The present invention provides a family of bacterial acyl glucosaminylinositol amidases with amidase activity against S-conjugate amides, particularly mycothiol-derived S-conjugate amides. The invention amidases are characterized by a highly conserved 20 amino acid N-terminal region and four highly conserved histidine-containing regions and by having amidase activity, particularly amide hydrolase activity. The invention further provides methods for using the invention amidases in drug screening assays to determine compounds with antibiotic activity or compounds that inhibit activity or production of endogenous acyl glucosaminyl inositol amidase in bacteria. The invention further provides methods for detoxifying a toxic substance by contacting the toxic substance with an invention amidase, for example, by expression of the amidase under environmental conditions in a bacterium.