Three-Dimensional Observations of an Aperiodic Oscillatory Gliding Behavior in Myxococcus xanthus Using Confocal Interference Reflection Microscopy

ABSTRACT The deltaproteobacterium Myxococcus xanthus is a model for bacterial motility and has provided unprecedented insights into bacterial swarming behaviors. Fluorescence microscopy techniques have been invaluable in defining the mechanisms that are involved in gliding motility, but these have almost entirely been limited to two-dimensional (2D) studies, and there is currently no understanding of gliding motility in a three-dimensional (3D) context. We present here the first use of confocal interference reflection microscopy (IRM) to study gliding bacteria, revealing aperiodic oscillatory behavior with changes in the position of the basal membrane relative to the substrate on the order of 90 nm in vitro. First, we use a model planoconvex lens specimen to show how topological information can be obtained from the wavelength-dependent interference pattern in IRM. We then use IRM to observe gliding M. xanthus bacteria and show that cells undergo previously unobserved changes in their adhesion profile as they glide. We compare the wild type with mutants that have reduced motility, which also exhibit the same changes in the adhesion profile during gliding. We find that the general gliding behavior is independent of the proton motive force-generating complex AglRQS and suggest that the novel behavior that we present here may be a result of recoil and force transmission along the length of the cell body following firing of the type IV pili. IMPORTANCE 3D imaging of live bacteria with optical microscopy techniques is a challenge due to the small size of bacterial cells, meaning that previous studies have been limited to observing motility behavior in 2D. We introduce the application of confocal multiwavelength interference reflection microscopy to bacteria, which enables visualization of 3D motility behaviors in a single 2D image. Using the model organism Myxococcus xanthus, we identified novel motility behaviors that are not explained by current motility models, where gliding bacteria exhibit aperiodic changes in their adhesion to an underlying solid surface. We concluded that the 3D behavior was not linked to canonical motility mechanisms and that IRM could be applied to study a range of microbiological specimens with minimal adaptation to a commercial microscope.

A mathematical model of the locomotion of bacteria near an inclined solid substrate: effects of different waveforms and rheological properties of couple-stress slime

Morphological mutations in bacterial cell make them the most miscellaneous microscopic group. Their non-flagellated species known as gliding bacteria exhibit self-powered motion and leave an adhesive trail of slime. The self-propelled motion in some gliding bacteria is achieved as a result of backward surface wave in the cell envelope. Motivated by this fact, an undulating surface on a layer of couple-stress fluid is used to model the motion of such gliding bacteria. Five different wave profiles, namely, sawtooth, sinusoidal, triangular, trapezoidal, and square profiles are used to model the waveform of the undulating wave in the outer cell surface. The inclination of the surface is also integrated into the model. The flow equations are set up under the lubrication assumption. Stream function is derived as an elementary function of an organism’s speed, undulation amplitude, and couple-stress parameter with its flow rate. Speed of the glider and flow rate (satisfying equilibrium conditions) are computed by employing modified Newton–Raphson method. These refined values are further utilized to compute the power dissipation. Effects of different waveforms, inclination angle, gravitational and couple-stress parameters on the speed of the microorganism and rate of energy expended are also quantified. Slime velocity is also plotted for fixed glider. In addition, making use of the obtained realistic set of values of the organism’s speed, flow rate, occlusion parameter, and couple-stress parameter, streamline patterns of the slime are plotted and discussed in detail.

Cargo transport shapes the spatial organization of a microbial community

The human microbiome is an assemblage of diverse bacteria that interact with one another to form communities. Bacteria in a given community are arranged in a 3D matrix with many degrees of freedom. Snapshots of the community display well-defined structures, but the steps required for their assembly are not understood. Here, we show that this construction is carried out with the help of gliding bacteria. Gliding is defined as the motion of cells over a solid or semisolid surface without the necessity of growth or the aid of pili or flagella. Genomic analysis suggests that gliding bacteria are present in human microbial communities. We focus on Capnocytophaga gingivalis, which is present in abundance in the human oral microbiome. Tracking of fluorescently labeled single cells and of gas bubbles carried by fluid flow shows that swarms of C. gingivalis are layered, with cells in the upper layers moving more rapidly than those in the lower layers. Thus, cells also glide on top of one another. Cells of nonmotile bacterial species attach to the surface of C. gingivalis and are propelled as cargo. The cargo cell moves along the length of a C. gingivalis cell, looping from one pole to the other. Multicolor fluorescent spectral imaging of cells of different live but nonmotile bacterial species reveals their long-range transport in a polymicrobial community. A swarm of C. gingivalis transports some nonmotile bacterial species more efficiently than others and helps to shape the spatial organization of a polymicrobial community.

Cargo Transport Shapes the Spatial Organization of a Microbial Community

The human microbiome is an assemblage of diverse bacteria that interact with one another to form communities. Bacteria in a given community are arranged in a three-dimensional matrix with many degrees of freedom. Snapshots of the community display well-defined structures, but the steps required for their assembly are not understood. Here, we show that this construction is carried out with the help of gliding bacteria. Gliding is defined as the motion of cells over a solid or semi-solid surface without the necessity of growth or the aid of pili or flagella. Genomic analysis suggests that gliding bacteria are present in human microbial communities. We focus on Capnocytophaga gingivalis which is present in abundance in the human oral microbiome. Tracking of fluorescently-labeled single cells and of gas bubbles carried by fluid flow shows that swarms of C. gingivalis are layered, with cells in the upper layers moving more rapidly than those in the lower layers. Thus, cells also glide on top of one another. Cells of non-motile bacterial species attach to the surface of C. gingivalis and are propelled as cargo. The cargo cell moves along the length of a C. gingivalis cell, looping from one pole to the other. Multi-color fluorescent spectral imaging of cells of different live but non-motile bacterial species reveals their long-range transport in a polymicrobial community. A swarm of C. gingivalis transports some non-motile bacterial species more efficiently than others and helps shape the spatial organization of a polymicrobial community.SignificanceWe describe a situation in which bacteria typical of the human oral microbiome are organized spatially by gliding cells, species of Capnocytophaga, that move backwards and forwards over the substratum. The mobile adhesins that pull the cells over the substratum also attach to cells of non-motile bacterial species, which are carried up and down the motile cells as cargo. The synchronized transport of non-motile cargo bacteria helps shape a polymicrobial community.

Dissipative shocks behind bacteria gliding

Gliding is a means of locomotion on rigid substrates used by a number of bacteria, including myxobacteria and cyanobacteria. One of the hypotheses advanced to explain this motility mechanism hinges on the role played by the slime filaments continuously extruded from gliding bacteria. This paper solves, in full, a non-linear mechanical theory that treats as dissipative shocks both the point where the extruded slime filament comes into contact with the substrate, called the filament’s foot , and the pore on the bacterium outer surface from where the filament is ejected. I prove that kinematic compatibility for shock propagation requires that the bacterium uniform gliding velocity (relative to the substrate) and the slime ejecting velocity (relative to the bacterium) must be equal, a coincidence that seems to have already been observed.