Bile salt hydrolase is a cysteine hydrolase that catalyzes the deconjugation of amino acids from bile acid amidates. The catalytic residue, cysteine 2, must be in a thiolate form for hydrolysis to occur. Using a chemical probe, CA-Dim (file name suffix 'D'), we identified cysteine 2 reversibly forms a sulfinic acid, whose profiling was enabled by this probe. We profiled this posttranslation modification in the murine fecal microbiome, comparing to a DMSO vehicle control (file name suffix 'C') and a thiolate-selective probe, CA-FMK (file name suffix 'F').