Chromogenic hydroxyanthraquinone-based enzyme substrates for the detection of microbial β-d-galactosidase, β-d-glucuronidase and β-d-ribosidase†

Di-β-D-galactopyranoside derivatives of quinizarin (1,4-dihydroxyanthraquinone) and anthrarufin (1,5-dihydroxyanthraquinone) were evaluated as microbial enzyme substrates in Columbia agar medium for the detection of clinically important microorganisms. Furthermore, these substrates were evaluated both in the presence and absence of iron salts which could chelate to the aglycone after microbial hydrolysis of the substrate. The quinizarin-based substrate resulted in the formation of black microbial colonies in the presence of iron salts and orange colonies in their absence. In contrast, yellow-coloured microbial colonies were observed with the anthrarufin-based substrate regardless of whether iron salts were present or not. 1-Hydroxyanthraquinone-β-D-galactopyranoside also resulted in yellow-coloured microbial colonies in the absence of iron salts and an extended study of this substrate using 38 clinical strains of E. coli indicated its potential for identifying this microorganism when compared to a commercially available indoxyl based substrate. 1-Hydroxyanthraquinone-β-D-glucopyranuronide was also evaluated for E. coli detection, but this substrate was deemed less effective than its indoxyl-based counterpart. 1-Hydroxyanthraquinone-β-D-ribofuranoside was evaluated for its potential to detect Pseudomonas aeruginosa and this substrate shows promise for this application.