Image artifacts in Single Molecule Localization Microscopy: why optimization of sample preparation protocols matters

Single molecule localization microscopy (SMLM) techniques allow for sub-diffraction imaging with spatial resolutions better than 10 nm reported. Much has been discussed relating to different variations of SMLM and all-inclusive microscopes can now be purchased, removing the need for in-house software or hardware development. However, little discussion has occurred examining the reliability and quality of the images being produced, as well as the potential for overlooked preparative artifacts. As a result of the up to an order-of-magnitude improvement in spatial resolution, substantially more detail is observed, including changes in distribution and ultrastructure caused by the many steps required to fix, permeabilize, and stain a sample. Here we systematically investigate many of these steps including different fixatives, fixative concentration, permeabilization concentration and timing, antibody concentration, and buffering. We present three well-optimized fixation protocols for staining microtubules, mitochondria and actin in a mammalian cell line and then discuss various artifacts in relation to images obtained from samples prepared using the protocols. The potential for such errors to go undetected in SMLM images and the complications in defining a 'good' image using previous parameters applied to confocal microscopy are also discussed.