Conversations with Arnold Mathijssen: Strong vortex formation and biofilm nutrient supplies

Biofilms represent a persistent and often dangerous threat. Chemical, oil, and water companies have had to deal with the often corrosive effects of biofilms forming in piping. Dentists (and their patients) have long dealt with the damage plaque causes on the enamel coating teeth. More recently, healthcare settings have struggled to dislodge biofilms in tubing and equipment that often serve as sources of antibiotic resistant bacteria and long term persisters. Biofilms even coat human lungs, making treatment difficult.

Stanford University’s Arnold Mathijssen has investigated how the bacteria forming biofilms feed themselves by manipulating their surroundings. He set aside time to discuss his research with SCINQ.

SCIENTIFIC INQUIRER: How did you come about studying how biofilms affect the flow of surrounding fluids?

ARNOLD MATHIJSSEN: Since my PhD (with Prof Julia Yeomans FRS, at Oxford University) I have been interested in bacterial dynamics on surfaces. We looked at bacteria swimming upstream in pipes in freshwater contamination studies, how pathogens accumulate on surfaces, and how they behave in thin liquid films like a layer of sweat on our skins.

Bacteria tend to form biofilms when they aggregate on surfaces. These sticky colonies act as a natural protection mechanism for bacteria against viruses, but also allow them to become more resistant against antibiotics. However, tightly packed bacterial colonies also run out of food and resources. Therefore we were interested in understanding their pathways of nutrient supply. If we can interrupt these pathways, then some biofilms can possibly be prevented from forming.

SI: How did you design the experiment? What conditions, behaviors, functions did you need to take into consideration while setting parameters for your model?

AM: We considered a large amount of individual bacteria swimming along a flat surface. Every cell generates a flow as it swims, so this swarming community can be considered as an “active carpet” that stirs the liquid. We envisaged that this stirring woud be strong close to the surface and weaker further away. From a physics point of view, that means the diffusion coefficient of nutrients would become a function of distance from the active carpet.

To prove this, we started a large computer simulation where we computed the flows added from millions of bacteria, all swimming parallel to this surface. The parameters of this simulation were easy to determine because many experiments have been performed by other colleagues that measure the flows generated by individual cells. These experiments are very well understood by hydrodynamic theory, some of which I also developed while doing my PhD. So the only parameters left to tune were the number of cells and their positions and orientations, the biofilm patterns, that is the ‘architecture’ of the active carpet.

SI: What did the model you designed reveal about how biofilms interact with surrounding liquids?

AM: To me the largest surprise is the discovery that a colony of bacteria can generate a strong vortex flow together. Specifically, that each individual bacterium is swimming in a random direction but the sum of their random flows adds up to a non-random vortex. The bacteria can make use of this flow (100x larger than the cell size) to attract nutrients or other resources like oxygen or fresh water.

SI: Did your results confirm your initial hypothesis regarding the flow of microfluids around biofilms?

AM: Not exactly. We initially only expected to find a diffusive-type of flow being generated by the bacteria, which would decay quite rapidly with distance from the active carpet. However, we found that the flows added up to a non-random drift, that reached out much further from the carpet and also adds up over time.

SI: Does different arrangements of bacterial flagella make a difference in the overall flow of fluids?

AM: Yes! Once we found that a simple random pattern of bacteria can generate these long-ranged flows, we started looking at more complex arrangements. For example, a circular pattern where all the cells move around a single point gives attractive flows towards the carpet. But when the cells are aligned in an aster, all pointing towards one point, then they can actually make the liquid move upwards from the carpet. We developed a more general theory for this that allows researchers to consider any type of arrangement that combines different local patterns into a global architecture.

SI: Does the strength of the current created by the bacteria in biofilms vary, depending on conditions surrounding it?

AM: The viscosity of the surrounding water has a strong impact on the current strengths, because it determines how much energy is required to move a viscous liquid around. Variations in temperature or chemical conditions can also affect the activity of the bacteria.

SI: Can you discuss some of the different patterns and how they can be used to determine flow?

AM: The general rule is that for randomly oriented bacteria, the flows are being attracted down to the active carpet. However, everything that comes down has to go back up again somewhere, because the liquid cannot pass through the surface. So in a pattern with biofilm arms branching outwards, the flow will come down to the arms and up between the arms. In a circular patterns the flow comes down all along the inner circles and eventually has to go up again at the outer edges of the colony. For an aster pattern the flow direction is reversed.

SI: How can your findings be applied in biomedical settings?

AM: Our research could help understand how the nutrient supply of bacterial colonies could be interrupted, particularly in the early phases of biofilm formation when the bacteria are swimming on the surface and starting to make colonies. Therefore it is not a direct solution to the health problem of antibiotic resistance, but certainly a step in the right direction.

It is probably too early to say how much impact this work could have, but it is an important discovery that doctors and researchers can follow up on. Bacteria could be seen as some of the simplest organisms in biology and yet scientists keep discovering more and more complex mechanisms they use.

For more information about Arnold Mathijssen and his research visit his Stanford University page. You can also follow him on Twitter @AJTM_M.

IMAGE SOURCE: Creative Commons

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