Eavesdropping on the chemical conversations of microbes requires that researchers keep tabs on a bacterium’s “ears,” namely the protein receptors involved in detecting chemical words. Chemists in Israel have found a way to fluorescently label these receptors in live bacterial cells (J. Am. Chem. Soc., DOI: 10.1021/ja200455d). To develop the label, Michael M. Meijler and his colleagues at Ben-Gurion University of the Negev started with (3-oxododecanoyl)homoserine lactone. Pseudomonas aeruginosa, the bacteria behind many antibiotic-resistant hospital-acquired infections, communicate with one another via this lactone. To covalently link the lactone to the P. aeruginosa receptors that recognize it, the researchers added an isothiocyanate to the end of the lactone’s fatty acid chain. The isothiocyanate traps the lactone on an exposed cysteine residue side chain in the receptors’ binding pocket. They also tagged the lactone with an amino-oxy fluorescent label using aniline-catalyzed oxime chemistry. When the cell was bombarded with the green fluorescent tag, the team found that the labeled receptors were concentrated at the two poles of these long, skinny bacteria.