A Colorimetric Assay to Enable High-Throughput Identification of Biofilm Exopolysaccharide-Hydrolyzing Enzymes

Expansion of the diversity of dispersin scaffolds

Microorganisms are known to secrete copious amounts of extracellular polymeric substances (EPS) that form complex matrices around the cells to shield them against external stresses, to maintain structural integrity and to influence their environment. Many microorganisms also secrete enzymes that are capable of remodelling or degrading EPS in response to various environmental cues. One key enzyme class is the poly-β-1,6-linked N-acetyl-D-glucosamine (PNAG)-degrading glycoside hydrolases, of which the canonical member is dispersin B (DspB) from CAZy family GH20. We sought to test the hypothesis that PNAG-degrading enzymes would be present across family GH20, resulting in expansion of the sequence and structural space and thus the availability of PNAGases. Phylogenetic analysis revealed that several microorganisms contain potential DspB-like enzymes. Six of these were expressed and characterized, and four crystal structures were determined (two of which were in complex with the established GH20 inhibitor 6-acetamido-6-deoxy-castanospermine and one with a bespoke disaccharide β-1,6-linked thiazoline inhibitor). One enzyme expressed rather poorly, which restricted crystal screening and did not allow activity measurements. Using synthetic PNAG oligomers and MALDI-TOF analysis, two of the five enzymes tested showed preferential endo hydrolytic activity. Their sequences, having only 26% identity to the pioneer enzyme DspB, highlight the considerable array of previously unconsidered dispersins in nature, greatly expanding the range of potential dispersin backbones available for societal application and engineering.

Insights into biofilm architecture and maturation enable improved clinical strategies for exopolysaccharide-targeting therapeutics

Polysaccharide intercellular adhesin (PIA), an exopolysaccharide composed of poly-N-acetyl glucosamine (PNAG), is an essential component in many pathogenic biofilms. Partial deacetylation of PNAG is required for biofilm formation, but limited structural knowledge hinders therapeutic development. Employing a new monoclonal antibody (TG10) that selectively binds highly deacetylated PNAG and an antibody (F598) in clinical trials that binds highly acetylated PNAG, we demonstrate that PIA within the biofilm contains distinct regions of highly acetylated and deacetylated exopolysaccharide, contrary to the previous model invoking stochastic deacetylation throughout the biofilm. This discovery led us to hypothesize that targeting both forms of PNAG would enhance efficacy. Remarkably, TG10 and F598 synergistically increased in vitro and in vivo activity, providing 90% survival in a lethal Staphylococcus aureus challenge murine model. Our advanced model deepens the conceptual understanding of PIA architecture and maturation and reveals improved design strategies for PIA-targeting therapeutics, vaccines, and diagnostic agents.

Stereoselective chemical N-glycoconjugation of amines via CO2 incorporation

Chemical N-glycoconjugation can provide a unique way to tailor the properties of the ubiquitous amines for further expending their diverse functions and applications. Nevertheless, effective methodology for glycoconjugation of amines remains largely underdeveloped. Inspired by a biotransformation pathway of amine-containing drugs in vivo, we have developed an effective protocol that enables one-step chemical N-glycoconjugation of amines in high stereoselectivity under mild conditions. This protocol involves conversion of the amine moiety into the corresponding carbamate anion under CO2 atmosphere and a subsequent SN2 type reaction with glycosyl halides. This work provides an example of using CO2 as the coupling unit in chemical glycoconjugation reactions. A case study on the resulting N-glycoconjugates of Crizotinib, an anticancer drug, demonstrates a quick cleavage of the glucosyl carbamate linkage, testifying that this N-glyconjugation method could serve as a general approach to procure novel prodrugs.

The promotion of biofilm dispersion: a new strategy for eliminating foodborne pathogens in the food industry

Food safety is a critical global concern due to its direct impact on human health and overall well-being. In the food processing environment, biofilm formation by foodborne pathogens poses a significant problem as it leads to persistent and high levels of food contamination, thereby compromising the quality and safety of food. Therefore, it is imperative to effectively remove biofilms from the food processing environment to ensure food safety. Unfortunately, conventional cleaning methods fall short of adequately removing biofilms, and they may even contribute to further contamination of both equipment and food. It is necessary to develop alternative approaches that can address this challenge in food industry. One promising strategy in tackling biofilm-related issues is biofilm dispersion, which represents the final step in biofilm development. Here, we discuss the biofilm dispersion mechanism of foodborne pathogens and elucidate how biofilm dispersion can be employed to control and mitigate biofilm-related problems. By shedding light on these aspects, we aim to provide valuable insights and solutions for effectively addressing biofilm contamination issues in food industry, thus enhancing food safety and ensuring the well-being of consumers.

Chemical biology tools to probe bacterial glycans

Bacterial cells are covered by a complex carbohydrate coat of armor that allows bacteria to thrive in a range of environments. As a testament to the importance of bacterial glycans, effective and heavily utilized antibiotics including penicillin and vancomycin target and disrupt the bacterial glycocalyx. Despite their importance, the study of bacterial glycans lags far behind their eukaryotic counterparts. Bacterial cells use a large palette of monosaccharides to craft glycans, leading to molecules that are significantly more complex than eukaryotic glycans and that are refractory to study. Fortunately, chemical tools designed to probe bacterial glycans have yielded insights into these molecules, their structures, their biosynthesis, and their functions.

Strategy to combat biofilms: a focus on biofilm dispersal enzymes

Bacterial biofilms, which consist of three-dimensional extracellular polymeric substance (EPS), not only function as signaling networks, provide nutritional support, and facilitate surface adhesion, but also serve as a protective shield for the residing bacterial inhabitants against external stress, such as antibiotics, antimicrobials, and host immune responses. Biofilm-associated infections account for 65-80% of all human microbial infections that lead to serious mortality and morbidity. Tremendous effort has been spent to address the problem by developing biofilm-dispersing agents to discharge colonized microbial cells to a more vulnerable planktonic state. Here, we discuss the recent progress of enzymatic eradicating strategies against medical biofilms, with a focus on dispersal mechanisms. Particularly, we review three enzyme classes that have been extensively investigated, namely glycoside hydrolases, proteases, and deoxyribonucleases.

Mapping protein-exopolysaccharide binding interaction in Staphylococcus epidermidis biofilms by live cell proximity labeling

Bacterial biofilms consist of cells encased in an extracellular polymeric substance (EPS) composed of exopolysaccharides, extracellular DNA, and proteins that are critical for cell-cell adhesion and protect the cells from environmental stress, antibiotic treatments, and the host immune response. Degrading EPS components or blocking their production have emerged as promising strategies for prevention or dispersal of bacterial biofilms, but we still have little information about the specific biomolecular interactions that occur between cells and EPS components and how those interactions contribute to biofilm production. Staphylococcus epidermidis is a leading cause of nosocomial infections as a result of producing biofilms that use the exopolysaccharide poly-(1→6)-β-N-acetylglucosamine (PNAG) as a major structural component. In this study, we have developed a live cell proximity labeling approach combined with quantitative mass spectrometry-based proteomics to map the PNAG interactome of live S. epidermidis biofilms. Through these measurements we discovered elastin-binding protein (EbpS) as a major PNAG-interacting protein. Using live cell binding measurements, we found that the lysin motif (LysM) domain of EbpS specifically binds to PNAG present in S. epidermidis biofilms. Our work provides a novel method for the rapid identification of exopolysaccharide-binding proteins in live biofilms that will help to extend our understanding of the biomolecular interactions that are required for bacterial biofilm formation.

Multifunctional fluorescent probes for high-throughput characterization of hexosaminidase enzyme activity

Microbial polysaccharides composed of N-acetylglucosamine (GlcNAc), such as chitin, peptidoglycan and poly-β-(1 → 6)-GlcNAc (dPNAG), play a critical role in maintaining cell integrity or in facilitating biofilm formation in numerous fungal and bacterial pathogens. Glycosyl hydrolase enzymes that catalyze the degradation of these β-GlcNAc containing polysaccharides play important roles in normal microbial cell physiology and can also be exploited as biocatalysts with applications as anti-fungal, anti-bacterial, or biofilm dispersal agents. Assays to rapidly detect and characterize the activity of such glycosyl hydrolase enzymes can facilitate their development as biocatalyst, however, currently available probes such as 4-methylumbelliferyl-β-GlcNAc (4MU-GlcNAc) are not universally accepted as substrates, and their fluorescent signal is sensitive to changes in pH. Here, we present the development of a new multifunctional fluorescent substrate analog for the detection and characterization of hexosaminidase enzyme activity containing a 7-amino-4-methyl coumarin (AMC) carbamate aglycone. This probe is widely tolerated as a substrate for exo-acting β-hexosaminidase, family 19 endo-chitinase, and the dPNAG hydrolase enzyme Dispersin B (DspB) and enables detection of hexosaminidase enzyme activity via either single wavelength fluorescent measurements or ratiometric fluorescent detection. We demonstrate the utility of this probe to screen for recombinant DspB activity in Escherichia coli cell lysates, and for the development of a high-throughput assay to screen for DspB inhibitors.

Anionic amino acids support hydrolysis of poly-β-(1,6)-N-acetylglucosamine exopolysaccharides by the biofilm dispersing glycosidase Dispersin B

The exopolysaccharide poly-β-(1→6)-N-acetylglucosamine (PNAG) is a major structural determinant of bacterial biofilms responsible for persistent and nosocomial infections. The enzymatic dispersal of biofilms by PNAG-hydrolyzing glycosidase enzymes, such as Dispersin B (DspB), is a possible approach to treat biofilm-dependent bacterial infections. The cationic charge resulting from partial de-N-acetylation of native PNAG is critical for PNAG-dependent biofilm formation. We recently demonstrated that DspB has increased catalytic activity on de-N-acetylated PNAG oligosaccharides, but the molecular basis for this increased activity is not known. Here, we analyze the role of anionic amino acids surrounding the catalytic pocket of DspB in PNAG substrate recognition and hydrolysis using a combination of site-directed mutagenesis, activity measurements using synthetic PNAG oligosaccharide analogs, and in vitro biofilm dispersal assays. The results of these studies support a model in which bound PNAG is weakly associated with a shallow anionic groove on the DspB protein surface with recognition driven by interactions with the -1 GlcNAc residue in the catalytic pocket. An increased rate of hydrolysis for cationic PNAG was driven, in part, by interaction with D147 on the anionic surface. Moreover, we identified that a DspB mutant with improved hydrolysis of fully acetylated PNAG oligosaccharides correlates with improved in vitro dispersal of PNAG-dependent Staphylococcus epidermidis biofilms. These results provide insight into the mechanism of substrate recognition by DspB and suggest a method to improve DspB biofilm dispersal activity by mutation of the amino acids within the anionic binding surface.