Biofilms. They’re everywhere. Coating our teeth. Lining the tubes that supply our water. Corroding the pipes transporting gas across thousands of miles. And yet, despite decades of research, they’ve been unwilling to reveal their secrets to researchers. Much of this has been due to technological limitations. For a long time the only way to study the internal structures of biofilms has been through cystallographic methods which required that they be dried out, killing them in the process. It offered a snapshot, akin to a single frame of a three hour movie.
But things are changing. Today, countless studies offer fresh insights into the dynamic nature of biofilms. They are revealing a microscopic world where all types of bacteria interact in complex and unexpected ways.
Oliver Lieleg, of the Technical University of Munich, makes it his business to decipher the mysteries contained within the microscopic confines of biofilms. Recently, he has fixed his gaze on the outside – specifically, the surface that acts as protective layer.
Dr. Lieleg set aside some time from his busy schedule to field some question SCINQ posed to him.
SCIENTIFIC INQUIRER: Before going into the specifics of your research, can you just highlight some of the challenges of studying bacterial biofilms?
OLIVER LIELEG: In natural settings, bacterial biofilms can be formed on many surfaces. They grow in pipes, on teeth and even in catheters. Although such biofilms are ubiquitous, little is known about the physico-chemical principles that render those biofilms so sturdy and robust. One huge problem is that removing biofilms from the surfaces they grow on is really difficult.
Growing biofilms under lab conditions can be a lengthy process requiring several days up to weeks. We need different growth conditions in order to obtain biofilms in the lab. Then we can study their behavior in response to different environmental challenges such as chemicals, temperature changes or mechanical stress. Another challenge is that biofilms typically contain many different bacterial species at the same time. In the lab, we typically focus on biofilms generated by a specific type of bacteria. We mainly use Bacillus subtilis, a harmless, apathogenic bacterium from the soil. Since we work with this model organism to study the material properties of biofilms, a necessary next step is to confirm our results with more complex biofilms as they are found in natural or industrial settings.
SCINQ: How was your experiment designed and what was its objective?
OL: When performing previous experiments regarding the erosion resistance of biofilms in the presence of water flux, we noticed that several of the biofilms variants we grew resisted wetting very efficiently. However, we also realized that, after immersing the biofilms into water, water droplets were rolling off the surface of some biofilms variants whereas, on other samples, the droplets sticked to surface – without spreading. We found this behavior curious and asked if this behavior has been observed for other materials before. This was indeed the case: lotus leaves and rose petals behave the same. The goal of our study was to test if similar physical mechanisms are responsible for the wetting resistance of biofilms as for the wetting resistance of the plant leaves.
SCINQ: You correlate the wetting behaviors of lotus leaves and rose petals with biofilms. Why?
OL: Those two plant leaves are very well known for their hydrophobic behavior. The difference between both is that, for lotus leaves, rain drops roll of the surface whereas, for rose petals, they stick to the surface but do not spread. We were very happy to see that the analogy between the plant leaves and biofilms works very well – not only in terms of their hydrophobic behavior but also with regard to their topology. In the end, this is quite surprising considering that we are comparing a solid plant structure with soft, slimy biofilms.
SCINQ: What did your research reveal regarding biofilm wetting behaviors?
OL: One key result was that the surface structure – what we refer to as topology – is a main factor which decides how a biofilm behaves when brought in contact with a water droplet. Smooth biofilms behave hydrophilic, whereas rough biofilms behave hydrophobic. However, Which type of hydrophobic behavior occurs depends on the detailed surface features of the biofilm colony. One parameter we use to link the surface morphology of the different biofilm colonies to their wetting behavior is the ratio of the net surface to the projected surface (i.e. the area the biofilm covers). We found that, for low values, the biofilm colony is hydrophilic;at intermediate values the biofilm colony is hydrophobic/rose petal-like (i.e. water droplets remain spherical but stick to the surface when the surface is tilted) and at high values the biofilm colony is hydrophobic/lotus leaf-like (i.e. water droplets roll off the surface when the colony is tilted). One surprising aspect was that one and the same bacterial strain is, in principle, able to build colonies with all three variants of wetting behavior. Which type of wetting is observed depends on conditions, e.g., on the nutrient available to the bacteria during biofilm growth.
SCINQ: Was this in line with pre-experiment assumptions? What are the implications of your findings?
OL: When we started our research, it was already known that certain biofilms can be strongly hydrophobic. However, most of previous biofilm research did not distinguish in detail between the two variants of hydrophobic behavior known from lotus leaves and rose petals. One result we show in our study is that, for lotus-like hydrophobic behavior, there are small water bubbles separating the biofilm surface from the water droplet. This is again a very nice analogy to what is known for lotus leaves, where the same effect occurs. For rose-petals, the water droplet is in full contact with the underlying rose surface, at least locally; this is why the droplets behave so differently when the plant leaves (or the biofilm colonies) are tilted. In the end, this also tells us that lotus-like biofilms are even more difficult to fight than rose-like biofilms: Imagine trying to disinfect a biofilm colony with a chemical solution, but the chemical does not really reach the biofilm bacteria since there are microscopic air bubbles preventing contact with the solution. This is a problem that I feel is not appreciated enough.
SCINQ: On a personal level, what brought you to choose a life in the sciences and what attracted you to biomechanics?
OL: Actually, I am a trained physicist who became interested in biological matter during the 2nd half of my diploma studies. Then, I did my PhD research in the field of cellular and molecular biophysics and became hooked on the fascinating tricks nature uses to tune the material properties of biopolymer networks. During my postdoc time at Harvard I became interested in slimy, gooey materials and started working with mucus and biofilms – two biological materials that still fascinate me with surprising properties. Nature had so much more time than mankind to come up with smart solutions for tricky problems. There is still so much we can learn, and figuring out how nature gets things done is lots of fun.
SCINQ: Can you place your latest research into the context of your past investigations? How does it influence future inquiries?
OL: We are trying to understand the complex material properties of biofilms; this includes their mechanical behavior as well as their permeability and wetting resistance. Furthermore, we are trying to create bio-inspired artificial materials that mimic the properties of their biological counterparts or to find medical or technical applications for biopolymers which we purify from biological, slimy materials – a typical bioengineering approach I picked up during my time at MIT. One example is our bio-hybrid mortar (which contains freeze-dried biofilm) which exhibits lotus-like properties.
The next step in our research is to ask if a fully grown biofilm can be “reprogrammed” such that its surface topology is changed, e.g. by switching the nutrient conditions. The idea is to force the biofilm to switch its wetting resistance from lotus-like to rose-like – the latter should not be “as bad” as the former. Whether this can be achieved or not is not clear yet.
SCINQ: What role do you believe the Scientist should play in the modern world?
OL: I think that, in a time where “alternative facts” become more and more popular, it is crucial to remind everyone that educated decisions can only be based on thoroughly acquired data. As a scientist, I try to teach my students to distinguish between hypotheses, speculations and conclusions and to mark them as such when making written statements. Also, being critical about your results, questioning your own point of view and being open to adjusting your own opinion (if proper argumentation is provided) are skills that are elementary not only to scientists but also for many other fields of today’s life. I hope that the students we train at universities go out into the world with such an open and self-critical attitude – even if they do not pursue science in their later life.
SCINQ: If you weren’t a scientist, what would you be?
OL: Maybe a landscape architect or a gardener? But then I might end up cross-breeding roses, which would directly lead back to science…
For more information about Dr. Oliver Lieleg and his research visit his lab page at TUM.
For more information about biofilms and their complex role in nature watch this video.