Bacteria are not too small for spatial sensing of chemical gradients: An experimental evidence

Abstract

By analyzing the chemotactic behavior of a recently described marine bacterial species, we provide experimental evidence that bacteria are not too small for sensing chemical gradients spatially. The bipolar flagellated vibrioid bacteria (typical size 2 ؋ 6 m) exhibit a unique motility pattern as they translate along as well as rotate around their short axis, i.e., the pathways of the cell poles describe a double helix. The natural habitat of the bacteria is characterized by steep oxygen gradients where they accumulate in a band at their preferred oxygen concentration of Ϸ2 M. Single cells leaving the band toward the oxic region typically return to the band within 16 s following a U-shaped track. A detailed analysis of the tracks reveals that the cells must be able to sense the oxygen gradient perpendicular to their swimming direction. Thus, they can detect oxygen gradients along a distance of Ϸ5 m corresponding to the extension of their long axis. The observed behavior can be explained by the presence of two independent sensor regions at either cell pole that modulate the rotation speed of the polar flagellar bundles, i.e., the flagellar bundle at the cell pole exposed to higher oxygen concentration is rotating faster than the other bundle. A mathematical model based on these assumptions reproduces the observed swimming behavior of the bacteria. F lagellar motility is widespread among prokaryotes living in aquatic environments, and it is estimated that 20% of marine planktonic bacteria are motile (1). Generally, bacteria exhibit motility to find places in their environment where they can maximize their substrate or energy uptake. To achieve this, they have to modulate their motility in response to environmental variables (2). When cell motility is modulated via sensing of chemical signals in the environment, the resulting motility behavior is termed chemotaxis. Best understood is the chemotactic behavior of the flagellated enteric bacterium Escherichia coli (reviewed in refs. 3-5). More or less straight swimming paths (caused by synchronous counter-clockwise rotation of all flagella) are interrupted periodically by random direction changes: so-called tumbles caused by short-term direction reversals of one or more flagella. Chemical attractants or repellants bind to specific receptor protein complexes at the cell surface, which subsequently inhibit or promote, respectively, the phosphorylation of an intracellular freely diffusing signaling protein. The flagellar motors respond to increased phosphorylation levels of the signaling protein with more frequent reversals of flagellar rotation causing more frequent tumbling of the bacteria. Additionally, a dynamic methylation of the receptor protein complexes provides an adaptation mechanism, ensuring that the chemotactic behavior is functioning over a wide range of attractant or repellant concentrations. If a bacterium experiences increasing attractant (or decreasing repellant) concentrations along its swimming path, the described mechanism results in less-frequent ran...

Ecology and Physics of Bacterial Chemotaxis in the Ocean

“…Gradient sensing is typically achieved by temporal comparisons: in E. coli, the receptor occupancy in the recent past is compared to that in the more distant past (166). Spatial sensing, where a bacterium instantaneously compares concentrations over the length of its body, is believed to be more rare but not impossible (46,193). The intracellular transfer of information involved in gradient sensing has been remarkably well characterized and is reviewed elsewhere (152,187,202).…”

Computational and experimental study of chemotaxis of an ensemble of bacteria attached to a microbead

“…A bacterium is set to increase its running time and therefore decrease its tumbling probability, according to Eq. (3), when the chemical concentration it senses exceeds 5 nmol [14]. When in presence of a negative chemical gradient or in an isotropic medium, the bacterium maintains a constant tumbling probability of 0.1.…”

Continuous-Flow Capillary Assay for Measuring Bacterial Chemotaxis

“…Assays that examine the motion of individual bacteria are useful for elucidating the sensing mechanism of a bacterium (16,24,26,35,41,62,64,66) and the swimming strategy bacteria use in a gradient (8,14,39,44,(67)(68)(69)71). Assays that measure the motion of a population of bacteria (populationscale assays), on the other hand, are useful for determining bulk bacterial-transport parameters that can be used in population-scale chemotaxis models.…”