Thermus thermophilus polyploid cells directly imaged by X-ray laser diffraction

Thermus thermophilus is reportedly polyploid and carries four to five identical genome copies per cell, based on molecular biological experiments. To directly detect polyploidy in this bacterium, we performed live cell imaging by X-ray free-electron laser (XFEL) diffraction and observed its internal structures. The use of femtosecond XFEL pulses enables snapshots of live, undamaged cells. For successful XFEL imaging, we developed a bacterial culture method using a starch- and casein-rich medium that produces a predominance of rod-shaped cells shorter than the focused XFEL beam size, which is slightly smaller than 2 µm. When cultured in the developed medium, the length of T. thermophilus cells, which is typically ~4 µm, was less than half its usual length. We placed living cells in a micro-liquid enclosure array and successively exposed each enclosure to a single XFEL pulse. A cell image was successfully obtained by the coherent diffractive imaging technique with iterative phase retrieval calculations. The reconstructed cell image revealed five peaks, which are most likely to be nucleoids, arranged in a row in the polyploid cell without gaps. This study demonstrates that XFELs offer a novel approach for visualizing the internal nanostructures of living, micrometer-sized, polyploid bacterial cells.

From single bacterial cell imaging towards in vivo single-molecule biochemistry studies

Bacteria as single-cell organisms are important model systems to study cellular mechanisms and functions. In recent years and with the help of advanced fluorescence microscopy techniques, immense progress has been made in characterizing and quantifying the behavior of single bacterial cells on the basis of molecular interactions and assemblies in the complex environment of live cultures. Importantly, single-molecule imaging enables the in vivo determination of the stoichiometry and molecular architecture of subcellular structures, yielding detailed, quantitative, spatiotemporally resolved molecular maps and unraveling dynamic heterogeneities and subpopulations on the subcellular level. Nevertheless, open challenges remain. Here, we review the past and current status of the field, discuss example applications and give insights into future trends.