For decades, research into bacterial behavior has largely focused on biochemical interactions. Free-swimming bacteria, such as Escherichia coli, Salmonella enterica, and Pseudomonas aeruginosa are among the most extensively studied. Through careful work carried out by a vibrant community, we know that binding of ligands to cell surface receptors triggers intracellular signaling cascades that, among other things, control the direction of rotation of the flagellar motor, which is a large molecular machine that powers motility in many bacteria (Wadhwa and Berg, 2021). More recently, however, it has become clear that physical forces also supply critical information in bacterial cells, where they regulate a variety of behaviors, including swarming, biofilm formation, and pathogenesis (Dufrêne and Persat, 2020). Mechanosensing enables bacteria to respond to a rich landscape of physical cues from their immediate environment, such as surfaces, flows, and osmolarity. This, in turn, enables them to adapt to their specific environmental niches and therefore improve fitness.

We are at the onset of a new field of mechanomicrobiology that investigates the interactions between bacteria and their mechanical environment. We are only beginning to understand the many ways in which physical forces, particularly at the single cell level, modulate behavior in bacteria. New problems require new tools, and new tools often reveal new biology. Mechanical manipulation of single bacteria is challenging due to their small size, and we invest part of our effort into developing techniques to meet these challenges.

Past research

Our past research focused on how the bacterial flagellar motor adapts to large changes in mechanical load (Wadhwa, Phillips, and Berg 2019; Wadhwa, Tu, and Berg, 2021). We adopted the technique of electrorotation to apply a high-frequency rotating electric field to single bacteria, causing an external torque on the cell. Using this novel approach, we showed that the motor adapts to the external load by rebuilding itself; it adds or removes force-generating (stator) units. Force generated by the stator units controls their unbinding, forming a positive feedback loop that leads to autoregulation of the assembly. We have collaborated with Rob Phillips’ and Yuhai Tu’s groups to develop theoretical models for load-dependent motor remodeling (Wadhwa, Phillips, and Berg 2019; Wadhwa et al., 2022). This work quantitatively describes how a mechanical stimulus from the environment (change in load on the motor) is converted into a biochemical change within the cell (stator remodeling). We hypothesize that this mechanochemical coupling forms an input to an intracellular signaling pathway that enables flagella-mediated surface sensing in bacteria.

The biophysical nature of the flagellar motor remains a topic of deep interest. We recently collaborated with colleagues in Denmark and Germany to ask how the flagellar motor generates rotation (Hu et al., 2021). Despite decades of research, the mechanism of force production in the motor had remained unknown. To fill this gap, Nicholas Taylor’s group at the University of Copenhagen used cryo-EM to obtain the high-resolution structure of stator units. We proposed a model of force production in which rotation within a stator unit drives the motor (Santiveri et al., 2020). So, the flagellar motor is driven by even smaller rotary motors. Susan Lea’s group at Oxford independently came to the same conclusions at around the same time (Deme et al., 2020).

Current research

Our overarching goal is to understand the interaction between physical forces and cellular behavior, with special focus on motile bacteria and the molecular machinery that makes them move. We are currently working on three questions in more detail:

  • How do bacterial flagella act as mechanosensors during surface sensing?
    • During early stages of biofilm formation, bacterial cells undergo a planktonic-to-sessile transition upon detecting surface contact. But how do single cells “know” that they are on a surface? Existing evidence shows that in addition to powering motility, the bacterial flagellar motor acts as a mechanosensor during surface contact. Our work on load-dependent remodeling provides a plausible mechanism for mechanosensing by the motor. We combine biophysical manipulation of the motor with high-throughput genetics tools to tease apart the signaling pathways that connect the motor with other processes in the cell.

  • How do bacteria sense flows?
    • Bacteria live in a fluid environment and as such they are constantly exposed to hydrodynamic signals from their surroundings. How bacteria sense and respond to these signals remains unknown. We combine microfluidic manipulation of bacteria with quantitative imaging to investigate how single bacterial cells interact with their immediate hydrodynamic environment.

  • How do bacteria deal with osmotic stress?
    • Osmotic stress is one of the most common types of mechanical stress that bacteria experience. How bacteria adapt to sudden downshifts in the external osmolarity is well characterized. In contrast, we know much less about how bacteria sense and respond to sudden upshifts in osmolarity. In the past, bacterial response to osmotic insults has typically been studied over a timescale of several minutes to hours. We conduct experiments that apply rapid osmotic shocks to single bacteria and measure their response at very high temporal resolution.

To answer these questions, we develop and apply multidisciplinary approaches to precisely manipulate and quantify the workings of single bacterial cells. These include single-cell biophysics techniques (such as electrorotation), quantitative microscopy, high-throughput genetics, and theoretical modeling. Using these approaches, we hope to gain novel insights into how physical forces affect the lives of bacteria in their natural environment and during infections.