N-acetyl-β-hexosaminidase activity is important for chitooligosaccharide metabolism and biofilm formation in Burkholderia pseudomallei

Burkholderia pseudomallei is a saprophytic Gram-negative bacillus that can cause the disease melioidosis. Although B. pseudomallei is a recognised member of terrestrial soil microbiomes, little is known about its contribution to the saprophytic degradation of polysaccharides within its niche. For example, while chitin is predicted to be abundant within terrestrial soils the chitinolytic capacity of B. pseudomallei is yet to be defined. This study identifies and characterises a putative glycoside hydrolase, bpsl0500, which is expressed by B. pseudomallei K96243. Recombinant BPSL0500 was found to exhibit activity against substrate analogues and GlcNAc disaccharides relevant to chitinolytic N-acetyl-β-d-hexosaminidases. In B. pseudomallei, bpsl0500 was found to be essential for both N-acetyl-β-d-hexosaminidase activity and chitooligosaccharide metabolism. Furthermore, bpsl0500 was also observed to significantly affect biofilm deposition. These observations led to the identification of BPSL0500 activity against model disaccharide linkages that are present in biofilm exopolysaccharides, a feature that has not yet been described for chitinolytic enzymes. The results in this study indicate that chitinolytic N-acetyl-β-d-hexosaminidases like bpsl0500 may facilitate biofilm disruption as well as chitin assimilation, providing dual functionality for saprophytic bacteria such as B. pseudomallei within the competitive soil microbiome.

Synthesis of defined mono-de-N-acetylated β-(1→6)-N-acetyl-d-glucosamine oligosaccharides to characterize PgaB hydrolase activity

Many clinically-relevant biofilm-forming bacterial strains produce partially de-N-acetylated poly-β-(1→6)-N-acetyl-d-glucosamine (dPNAG) as an exopolysaccharide. In Gram-negative bacteria, the periplasmic protein PgaB is responsible for partial de-N-acetylation of PNAG prior to its export to the extracellular space. In addition to de-N-acetylase activity found in the N-terminal domain, PgaB contains a C-terminal hydrolase domain that can disrupt dPNAG-dependent biofilms and hydrolyzes dPNAG but not fully acetylated PNAG. The role of this C-terminal domain in biofilm formation has yet to be determined in vivo. Further characterization of the enzyme's hydrolase activity has been hampered by a lack of specific dPNAG oligosaccharides. Here, we report the synthesis of a defined mono de-N-acetylated dPNAG penta- and hepta-saccharide. Using mass spectrometry analysis and a fluorescence-based thin-layer chromatography (TLC) assay, we found that our defined dPNAG oligosaccharides are hydrolase substrates. In addition to the expected cleavage site, two residues to the reducing side of the de-N-acetylated residue, minor cleavage products on the non-reducing side of the de-N-acetylation site were observed. These findings provide quantitative data to support how PNAG is processed in Gram-negative bacteria.