The Physics Arxiv has a new preprint which I find...interesting. It's pretty simple to describe the setup - the researchers simply put a actively growing bacterial culture in an standard dynamic mechanical analyzer and looked at what happened over time as the colony multiplied via division [*]. I say that this is "interesting" because while the rheology details aren't quite right, the overall results are still pretty fascinating. Here's the primary finding:
The researchers had expected such non-linear results as they were working with Staphylococcus aureus, a microbe known to produce adhesins (yes, adhesins, not adhesions). These are proteins on their cell walls that allow them to adhere to surfaces and each other. Adhesin production is greatest when the growth of the CFU's is the greatest. After the cell density is high enough that a more-or-less continuous network of cells is formed, the adhesin production then drops off (I suspect that this is to allow cells to move off and infect new areas, much like when cancer cell metastasizes).
The researchers are proposing that it is largely the adhesin production cycle that is being shown in the rheological data. During the initial production stages, the viscosity rises are the cells stick to each other and the rheometer's plates. Later, once the adhesin production falls, the shear in the rheometer is able to break apart the cell-cell and cell-wall interactions and destroy the network. Getting both shear-thickening and shear-thinning behavior from a single sample is pretty difficult with the inert materials that a rheologist commonly faces, but for biological samples, it's all in a day's work.
[*] More proof that microbiologists are bad at math - they think multiplication and division are one and the same.