P. aeruginosa and B. subtilis. While these kinds of structures have been the subjects of both aesthetic admiration and scientific scrutiny, a very minute, crucial detail has evaded our attention… until now.
Enter Escherichia coli: a gram-negative bacterium notorious for its disease-causing strains, though many other varieties are harmless and even beneficial. In fact, it is commonly found as a commensal bacterium in the intestinal tracts of healthy mammals, assisting with the breakdown of carbon compounds. Research has even suggested that these commensal strains can have probiotic properties, preventing their pathogenic relatives from taking over in the gut. However, this is far from everything E. coli can do. Recently, it was found that E. coli is the architect of a subtler, though no less striking, type of biofilm structure. In a 2020 study, Rooney et al. made the exciting discovery that E. coli biofilms create swirling channels that are used for the uptake of nutrients from the environment. Unlike the folds and ridges visible to the naked eye on the surfaces of P. aeruginosa and B. subtilis colonies, the E. coli channels are microscopic and exist within the body of the biofilm. Rooney et al. were able to capture pictures of these channels for the first time in stunning detail using a specialized microscope called the Mesolens, which allows for imaging of bacteria and biofilms in one single view. This eliminates the need to make a composite of multiple smaller images, which can be a hindrance when doing research on such a small scale.
Thanks to the power of the Mesolens, several key features of these channels were identified. Firstly, they form a network that extends across the entire biofilm— from the center to the edge. Secondly, they are not lateral, enclosed structures like capillaries, but open 3D structures that more closely resemble “canyons and ravines”. Thirdly, Rooney et al. used the process of elimination with several imaging techniques to figure out the actual composition of the channels. They ruled out dead cells, EPS, and lipids, finally determining that the channels were filled with a protein-based matrix.
The microscopic channels in E. coli biofilms (Figure 1, Rooney et al. 2020) can be likened to canyons cutting through a landscape (Cam and Nicole Wears)
Further experimentation revealed more fascinating things about this channel network. The biofilm was mixed up to disrupt its structure, and then left to grow again. Ten hours later, the channels had reformed in areas of new growth, indicating that they are an inherent property of the biofilm. In addition, when two different strains of E. coli were cultivated together, their channels remained relegated to their respective colonies— in other words, they did not cross over to form a network with the other strain.
So, how exactly do these channels work, and what could their function be in nature? Rooney et al. sought to answer this question by introducing microscopic, magenta fluorescent particles to an E. coli biofilm engineered to express green fluorescent protein (GFP). This offers us visually spectacular imagery depicting the channels in the process of carrying microscopic particles towards the center of the biofilm (seen below in Figure 5). Once it was clear that these channels function to transport substances from the environment into the colony, the next step was to see if this could possibly be used for nutrient acquisition. A new E. coli strain was made with a GFP that is induced by arabinose, a common source of carbon for bacteria (If you want to learn more about GFP and arabinose in bacteria, check out this experiment from a course at Laney College). When this strain was grown on a medium with L-arabinose as the only nutrient, the biofilm fluoresced the most around the channels. This suggests that L-arabinose was most concentrated inside the channels, which in turn tells us that these channels are used for the uptake of nutrients.
Figure 5. Magnified images depicting the transport of microscopic particles (magenta) from the edge of an E. coli biofilm (green) towards the center via its complex network of channels (Rooney et al. 2020).
These results have multiple significant implications. Firstly, they challenge our previous understanding of how bacterial biofilms work. It was previously believed that biofilms acquired nutrients through passive diffusion. This discovery indicates the contrary, and supports the hypothesis that bigger biofilms require some sort of active transport system to distribute nutrients to their cells at the center. Rooney et al. also note the fractal qualities of these channels, as they form intricate, repetitive patterns within the biofilm. They are unique from fractals that are formed by multi-strain colonies because they do not consist of living cells, and they do not form an integrated network between strains.
Another potential impact of these results is on the field of medicine. When it comes to dealing with pathogenic E. coli, biofilms have posed an issue due to the protection they provide colonies from antibiotics. However, their transport channels may be a brand new way to overcome this obstacle and successfully introduce antimicrobial agents straight to a colony. It is highly feasible that future research will investigate other virulent strains of bacteria to determine if their biofilms also possess this potential weak spot. This study opens a whole new door to the improvement of medicine, and it provides us with an exciting new way of understanding how bacteria so cleverly thrive in the natural world. So next time you think about what’s going on inside your gut, give a salute to the artistic E. coli— they’re busy in there making a masterpiece.
Dear E. coli, welcome to the fractal family! Fractals can be found in nature on both the macro and micro levels. Left to right: Aerial view of Isla de Enmedio, Spain (Grist Magazine); Channels in an E. coli biofilm; Tree branches (Colin Drysdale).
About the Author:
Naomi Eylath '23 is a Biological Sciences major. Her love of biology is fueled by her passion for appreciating and unravelling the intricacies of the natural world. As of Spring 2023, she has completed her honors thesis investigating the genetic underpinnings of inflated calyx syndrome, a floral morphological novelty, in Physalis grisea (groundcherry). During her free time, Naomi enjoys playing the violin, Dungeons and Dragons, bird watching, and folk dancing.
No comments:
Post a Comment