Single-molecule and super-resolution microscopy of bacterial cells [electronic resource]
Keywords: Chemistry.

Single molecules were first detected at low temperatures twenty-six years ago in the laboratory of W. E. Moerner. Subsequent technological advances have allowed researchers to study single molecules at room temperatures and within living cells, providing novel biological insight about underlying spatial and dynamical heterogeneity. By combining single molecule detection with the ability to control the emissive state of the fluorescent label (also called "active control"), a suite of super-resolution imaging techniques has been developed. These single-molecule-based super-resolution imaging strategies leverage the fluorescence microscope's ability to non-invasively study multiple targets within living cells, while bridging the resolution gap between optical and electron microscopies. In large part, future advances to improve single molecule and super-resolution imaging require better fluorophore and labeling technologies. Utilizing fluorophore with higher photon yields will increase the resolution of super-resolution images and the data acquisition speed. Additionally, a greater library fluorophores with different of colors and sensing capabilities will enable application to more imaging targets. Currently, many single molecule and super-resolution experiments within living systems use fluorescent proteins because the labeling of target proteins is more straightforward. However, the limited photon yield of fluorescent proteins often results in tantalizingly fuzzy super-resolution images. Imaging the same targets, labeled instead with brighter organic emitters, could provide more image detail, but better fluorogenic and genetically encoded labeling schemes must be developed and discovered. The first chapter of this dissertation will introduce and discuss the historical context and basic principles of single molecule and super-resolution imaging. Chapter 2 will then describe the general experimental procedures necessary for quantitative single molecule and super-resolution imaging, including quantifying the number of photons detected (and emitted) from a single molecule, as well as the preparation of bacterial samples for fluorescence microscopy. Later chapters apply these fundamental experimental measurements to study bacterial biology and fluorophore photophysics. Chapters 3 and 4 concern the development and characterization of organic emitters suitable for single molecule or super-resolution imaging, work achieved with the synthetic collaboration of organic chemists in the laboratory of Professor Robert J. Twieg at Kent State University. Chapter 3 discusses the optimization of rhodamine spirolactam photoswitching such that activation could occur at visible wavelengths. The optimized rhodamine spirolactams were then covalently attached to the surface of bacterial cells and imaged with three-dimensional super-resolution. Images of the bacterial cell surface demonstrates a marked improvement in labeling uniformity, specificity, and density compared to previous methods which labeled the surface with the transient binding of a membrane sensitive dye. Chapter 4 introduces a novel enzyme-based strategy to control the fluorescence from nitro-aryl fluorogens. A proof-of-principle experiment demonstrated that endogenous nitroreductase enzymes within bacterial cells could catalyze the fluorescence-activating reaction, thus generating free fluorophores, which were detectable on the single-molecule-level within the cell. Lastly, chapter 5 summarizes three-dimensional imaging experiments (performed in collaboration with the laboratory of Professor Lucy Shapiro in the Department of Developmental Biology at Stanford University) of components of the bacterial gene expression machinery labeled with fluorescent proteins. Super-resolution imaging is ideally suited to the small size scale of bacterial cells, and a wealth of biological insights remains to be discovered. Simultaneously improving fluorophore photon yield, specificity, and active control strategies will have a profound impact on super-resolution precision and speed.