Biology subjects

Poon, W. C. K.

Publications and source records attributed to Poon, W. C. K..

6 recordsLinked to original sources

bioRxiv2026

Metabolic feedback during bacterial fermentation is a motility brake

We study an unexpectedly fast decay of motility in dense suspensions of Escherichia coli bacteria supplied with excess glucose under anaerobic conditions. The decrease in swimming speed occurs on a timescale inversely proportional to the cell concentration, and is associated with the secretion of organic acids by the bacteria. We show that the decay is driven by the progressive accumulation of non-ionised organic acids in the medium, and develop a chemical kinetic model that successfully predicts the swimming speed variations over a range of conditions in the presence of these acids. We further measure the internal pH of E. coli cells exposed to organic acids, and find that the speed decay coincides with sharp declines in internal pH and metabolic rate. Our findings identify an additional layer of motility control that can arise in complex environments even when motility genes are expressed and energy sources are abundant. This mechanism is likely relevant for understanding bacterial motility in habitats such as the human gut, where high densities of bacteria and organic acids are common.

microbiology↗

bioRxiv2026

Why Life is Hot

The process of evolution by natural selection leads to phenotypes of increasing fitness. For cellular chemical reaction networks, this means optimising a variety of fitness functions such as robustness, precision, or sensitivity to external stimuli. We argue that these diverse goals can be achieved by a versatile, generic mechanism: coupling chemical reaction networks to reservoirs that are strongly out of equilibrium. Using theory and numerics we show that this mechanism of optimisation comes at the price of significant heat dissipation. We compute the heat flux caused by kinetic proofreading in Escherichia coli and show that it constitutes a significant fraction of the total heat flux experimentally measured in this model organism. We then demonstrate that the degree of optimality achievable saturates, and that Nature appears to operate near saturation despite high energetic costs. We argue that `life is hot' largely because of the need for a versatile mechanism to optimise a variety of fitness functions.

biophysics↗

bioRxiv2025

Medium-assisted tumbling controls bacteria exploration in a complex fluid

In nature, many fluids that harbor bacterial populations or protect against microbial contamination exhibit non-Newtonian rheology. To study the spatial exploration of E.coli bacteria, a model multi-flagellated microorganism, in such complex environments, we design a motility medium with tunable macroscopic rheology. By increasing the solid charge in soft carbomer grains, we transition from a Newtonian viscous suspension to a yield-stress fluid. Using a 3D Lagrangian tracking device, we collect many individual bacterial tracks and characterize changes in motility properties such as swimming speed, persistence times and diffusivity for both a wild-type and a smooth runner mutant, up to the formation of a motility barrier at higher carbomer concentrations. We show that the presence of local mechanical disorder and resistance to penetration essentially override the biologically driven run-and-tumble navigation process. This "medium-assisted" exploration scenario is characterized by directional switching and stop-and-go kinematics and is closely related to the flexibility of the flagellar bundle.

biophysics↗

bioRxiv2025

Environmental dependence of colony morphologies in Labyrinthula species

Labyrinthula species are protist organisms found across a variety of marine environments whose defining characteristic is the secretion of an extracellular ectoplasmic net. Under certain conditions, colonies form a spatial network of tracks through which cells move bidirectionally. We show that this network morphology depends on the presence of a liquid overlay, with air exposed colonies exhibiting instead a dense, aggregated morphology. We demonstrate dynamic restructuring between these two morphologies upon addition or removal of the liquid overlay, and investigate growth behaviour under varying nutrient conditions. Given the inter-tidal environment of certain seagrass species colonised by Labyrinthula, our results may shed light on the relationship between this organism and its seagrass host, for which it is an opportunistic pathogen associated with seagrass wasting disease.

biophysics↗

bioRxiv2024

Nonlinear dependency of the bacterial flagellar motor speed on proton motive force and its consequences for swimming

The bacterial flagellar motor enables bacteria to swim by rotating helical flagellar filaments that form a bundle at the back of the cell. Escherichia colis motor uses the energy stored in the electrochemical gradient of protons, the proton motive force (PMF), to generate the torque driving this rotation. Until now, motor speed was thought to be proportional to the PMF, irrespective of the viscous load on the motor, and across the physiological range of PMF values. Here, we show that the PMF-speed relationship is non-linear in the high-torque regime. Because saturation in the relationship occurs in the physiologically relevant range, it challenges all current models of motor function that assume a tight coupling between the motor rotation and PMF across the entire physiological range. Furthermore, we experimentally determine the load on the motor experienced by swimming cells, and show that free swimming occurs close to or within the saturation regime, making the observed limiting torque evolutionary relevant.

microbiology↗

bioRxiv2018

High-throughput characterisation of bull semen motility using differential dynamic microscopy

We report a high-throughput technique for characterising the motility of spermatozoa using differential dynamic microscopy. A large field of view movie (~ 10mm2) records thousands of cells (e.g. {approx} 5000 cells even at a low cell density of 20 x 106 cells/ml) at once and yields averaged measurements of the mean ({upsilon}) and standard deviation ({sigma}) of the swimming speed, a head oscillation amplitude (A0) and frequency (f0), and the fraction of motile spermatozoa (). Interestingly, the measurement of relies on the swimming spermatozoa enhancing the motion of the non-swimming population. We demonstrate the ease and rapidity of our method by performing on-farm characterisation of bull spermatozoa motility, and validate the technique by comparing laboratory measurements with tracking. Our results confirm the long-standing theoretical prediction that [Formula] for swimming spermatozoa.

biophysics↗