Imaging the Separation Distance between the Attached Bacterial Cells and the Surface with a Total Internal Reflection Dark-Field Microscope

Investigation of Three-Dimensional Bacterial Adhesion Manner on Model Organic Surfaces Using Quartz Crystal Microbalance with Energy Dissipation Monitoring

Bacterial biofilms reduce the performance and efficiency of biomedical and industrial devices. The initial step in forming bacterial biofilms is the weak and reversible attachment of the bacterial cells onto the surface. This is followed by bond maturation and secretion of polymeric substances, which initiate irreversible biofilm formation, resulting in stable biofilms. This implies that understanding the initial reversible stage of the adhesion process is crucial to prevent bacterial biofilm formation. In this study, we analyzed the adhesion processes of E. coli on self-assembled monolayers (SAMs) with different terminal groups using optical microscopy and quartz crystal microbalance with energy dissipation (QCM-D) monitoring. We found that a considerable number of bacterial cells adhere to hydrophobic (methyl-terminated) and hydrophilic protein-adsorbing (amine- and carboxy-terminated) SAMs forming dense bacterial adlayers while attaching weakly to hydrophilic protein-resisting SAMs [oligo(ethylene glycol) (OEG) and sulfobetaine (SB)], forming sparse but dissipative bacterial adlayers. Moreover, we observed positive shifts in the resonant frequency for the hydrophilic protein-resisting SAMs at high overtone numbers, suggesting how bacterial cells cling to the surface using their appendages as explained by the coupled-resonator model. By exploiting the differences in the acoustic wave penetration depths at each overtone, we estimated the distance of the bacterial cell body from different surfaces. The estimated distances provide a possible explanation for why bacterial cells tend to attach firmly to some surfaces and weakly to others. This result is correlated to the strength of the bacterium-substratum bonds at the interface. Elucidating how the bacterial cells adhere to different surface chemistries can be a suitable guide in identifying surfaces with a more significant probability of contamination by bacterial biofilms and designing bacteria-resistant surfaces and coatings with excellent bacterial antifouling characteristics.

Roadmap on emerging concepts in the physical biology of bacterial biofilms: from surface sensing to community formation

Bacterial biofilms are communities of bacteria that exist as aggregates that can adhere to surfaces or be free-standing. This complex, social mode of cellular organization is fundamental to the physiology of microbes and often exhibits surprising behavior. Bacterial biofilms are more than the sum of their parts: single-cell behavior has a complex relation to collective community behavior, in a manner perhaps cognate to the complex relation between atomic physics and condensed matter physics. Biofilm microbiology is a relatively young field by biology standards, but it has already attracted intense attention from physicists. Sometimes, this attention takes the form of seeing biofilms as inspiration for new physics. In this roadmap, we highlight the work of those who have taken the opposite strategy: we highlight the work of physicists and physical scientists who use physics to engage fundamental concepts in bacterial biofilm microbiology, including adhesion, sensing, motility, signaling, memory, energy flow, community formation and cooperativity. These contributions are juxtaposed with microbiologists who have made recent important discoveries on bacterial biofilms using state-of-the-art physical methods. The contributions to this roadmap exemplify how well physics and biology can be combined to achieve a new synthesis, rather than just a division of labor.

High-performance imaging of cell-substrate contacts using refractive index quantification microscopy

Non-invasive imaging of living cells is an advanced technique that is widely used in the life sciences and medical research. We demonstrate a refractive index quantification microscopy (RIQM) that enables label-free studies of glioma cell-substrate contacts involving cell adhesion molecules and the extracellular matrix. This microscopy takes advantage of the smallest available spot created when an azimuthally polarized perfect optical vortex beam (POV) is tightly focused with a first-order spiral phase, which results in a relatively high imaging resolution among biosensors. A high refractive index (RI) resolution enables the RI distribution within neuronal cells to be monitored. The microscopy shows excellent capability for recognizing cellular structures and activities, demonstrating great potential in biological sensing and live-cell kinetic imaging.