A new mode of swimming in singly flagellated Pseudomonas aeruginosa

Torque-speed relationship of the flagellar motor with dual-stator systems in Pseudomonas aeruginosa

The single polar flagellar motor in Pseudomonas aeruginosa is equipped with two stator systems, MotAB and MotCD, both driven by H+ ions. The torque-speed relationship for flagellar motors with two stator systems has not been explored previously. In this study, we developed a method that utilizes optical trapping and fluorescence labeling to measure the torque-speed relationships for the wild-type P. aeruginosa motor with dual stators and mutant strains with a single stator system, revealing surprising differences in them. Moreover, we found that the MotAB stators exhibit slip-bond behavior in load dependence, contrasting with the catch-bond behavior of the MotCD stators and Escherichia coli stators. Further examination of the solvent isotope and pH effects on the torque-speed relationships of these stator systems provided additional insights into their dynamics. Interestingly, we discovered that the torque of the wild-type motor is similar to the combined torque of motors with MotAB or MotCD stators, indicating an additive contribution from the two stator types in the wild-type motors. These findings underscore the enhanced adaptability of P. aeruginosa to a wide range of external environments with varying load conditions.IMPORTANCEWe developed a novel method to measure the flagellar motor torque-speed relationship by trapping a swimming bacterium using optical tweezers. Using the P. aeruginosa flagellar motor as a model system to investigate motor dynamics with dual stator types, we measured the torque-speed relationships for wild-type motors with dual stator types and mutants with a single type. We found drastic differences that stem from the varying load dependencies of stator stability. These variations enable bacteria to rapidly adjust their stator composition in response to external load conditions. Interestingly, we observed that the torque of the wild-type motor is akin to the cumulative torque of motors with either stator type, indicating an additive contribution from the two stator types in wild-type motors. The methodology we established here can be readily employed to study motor dynamics in other flagellated bacteria.

MotY modulates proton-driven flagellar motor output in Pseudomonas aeruginosa

MotY homologs are present in a variety of monotrichous bacterial strains and are thought to form an additional structural T ring in flagellar motors. While MotY potentially plays an important role in motor torque generation, its impact on motor output dynamics remains poorly understood. In this study, we investigate the role of MotY in P. aeruginosa, elucidating its interactions with the two sets of stator units (MotAB and MotCD) using Förster resonance energy transfer (FRET) assays. Employing a newly developed bead assay, we characterize the dynamic behavior of flagellar motors in motY mutants, identifying MotY as the key functional protein to affect the clockwise bias of naturally unbiased motors in P. aeruginosa. Our findings reveal that MotY enhances stator assembly efficiency without affecting the overall assembly of the flagellar structure. Additionally, we demonstrate that MotY is essential for maintaining motor torque and regulating switching rates. Our study highlights the physiological significance of MotY in fine-tuning flagellar motor function in complex environments.

Mutation of Pseudomonas aeruginosa lasI/rhlI diminishes its cytotoxicity, oxidative stress, inflammation, and apoptosis on THP-1 macrophages

The management of Pseudomonas aeruginosa (P. aeruginosa) infections presents a substantial challenge to clinics and public health, emphasizing the urgent need for innovative strategies to address this issue. Quorum sensing (QS) is an intercellular communication mechanism that coordinates bacterial activities involved in various virulence mechanisms, such as acquiring host nutrients, facilitating biofilm formation, enhancing motility, secreting virulence factors, and evading host immune responses, all of which play a crucial role in the colonization and infection of P. aeruginosa. The LasI/R and RhlI/R sub-systems dominate in the QS system of P. aeruginosa. Macrophages play a pivotal role in the host's innate immune response to P. aeruginosa invasion, particularly through phagocytosis as the initial host defense mechanism. This study investigated the effects of P. aeruginosa's QS system on THP-1 macrophages. Mutants of PAO1 with lasI/rhlI deletion, as well as their corresponding complemented strains, were obtained, and significant downregulation of QS-related genes was observed in the mutants. Furthermore, the ΔlasI and ΔlasIΔrhlI mutants exhibited significantly attenuated virulence in terms of biofilm formation, extracellular polymeric substances synthesis, bacterial adhesion, motility, and virulence factors production. When infected with ΔlasI and ΔlasIΔrhlI mutants, THP-1 macrophages exhibited enhanced scavenging ability against the mutants and demonstrated resistance to cytotoxicity, oxidative stress, inflammatory response, and apoptosis induced by the culture supernatants of these mutant strains. These findings offer novel insights into the mechanisms underlying how the lasI/rhlI mutation attenuates cytotoxicity, oxidative stress, inflammation, and apoptosis in macrophages induced by P. aeruginosa.IMPORTANCEP. aeruginosa is classified as one of the ESKAPE pathogens and poses a global public health concern. The QS system of this versatile pathogen contributes to a broad spectrum of virulence, thereby constraining therapeutic options for serious infections. This study illustrated that the lasI/rhlI mutation of the QS system plays a prominent role in attenuating the virulence of P. aeruginosa by affecting bacterial adhesion, biofilm formation, extracellular polymeric substances synthesis, bacterial motility, and virulence factors' production. Notably, THP-1 macrophages infected with mutant strains exhibited increased phagocytic activity in eliminating intracellular bacteria and enhanced resistance to cytotoxicity, oxidative stress, inflammation, and apoptosis. These findings suggest that targeted intervention toward the QS system is anticipated to diminish the pathogenicity of P. aeruginosa to THP-1 macrophages.

Speed-dependent bacterial surface swimming

Solid surfaces submerged in liquid in natural environments alter bacterial swimming behavior and serve as platforms for bacteria to form biofilms. In the initial stage of biofilm formation, bacteria detect surfaces and increase the intracellular level of the second messenger c-di-GMP, leading to a reduction in swimming speed. The impact of this speed reduction on bacterial surface swimming remains unclear. In this study, we utilized advanced microscopy techniques to examine the effect of swimming speed on bacterial surface swimming behavior. We found that a decrease in swimming speed reduces the cell-surface distance and prolongs the surface trapping time. Both these effects would enhance bacterial surface sensing and increase the likelihood of cells adhering to the surface, thereby promoting biofilm formation. We also examined the surface-escaping behavior of wild-type Escherichia coli and Pseudomonas aeruginosa, noting distinct surface-escaping mechanisms between the two bacterial species.

IMPORTANCE

In the early phase of biofilm formation, bacteria identify surfaces and increase the intracellular level of the second messenger c-di-GMP, resulting in a decrease in swimming speed. Here, we utilized advanced microscopy techniques to investigate the impact of swimming speed on bacterial surface swimming, focusing on Escherichia coli and Pseudomonas aeruginosa. We found that an increase in swimming speed led to an increase in the radius of curvature and a decrease in surface detention time. These effects were explained through hydrodynamic modeling as a result of an increase in the cell-surface distance with increasing swimming speed. We also observed distinct surface-escaping mechanisms between the two bacterial species. Our study suggests that a decrease in swimming speed could enhance the likelihood of cells adhering to the surface, promoting biofilm formation. This sheds light on the role of reduced swimming speed in the transition from motile to sedentary bacterial lifestyles.

How P. aeruginosa cells with diverse stator composition collectively swarm

Swarming is a macroscopic phenomenon in which surface bacteria organize into a motile population. The flagellar motor that drives swarming in Pseudomonas aeruginosa is powered by stators MotAB and MotCD. Deletion of the MotCD stator eliminates swarming, whereas deletion of the MotAB stator enhances swarming. Interestingly, we measured a strongly asymmetric stator availability in the wild-type (WT) strain, with MotAB stators produced at an approximately 40-fold higher level than MotCD stators. However, utilization of MotCD stators in free swimming cells requires higher liquid viscosities, while MotAB stators are readily utilized at low viscosities. Importantly, we find that cells with MotCD stators are ~10× more likely to have an active motor compared to cells uses the MotAB stators. The spectrum of motility intermittency can either cooperatively shut down or promote flagellum motility in WT populations. In P. aeruginosa, transition from a static solid-like biofilm to a dynamic liquid-like swarm is not achieved at a single critical value of flagellum torque or stator fraction but is collectively controlled by diverse combinations of flagellum activities and motor intermittencies via dynamic stator utilization. Experimental and computational results indicate that the initiation or arrest of flagellum-driven swarming motility does not occur from individual fitness or motility performance but rather related to concepts from the "jamming transition" in active granular matter.IMPORTANCEIt is now known that there exist multifactorial influences on swarming motility for P. aeruginosa, but it is not clear precisely why stator selection in the flagellum motor is so important. We show differential production and utilization of the stators. Moreover, we find the unanticipated result that the two motor configurations have significantly different motor intermittencies: the fraction of flagellum-active cells in a population on average with MotCD is active ~10× more often than with MotAB. What emerges from this complex landscape of stator utilization and resultant motor output is an intrinsically heterogeneous population of motile cells. We show how consequences of stator recruitment led to swarming motility and how the stators potentially relate to surface sensing circuitry.

Bacterial surface swimming states revealed by TIRF microscopy

Motility near solid surfaces plays a key role in the life cycle of bacteria and is essential for biofilm formation, biofilm dispersal, and virulence. The alignment of the cell body with the surface during surface swimming impacts bacterial surface sensing. Here, we developed a high-throughput method for characterizing the orientation of the cell body relative to the surface using total internal reflection fluorescence (TIRF) microscopy. The angle between the cell body and the surface was determined by maximizing image cross-correlations between the TIRF image of the cell and a reference library. Utilizing this technique, we surprisingly identified six distinct surface swimming states of Pseudomonas aeruginosa according to the body alignment and the flagellar position. Furthermore, we observed that the near-surface swimming speed is greater in the pull state than in the push state, attributed to hydrodynamic effects near the liquid-solid interface. Hydrodynamic force analysis of the swimming states provided rich insights into the mechanics of bacterial surface swimming. Our technique is readily applicable to the study of surface motility across a wide spectrum of bacterial species.

Flagella and Cell Body Staining of Bacteria with Fluorescent Dyes

Bacteria can propel themselves by rotating a flagellum or a flagellar bundle. To image this thin structure in motile bacteria, the flagella can be vitally stained with fluorophores. This chapter describes a flagellar staining protocol with the additional possibility of visualizing the cell body. It offers the opportunity to track conformational changes of flagella and simultaneously track the positions of the cell bodies. The additional use of a filter increases the number of motile cells and improves the signal-to-noise ratio of images. The flagellar staining requires a prior introduction of a surface-exposed cysteine, which is not covered in this chapter.

Flagellar polymorphism-dependent bacterial swimming motility in a structured environment

Most motile bacteria use supramolecular motility machinery called bacterial flagellum, which converts the chemical energy gained from ion flux into mechanical rotation. Bacterial cells sense their external environment through a two-component regulatory system consisting of a histidine kinase and response regulator. Combining these systems allows the cells to move toward favorable environments and away from their repellents. A representative example of flagellar motility is run-and-tumble swimming in Escherichia coli, where the counter-clockwise (CCW) rotation of a flagellar bundle propels the cell forward, and the clockwise (CW) rotation undergoes cell re-orientation (tumbling) upon switching the direction of flagellar motor rotation from CCW to CW. In this mini review, we focus on several types of chemotactic behaviors that respond to changes in flagellar shape and direction of rotation. Moreover, our single-cell analysis demonstrated back-and-forth swimming motility of an original E. coli strain. We propose that polymorphic flagellar changes are required to enhance bacterial movement in a structured environment as a colony spread on an agar plate.

An unbroken network of interactions connecting flagellin domains is required for motility in viscous environments

In its simplest form, bacterial flagellar filaments are composed of flagellin proteins with just two helical inner domains, which together comprise the filament core. Although this minimal filament is sufficient to provide motility in many flagellated bacteria, most bacteria produce flagella composed of flagellin proteins with one or more outer domains arranged in a variety of supramolecular architectures radiating from the inner core. Flagellin outer domains are known to be involved in adhesion, proteolysis and immune evasion but have not been thought to be required for motility. Here we show that in the Pseudomonas aeruginosa PAO1 strain, a bacterium that forms a ridged filament with a dimerization of its flagellin outer domains, motility is categorically dependent on these flagellin outer domains. Moreover, a comprehensive network of intermolecular interactions connecting the inner domains to the outer domains, the outer domains to one another, and the outer domains back to the inner domain filament core, is required for motility. This inter-domain connectivity confers PAO1 flagella with increased stability, essential for its motility in viscous environments. Additionally, we find that such ridged flagellar filaments are not unique to Pseudomonas but are, instead, present throughout diverse bacterial phyla.

Differential Bending Stiffness of the Bacterial Flagellar Hook under Counterclockwise and Clockwise Rotations

The bacterial hook, as a universal joint coupling rotation of the flagellar motor and the filament, is an important component of the flagellum that propels the bacteria to swim. The mechanical properties of the hook are essential for the flagellum to achieve normal functions. In multiflagellated bacteria such as Escherichia coli, the hook must be compliant so that it can bend for the filaments to form a coherently rotating bundle to generate the thrust when the motor rotates counterclockwise (CCW), yet it also must be rigid so that the bundle can disrupt for the bacteria to tumble to change swimming direction when the motor rotates clockwise (CW). Here, by combining an elastic rod model with high-resolution bead assay to accurately measure the bending stiffness of the hook under CCW or CW rotation in vivo, we elucidate how the hook accomplishes this dual functionality: the hook stiffens under CW rotation, with bending stiffness under CW rotation twice as large as that under CCW rotation. This enables a robust run-and-tumble swimming motility for multiflagellated bacteria.

Distinct Screening Approaches Uncover PA14_36820 and RecA as Negative Regulators of Biofilm Phenotypes in Pseudomonas aeruginosa PA14

Pseudomonas aeruginosa commonly infects hospitalized patients and the lungs of individuals with cystic fibrosis. This species is known for forming biofilms, which are communities of bacterial cells held together and encapsulated by a self-produced extracellular matrix. The matrix provides extra protection to the constituent cells, making P. aeruginosa infections challenging to treat. We previously identified a gene, PA14_16550, which encodes a DNA-binding TetR-type repressor and whose deletion reduced biofilm formation. Here, we assessed the transcriptional impact of the 16550 deletion and found six differentially regulated genes. Among them, our results implicated PA14_36820 as a negative regulator of biofilm matrix production, while the remaining 5 had modest effects on swarming motility. We also screened a transposon library in a biofilm-impaired ΔamrZ Δ16550 strain for restoration of matrix production. Surprisingly, we found that disruption or deletion of recA increased biofilm matrix production, both in biofilm-impaired and wild-type strains. Because RecA functions both in recombination and in the DNA damage response, we asked which function of RecA is important with respect to biofilm formation by using point mutations in recA and lexA to specifically disable each function. Our results implied that loss of either function of RecA impacts biofilm formation, suggesting that enhanced biofilm formation may be one physiological response of P. aeruginosa cells to loss of either RecA function. IMPORTANCE Pseudomonas aeruginosa is a notorious human pathogen well known for forming biofilms, communities of bacteria that protect themselves within a self-secreted matrix. Here, we sought to find genetic determinants that impacted biofilm matrix production in P. aeruginosa strains. We identified a largely uncharacterized protein (PA14_36820) and, surprisingly, RecA, a widely conserved bacterial DNA recombination and repair protein, as negatively regulating biofilm matrix production. Because RecA has two main functions, we used specific mutations to isolate each function and found that both functions influenced matrix production. Identifying negative regulators of biofilm production may suggest future strategies to reduce the formation of treatment-resistant biofilms.

Porous marine snow differentially benefits chemotactic, motile, and nonmotile bacteria

Particulate organic carbon settling through the marine water column is a key process that regulates the global climate by sequestering atmospheric carbon. The initial colonization of marine particles by heterotrophic bacteria represents the first step in recycling this carbon back to inorganic constituents-setting the magnitude of vertical carbon transport to the abyss. Here, we demonstrate experimentally using millifluidic devices that, although bacterial motility is essential for effective colonization of a particle leaking organic nutrients into the water column, chemotaxis specifically benefits at intermediate and higher settling velocities to navigate the particle boundary layer during the brief window of opportunity provided by a passing particle. We develop an individual-based model that simulates the encounter and attachment of bacterial cells with leaking marine particles to systematically evaluate the role of different parameters associated with bacterial run-and-tumble motility. We further use this model to explore the role of particle microstructure on the colonization efficiency of bacteria with different motility traits. We find that the porous microstructure facilitates additional colonization by chemotactic and motile bacteria, and fundamentally alters the way nonmotile cells interact with particles due to streamlines intersecting with the particle surface.

Role of the Two Flagellar Stators in Swimming Motility of Pseudomonas putida

In the soil bacterium Pseudomonas putida, the motor torque for flagellar rotation is generated by the two stators MotAB and MotCD. Here, we construct mutant strains in which one or both stators are knocked out and investigate their swimming motility in fluids of different viscosity and in heterogeneous structured environments (semisolid agar). Besides phase-contrast imaging of single-cell trajectories and spreading cultures, dual-color fluorescence microscopy allows us to quantify the role of the stators in enabling P. putida's three different swimming modes, where the flagellar bundle pushes, pulls, or wraps around the cell body. The MotAB stator is essential for swimming motility in liquids, while spreading in semisolid agar is not affected. Moreover, if the MotAB stator is knocked out, wrapped mode formation under low-viscosity conditions is strongly impaired and only partly restored for increased viscosity and in semisolid agar. In contrast, when the MotCD stator is missing, cells are indistinguishable from the wild type in fluid experiments but spread much more slowly in semisolid agar. Analysis of the microscopic trajectories reveals that the MotCD knockout strain forms sessile clusters, thereby reducing the number of motile cells, while the swimming speed is unaffected. Together, both stators ensure a robust wild type that swims efficiently under different environmental conditions. IMPORTANCE Because of its heterogeneous habitat, the soil bacterium Pseudomonas putida needs to swim efficiently under very different environmental conditions. In this paper, we knocked out the stators MotAB and MotCD to investigate their impact on the swimming motility of P. putida. While the MotAB stator is crucial for swimming in fluids, in semisolid agar, both stators are sufficient to sustain a fast-swimming phenotype and increased frequencies of the wrapped mode, which is known to be beneficial for escaping mechanical traps. However, in contrast to the MotAB knockout, a culture of MotCD knockout cells spreads much more slowly in the agar, as it forms nonmotile clusters that reduce the number of motile cells.

Wrapped Up: The Motility of Polarly Flagellated Bacteria

A huge number of bacterial species are motile by flagella, which allow them to actively move toward favorable environments and away from hazardous areas and to conquer new habitats. The general perception of flagellum-mediated movement and chemotaxis is dominated by the Escherichia coli paradigm, with its peritrichous flagellation and its famous run-and-tumble navigation pattern, which has shaped the view on how bacteria swim and navigate in chemical gradients. However, a significant amount-more likely the majority-of bacterial species exhibit a (bi)polar flagellar localization pattern instead of lateral flagella. Accordingly, these species have evolved very different mechanisms for navigation and chemotaxis. Here, we review the earlier and recent findings on the various modes of motility mediated by polar flagella. Expected final online publication date for the Annual Review of Microbiology, Volume 76 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.