Inhibition of bacterial α-glucosidases by castanospermine in pure cultures and activated sludge

Castanospermine (CAST) is a known and potent inhibitor of various α-glucosidases in eukaryotes. In this work, we elucidated whether CAST could also be used for determining bacterial α-glucosidase activity, when measured with 4-methylumbelliferyl-α-D-glucoside as a substrate, both in a complex bacterial community, in activated sludge and in pure cultures of bacterial isolates. We found that 140 µM CAST inhibited α-glucosidase activity by 30% in a pure culture of Pseudomonas stutzeri. The α-glucosidase activity in Chryseobacterium gleum was inhibited by 90% at a concentration of 150 µM CAST, whereas the α-glucosidase in Paracoccus denitrificans was resistant to the inhibitor. CAST (140 µM) reduced α-glucosidase activity in activated sludge by 40%, the respiration rate being reduced by only 12%. No significant inhibition of the respiration rate was observed in Ps. stutzeri or Pa. denitrificans, whereas the respiration rate in C. gleum grown in a medium containing starch was inhibited by 50% with 140 µM CAST. No effect of CAST was observed in C. gleum grown in a complex medium. This indicated that CAST, at the concentration used, did not cause a general negative effect on bacterial activity. The results suggest that the CAST assay may potentially be useful in determining whether α-glucosidase activity, starch, poly- and disaccharides contribute appreciably to the overall activity of a bacterial community. However, the assay should not be used for quantitative measurements of such activity.