Single-cell adhesion force mapping of a highly sticky bacterium in liquid

The intricate link between membrane lipid structure and composition and membrane structural properties in bacterial membranes

It is now evident that the cell manipulates lipid composition to regulate different processes such as membrane protein insertion, assembly and function. Moreover, changes in membrane structure and properties, lipid homeostasis during growth and differentiation with associated changes in cell size and shape, and responses to external stress have been related to drug resistance across mammalian species and a range of microorganisms. While it is well known that the biomembrane is a fluid self-assembled nanostructure, the link between the lipid components and the structural properties of the lipid bilayer are not well understood. This perspective aims to address this topic with a view to a more detailed understanding of the factors that regulate bilayer structure and flexibility. We describe a selection of recent studies that address the dynamic nature of bacterial lipid diversity and membrane properties in response to stress conditions. This emerging area has important implications for a broad range of cellular processes and may open new avenues of drug design for selective cell targeting.

Adhesion preference of the sticky bacterium Acinetobacter sp. Tol 5

Gram-negative bacterium Acinetobacter sp. Tol 5 exhibits high adhesiveness to various surfaces of general materials, from hydrophobic plastics to hydrophilic glass and metals, via AtaA, an Acinetobacter trimeric autotransporter adhesin Although the adhesion of Tol 5 is nonspecific, Tol 5 cells may have prefer materials for adhesion. Here, we examined the adhesion of Tol 5 and other bacteria expressing different TAAs to various materials, including antiadhesive surfaces. The results highlighted the stickiness of Tol 5 through the action of AtaA, which enabled Tol 5 cells to adhere even to antiadhesive materials, including polytetrafluoroethylene with a low surface free energy, a hydrophilic polymer brush with steric hindrance, and mica with an ultrasmooth surface. Single-cell force spectroscopy as an atomic force microscopy technique revealed the strong cell adhesion force of Tol 5 to these antiadhesive materials. Nevertheless, Tol 5 cells showed a weak adhesion force toward a zwitterionic 2-methacryloyloxyethyl-phosphorylcholine (MPC) polymer-coated surface. Dynamic flow chamber experiments revealed that Tol 5 cells, once attached to the MPC polymer-coated surface, were exfoliated by weak shear stress. The underlying adhesive mechanism was presumed to involve exchangeable, weakly bound water molecules. Our results will contribute to the understanding and control of cell adhesion of Tol 5 for immobilized bioprocess applications and other TAA-expressing pathogenic bacteria of medical importance.

Time-dependent plastic behavior of bacteria leading to rupture

A study of the mechanical response of bacteria is essential in designing an antibacterial surface for implants and food packaging applications. This research evaluated the mechanical response of Escherichia coli under different loading conditions. Indentation and prolonged creep tests were performed to understand their viscoelastic-plastic response. The results indicate that varying loading rates from 1 μm/s to 5 μm/s show an increase in modulus of 182% and 90%, calculated in the loading and unloading cycles, respectively, and a decrease in adhesion force by 42%. However, on varying loads from 5 nN to 25 nN, nominal change is observed in both modulus and adhesion force. The rupture curve at 100 nN load shows elastic and a small plastic deformation accompanied by a sharp peak indicating the cell wall rupture. The rupture force at the peak was found to be 34.38 ± 5.15 nN, irrespective of the loading rate, making it a failure criterion for bacteria rupture. The creep response of bacteria increases (for 6 s) and then remains constant (for 15 s) with time, indicating that a standard linear solid (SLS) model applies to this behavior. This work attempts to evaluate the mechanical properties of E. coli bacteria focusing on its rupture by contact killing mechanism.

Growth phase-dependent production of the adhesive nanofiber protein AtaA in Acinetobacter sp. Tol 5

AtaA, the sticky, long, and peritrichate nanofiber protein from Acinetobacter sp. Tol 5, mediates autoagglutination and is highly adhesive to various material surfaces, resulting in a biofilm. Although the production of the adhesive nanofiber protein is likely to require a large amount of energy and material sources, the relationship between AtaA fiber production and cell growth remains unknown. Here, we report the growth phase-dependent AtaA fiber production in Tol 5. We examined the ataA gene expression in different growth phases using a reporter gene assay with an originally developed reporter plasmid and using reverse transcription-quantitative polymerase chain reaction. Bacterial cells with surface-displayed AtaA at different growth phases were immunostained and analyzed using fluorescence flow cytometry and confocal laser scanning microscopy. The results indicate that Tol 5 modulated the amount of surface-displayed AtaA at the transcriptional level. AtaA production was low in the early growth phase but remarkably increased in the late growth phase, covering the whole bacterial cell with AtaA fibers in the stationary phase. Tol 5 displayed AtaA fibers poorly in the early growth phase and showed less autoagglutination and adhesiveness than those in the stationary phase. Although Tol 5 grew as fast as its ataA-deficient mutant in the early growth phase, the optical density of Tol 5 culture was slightly lower than that of the ataA-deficient mutant in the late growth phase. Based on these experimental results, we propose the growth-phase-dependent production of AtaA fiber for efficient and fast cell growth.

Identification of the adhesive domain of AtaA from Acinetobacter sp. Tol 5 and its application in immobilizing Escherichia coli

Cell immobilization is an important technique for efficiently utilizing whole-cell biocatalysts. We previously invented a method for bacterial cell immobilization using AtaA, a trimeric autotransporter adhesin from the highly sticky bacterium Acinetobacter sp. Tol 5. However, except for Acinetobacter species, only one bacterium has been successfully immobilized using AtaA. This is probably because the heterologous expression of large AtaA (1 MDa), that is a homotrimer of polypeptide chains composed of 3,630 amino acids, is difficult. In this study, we identified the adhesive domain of AtaA and constructed a miniaturized AtaA (mini-AtaA) to improve the heterologous expression of ataA. In-frame deletion mutants were used to perform functional mapping, revealing that the N-terminal head domain is essential for the adhesive feature of AtaA. The mini-AtaA, which contains a homotrimer of polypeptide chains from 775 amino acids and lacks the unnecessary part for its adhesion, was properly expressed in E. coli, and a larger amount of molecules was displayed on the cell surface than that of full-length AtaA (FL-AtaA). The immobilization ratio of E. coli cells expressing mini-AtaA on a polyurethane foam support was significantly higher compared to the cells with or without FL-AtaA expression, respectively. The expression of mini-AtaA in E. coli had little effect on the cell growth and the activity of another enzyme reflecting the production level, and the immobilized E. coli cells could be used for repetitive enzymatic reactions as a whole-cell catalyst.

Recent advances in research on biointerfaces: From cell surfaces to artificial interfaces

Biointerfaces are regions where biomolecules, cells, and organic materials are exposed to environmental media or come in contact with other biomaterials, cells, and inorganic/organic materials. In this review article, six research topics on biointerfaces are described to show examples of state-of-art research approaches. First, biointerface design of nanoparticles for molecular detection is described. Functionalized gold nanoparticles can be used for sensitive detection of various target molecules, including chemical compounds and biomolecules, such as DNA, proteins, cells, and viruses. Second, the interaction between bacterial cell surfaces and material surfaces, including the introduction of advances in analytical methods and theoretical calculations, are explained as well as their applications to bioprocesses. Third, bioconjugation technologies for localizing functional proteins at biointerfaces are introduced, in particular, by focusing the potential of enzymes as a catalytic tool for designing different types of bioconjugates that function at biointerfaces. Forth topics is focusing on lipid-protein interaction in cell membranes as natural biointerfaces. Examples of membrane lipid engineering are introduced, and it is mentioned how their compositional profiles affect membrane protein functions. Fifth topic is the physical method for molecular delivery across the biointerface being developed currently, such as highly efficient nanoinjection, electroporation, and nanoneedle devices, in which the key is how to perforate the cell membrane. Final topic is the chemical design of lipid- or polymer-based RNA delivery carriers and their behavior on the cell interface, which are currently attracting attention as RNA vaccine technologies targeting COVID-19. Finally, future directions of biointerface studies are presented.