Melaugh, G. | bioRxiv Science

bioRxiv2022

Distinct types of multicellular aggregates in Pseudomonas aeruginosa liquid cultures

Pseudomonas aeruginosa forms suspended multicellular aggregates when cultured in liquid media. Such aggregates may be important in disease, and/or as a pathway to biofilm formation. The polysaccharide Psl and extracellular DNA (eDNA) have both been implicated in aggregation, but previous results depend strongly on the experimental conditions. Here we develop a quantitative microscopy-based method for assessing changes in the size distribution of suspended aggregates over time in growing cultures. For exponentially growing cultures of P. aeruginosa PAO1, we find that aggregation is mediated by cell-associated Psl, rather than by either eDNA or se-creted Psl. These aggregates arise de novo within the culture via a growth process that involves both collisions and clonal growth. They are "non-cheatable" since Psl non-producing cells do not aggregate with producers. In contrast, we find that stationary phase (overnight) cultures contain a different type of multicelullar aggregate, in which both eDNA and Psl mediate cohesion. Our findings suggest that the physical and bi-ological properties of multicellular aggregates may be very different in early-stage vs late-stage bacterial cultures.

microbiology↗

bioRxiv2022

Pinning transition in biofilm structure driven by active layer dynamics

Surface-attached communities of microbes, known as biofilms, are diverse in their morphologies. Characterising distinct types of biofilm spatial structure, and understanding how they emerge, can shed light on the fundamental biological and biophysical mechanisms involved, and can improve our understanding of evolution in biofilms. Here, we perform long-time individual-based simulations of growing biofilms. We observe distinct types of biofilm spatial structure depending on the parameters, and we classify these into three phases according to the behaviour of the active layer of growing cells at the biofilm interface. In the unpinned phase, the biofilm is smooth and the active layer is unbroken with no gaps. In the transiently pinned phase, short-lived gaps in the active layer arise, which can cause local parts of the biofilm interface to pin, or become stationary relative to the moving front. In the pinned phase these pinning sites persist, leading to fingering of the biofilm interface. We show that pinning arises due to the dynamical behaviour of active layer gaps, and observe that the relative magnitudes of the active layer thickness and the active layer fluctuations are important in this process. We demonstrate a direct connection between biofilm pinning and interface roughness, and we show that the pinning phase transition is well described by a control parameter that combines the average and standard deviation of the active layer thickness. Taken together, our work suggests a role for active layer dynamics in controlling pinning of the biofilm interface and hence biofilm morphology.

biophysics↗