Cell Size: Fat Makes Cells Fat

Optimal Cell Length for Exploration and Exploitation in Chemotactic Planktonic Bacteria

Elongated morphologies are prevalent among motile bacterioplankton in aquatic systems. This is often attributed to enhanced chemotactic ability, but how long is best? We hypothesized the existence of an optimal cell length for efficient chemotaxis resulting from shape-imposed physical constraints acting on the trade-off between rapid exploration versus efficient exploitation of nutrient sources. To test this hypothesis, we evaluated the chemotactic performance of elongated cephalexin-treated Escherichia coli towards α-methyl-aspartate in a microfluidic device creating linear, stable and quiescent chemical gradients. Our experiments showed cells of intermediate length aggregating most tightly to the chemoattractant source. A sensitivity analysis of an Individual-Based-Model replicating these results showed that 1) cells of intermediate length are optimal at transient states, whereas at steady state longest cells are best, 2) poor chemotactic performance of very short cells is caused by directionality loss, and 3) long cells are penalized by brief, slow runs. Finally, we evaluated chemotactic performance of cells of different length with simulations of a phycosphere, and found that long cells swimming in a run-and-reverse pattern with extended runs and moderate speeds are most efficient in this microenvironment. Overall, our results suggest that the stability of the chemical landscape plays a role in cell-size selection.

Quorum-sensing gene regulates hormetic effects induced by sulfonamides in Comamonadaceae

IMPORTANCE

Antibiotics can induce dose-dependent hormetic effects on bacterial cell proliferation, i.e., low-dose stimulation and high-dose inhibition. However, the underlying molecular basis has yet to be clarified. Here, we showed that sulfonamides play dual roles as a weapon and signal against Comamonas testosteroni that can modulate cell physiology and phenotype. Subsequently, through investigating the hormesis mechanism, we proposed a comprehensive regulatory pathway for the hormetic effects of Comamonas testosteroni low-level sulfonamides and determined the generality of the observed regulatory model in the Comamonadaceae family. Considering the prevalence of Comamonadaceae in human guts and environmental ecosystems, we provide critical insights into the health and ecological effects of antibiotics.

Quantitative Connection between Cell Size and Growth Rate by Phospholipid Metabolism

The processes involved in cell growth are extremely complicated even for a single cell organism such as Escherichia coli, while the relationship between growth rate and cell size is simple. We aimed to reveal the systematic link between them from the aspect of the genome-scale metabolic network. Since the growth rate reflects metabolic rates of bacteria and the cell size relates to phospholipid synthesis, a part of bacterial metabolic networks, we calculated the cell length from the cardiolipin synthesis rate, where the cardiolipin synthesis reaction is able to represent the phospholipid metabolism of Escherichia coli in the exponential growth phase. Combined with the flux balance analysis, it enables us to predict cell length and to examine the quantitative relationship between cell length and growth rate. By simulating bacteria growing in various nutrient media with the flux balance analysis and calculating the corresponding cell length, we found that the increase of the synthesis rate of phospholipid, the cell width, and the protein fraction in membranes caused the increase of cell length with growth rate. Different tendencies of phospholipid synthesis rate changing with growth rate result in different relationships between cell length and growth rate. The effects of gene deletions on cell size and growth rate are also examined. Knocking out the genes, such as Δ tktA, Δ tktB, Δ yqaB, Δ pgm, and Δ cysQ, affects growth rate largely while affecting cell length slightly. Results of this method are in good agreement with experiments.