A flat embedding method for transmission electron microscopy reveals an unknown mechanism of tetracycline

Transmission electron microscopy of cell sample sections is a popular technique in microbiology. Currently, ultrathin sectioning is done on resin-embedded cell pellets, which consumes milli- to deciliters of culture and results in sections of randomly orientated cells. This is problematic for rod-shaped bacteria and often precludes large-scale quantification of morphological phenotypes due to the lack of sufficient numbers of longitudinally cut cells. Here we report a flat embedding method that enables observation of thousands of longitudinally cut cells per single section and only requires microliter culture volumes. We successfully applied this technique to Bacillus subtilis, Escherichia coli, Mycobacterium bovis, and Acholeplasma laidlawii. To assess the potential of the technique to quantify morphological phenotypes, we monitored antibiotic-induced changes in B. subtilis cells. Surprisingly, we found that the ribosome inhibitor tetracycline causes membrane deformations. Further investigations showed that tetracycline disturbs membrane organization and localization of the peripheral membrane proteins MinD, MinC, and MreB. These observations are not the result of ribosome inhibition but constitute a secondary antibacterial activity of tetracycline that so far has defied discovery. Wenzel et al. developed a flat embedding method for TEM, which increases the fraction of appropriately orientated cells per image, making it easier to quantify cell morphology. They find that tetracycline-treated B. subtilis shows membrane lesions and that tetracycline action is via direct perturbation of bacterial cell membrane in addition to its known ribosome inhibition activity.

Automated summary

The flat embedding method developed by Wenzel et al. for TEM allows for the observation of a large number of longitudinally cut cells in microbiological samples. By applying this technique to various bacteria, they discovered unexpected membrane deformations in B. subtilis cells treated with tetracycline, which was not caused by ribosome inhibition but rather a secondary antibacterial activity of the antibiotic. This new method increases the fraction of appropriately orientated cells per image, facilitating the quantification of cell morphology.