 precise 
 imaging of the cell surface of fluorescently labeled bacteria 
 requires super - resolution methods because the size - scale of these 
 cells is on the order of the diffraction limit . in this work 
 , we present 
 a photocontrollable small - molecule rhodamine spirolactam 
 emitter suitable for non - toxic and specific labeling of the outer 
 surface of cells for three - dimensional ( 3d ) super - resolution ( sr ) 
 imaging . 
 conventional rhodamine spirolactams photoswitch 
 to the emitting form with uv light ; however , these wavelengths can 
 damage cells . 
 we extended photoswitching to visible wavelengths 
 > 400 nm by iterative synthesis and spectroscopic characterization 
 to optimize the substitution on the spirolactam . 
 further , an n - hydroxysuccinimide - functionalized derivative enabled 
 covalent labeling of amines on the surface of live caulobacter 
 crescentus cells . resulting 3d sr reconstructions of the 
 labeled cell surface reveal uniform and specific sampling with thousands 
 of localizations per cell and excellent localization precision in x , y , and z. the distribution 
 of cell stalk lengths ( a sub - diffraction - sized cellular structure ) 
 was quantified for a mixed population of cells . 
 pulse - chase experiments 
 identified sites of cell surface growth . 
 covalent labeling with the 
 optimized rhodamine spirolactam label provides a general 
 strategy to study the surfaces of living cells with high specificity 
 and resolution down to 1020 nm . 
Supplementary Material