Characterization of an α-glucosidase enzyme conserved in Gardnerella spp. isolated from the human vaginal microbiome

Gardnerella spp. in the vaginal microbiome are associated with bacterial vaginosis, a dysbiosis in which lactobacilli dominant microbial community is replaced with mixed aerobic and anaerobic bacteria including Gardnerella species. The co-occurrence of multiple Gardnerella species in the vaginal environment is common, but different species are dominant in different women. Competition for nutrients, particularly glycogen present in the vaginal environment, could play an important role in determining the microbial community structure. Digestion of glycogen into products that can be taken up by bacteria requires the combined activities of several enzymes collectively known as "amylases". In the present study, glycogen degrading abilities of Gardnerella spp. were assessed. We found that Gardnerella spp. isolates and filtered culture supernatants had amylase activity. Phylogenetic analyses predicted conserved Glycoside Hydrolase family 13 (GH13) members among Gardnerella spp. including a putative α-glucosidase. The gene for this enzyme was cloned and expressed, and recombinant protein was purified and functionally characterized. The enzyme was active on a variety of maltooligosaccharides over a broad pH range (4 - 8) with an optimum activity at pH 6-7. Glucose was released from maltose, maltotriose and maltopentose, however, no products were detected on thin layer chromatography (TLC) when the enzyme was incubated with glycogen. Our findings show that Gardnerella spp. produce a secreted α-glucosidase enzyme that can contribute to the complex and multistep process of glycogen breakdown by degrading smaller oligosaccharides into glucose, contributing to the pool of nutrients available to the vaginal microbiota.