Microbial catabolism-based grouping enables read-across of non-persistency for all constituents of hexyl glucoside and 2-ethylhexyl glucoside
Alkyl polyglucosides (APGs) are nonionic surfactants consisting of a hydrophobic alkyl chain and a hydrophilic (oligo)saccharide moiety linked through a glucosidic bond. APGs are complex multi-constituent mixtures. Assessment of persistency for all individual APG constituents by biodegradation testing is difficult or even impossible.  Read-across may be used as an alternative to testing. Ready biodegradability of the multi-constituents hexyl glucoside and 2-ethylhexyl glucoside was demonstrated in Closed Bottle tests inoculated with river water. To enable catabolism-based grouping the biodegradability of individual APG constituents was studied using bacteria isolated from river water. These bacteria were isolated by selection on prevalence using hexyl glucoside (multi-constituent), 2-ethylhexyl glucoside (multi-constituent), hexyl-ß-maltoside, glucose, and maltose as sole sources of carbon and energy. The first step in the biodegradation of APGs is the cleavage of the glucosidic bond. This hydrolysis step was demonstrated for all APG constituents of hexyl glucoside and 2-ethylhexyl glucoside by stoichiometric formation of hexanol and 2-ethylhexanol, respectively. The mixture of (oligo)glucosides formed was only partly degraded by the isolates, and but completely mineralized by microorganisms present in river water. Glucosidase activity in two of the isolated strains was constitutively present, mainly cellular bound , and ß-glucosidase activity was higher than the a-glucosidase activity. Read-across of ready biodegradability and herewith non-persistency for all constituents of hexyl glucoside (multi-constituent) and 2-ethylhexyl glucoside (multi-constituent)  is justified based on: the broad substrate specificity of glucosidases in the first biodegradation step resulting in stoichiometric formation of alcohols, and the subsequent rapid biodegradation of the formed hydrolysis products.