Whether you’ve Google-searched “biofilm” to learn more yourself, taken courses covering the subject, or are deeply embedded in biofilm-related research, you’ve probably encountered a model similar to the one below, which represents biofilm maturation. In the current model, a biofilm begins with a planktonic cell attaching to a surface, which multiplies and develops into a three-dimensional structure that includes both cells and extracellular matrix. Eventually, the biofilm releases planktonic cells, which can seed new biofilms by attaching and reinitiating the cycle. This model has been helpful in understanding the developmental stages of biofilm formation.
A new paper published in mBio addresses the question of these single planktonic cells. What if groups of cells break off? How would biofilms initiated from a cluster of aggregated cells differ from biofilms initiated from a single cell? How would cell aggregates compete against single cells during biofilm formation? These questions are addressed by first author Kasper Kragh, working with senior author Thomas Bjarnsholt.
In their natural settings, biofilms likely release both single cells and aggregated cells, reasoned the authors, and aggregated cells may better retain characteristics, such as adhesion abilities or stress responses, that are beneficial in a multicellular structure. Testing this both in silico and by studying Pseudomonas aeruginosa biofilm formation using a flow chamber – the in vitro device used to generate shear forces experienced in a catheter or waterway, where biofilms naturally occur – showed that the concentration of cells seeding a biofilm influences how competitive cell aggregates are against single cells. Measuring offspring numbers as fitness comparisons, the research team observed that the higher the initial seeding cell density, the better the aggregates competed.
Why? One of the differences between the single cells and the cell aggregates is their innate architecture. Aggregated cells are naturally a bit ‘taller’ than single cells, since cells sit atop one another and gain structural height. Based on the simulated and in vitro experiments, the researchers hypothesized that cells higher in the biofilm architecture were more fit, and produce more offspring. Single cells at higher elevation grew faster than those at lower elevation, and when single cells were elevated to the height of the biofilm, both single and aggregated cells grew at the same rates. Later experiments showed this was likely due to differences in access to oxygen, such that elevated cells can better compete for oxygen, and therefore reproduce faster.
Introduction of cell aggregates into biofilm development models complicates an already complicated system. In addition to seeding cell type and density, traits like exopolysaccharide production, motility, quorum sensing systems, and species mixture can quickly overwhelm a model with details. Without introducing these additional factors, the authors proposed a new model, roughly based on the established model, to account for the role of cell aggregates:
Scientific models are extremely useful even if they are all wrong: they integrate a lot of complex experiments and summarize our current understanding of how a system works. They often add a visual component for people who have different learning styles. And, importantly, good models are predictive – they grant assumptions that can be tested in future experiments. The authors argue that because the multicellular architecture is so important to biofilm nature, relevant models must consider the role played by cell aggregates in structural development.
What do you think, mBiosphere reader? Do you plan to use this new model in your next biofilm talk? Does it add depth to the research questions it introduces? Share your thoughts in the comments section!
-- Julie Wolf