18Imaging dense and diverse microbial communities has broad applications in basic microbiology 19 and medicine, but remains a grand challenge due to the fact that many species adopt similar 20 morphologies. While prior studies have relied on techniques involving spectral labeling, we have 21 developed an expansion microscopy method (µExM) in which cells are physically expanded 22 prior imaging and their expansion patterns depend on the structural and mechanical properties of 23 their cell walls, which vary across species and conditions. We use this phenomenon as a 24 quantitative and sensitive phenotypic imaging contrast orthogonal to spectral separation in order 25 to resolve bacterial cells of different species or in distinct physiological states. Focusing on host- 26 microbe interactions that are difficult to quantify through fluorescence alone, we demonstrate the 27 ability of µExM to distinguish species within a dense community through in vivo imaging of a 28 model gut microbiota, and to sensitively detect cell-envelope damage caused by antibiotics or 29 previously unrecognized cell-to-cell phenotypic heterogeneity among pathogenic bacteria as they 30 infect macrophages.
32Imaging of heterogeneous bacterial populations has broad applications in understanding the 33 complex microbiota that exist on and within our bodies, as well as complex host-microbial 34 interfaces, yet remains a significant challenge due to the lack of suitable tools for distinguishing 35 species and identifying altered physiological states [1][2][3]. Analyses to date have mostly relied on 36 spectral separation using fluorescence in situ hybridization (FISH) with probes designed to target 37 16S RNA sequences specific to certain taxa [4], or genetically engineered microbes that express 38 distinct fluorescent proteins [5]. However, these methods are generally insensitive to 39 physiological changes in bacterial cells that are often modulated by host environments and 40 believed to be critical for the growth and spatial organizations of microbes [6,7].
42The bacterial cell wall is a macromolecule responsible for shape determination in virtually all 43 bacteria. Although little is known about the molecular architecture of the cell wall in most non-44 model organisms, its dimensions can vary widely, with the wall typically thick (tens of 45 nanometers [8,9]) in Gram-positive species and thin (~2-4 nm) in Gram-negative species [10], 46 and their rigidity can vary across ~10-100 MPa for Young's modulus [11,12]. The cell wall also 47 has various biochemical compositions [13] and exhibit distinct spatial patterns of cross-linking 48 density [14], molecular organization [15,16], thickness [8,9], and stiffness [11,12], all of which 49 depend on species and cell physiology. Thus, cell wall mechanics can potentially provide a 50 contrast that is orthogonal to spectral separation in distinguishing species and even cellular 51 physiological states. However, while cell-wall structure and mechanics have been measured by 52 e...