Inhibition of efflux pumps aids small-molecule probe-based fluorescence labeling and imaging in the Gram-negative bacterium Escherichia coli

A major challenge in fluorescence imaging experiments, which are essential to determine protein activity, expression, and localization, is the penetration of small-molecule probes through the outer membrane permeability barrier of bacteria. Here, we describe a novel strategy for small-molecule probe-based fluorescence protein labeling and imaging in the Gram-negative bacterium Escherichia coli. We targeted a siderophore enterobactin biosynthetic enzyme EntE in E. coli. When coupled with an efflux pump inhibitor carbonyl cyanide m-chlorophenylhydrazone, small-molecule probes were able to efficiently enter the cells, leading to the fluorescence labeling and imaging of overproduced EntE in E. coli. This study demonstrates that the combination of small-molecule probes with appropriate efflux pump inhibitors may substantially enhance their interaction with the target proteins in live bacteria.