Hospital-acquired infections and multidrug resistant microbes represent a healthcare crisis where alternative treatments such as therapeutic antibodies and vaccines are needed. Poly-N-acetyl-D-glucosamine (PNAG) is a polysaccharide that is produced by many microbes, including bacteria, fungi, and protozoan parasites. Antibodies that interact with PNAG have demonstrated efficacy against a wide range of microbes. A human monoclonal antibody (mAb) F598 that binds PNAG has shown opsonic and protective activity against multiple microbial pathogens and is undergoing clinical assessment as a broad-spectrum antimicrobial therapy. We recently determined high resolution crystal structures of the unliganded F598 antigen binding fragment (Fab) and its complexes with N-acetyl-D-glucosamine (GlcNAc) and a PNAG oligosaccharide [Soliman et al (2018) J Biol Chem 293:5079]. Recognition of PNAG occurs in a large groove-shaped binding site that traverses the entire light and heavy chain interface and accommodates at least 5 GlcNAc residues. The Fab-GlcNAc complex revealed a deep binding pocket with near identical positioning of the monosaccharide and a core GlcNAc of the oligosaccharide suggesting an anchored binding mechanism of PNAG by the F598 mAb. Although intact F598 IgG is required for effector functions, the Fab used here retained binding to PNAG on the surface of an antibiotic-resistant biofilm-forming strain of Staphylococcus aureus. A model of intact F598 IgG binding to two pentasaccharide epitopes shows the antibody can span across an extended PNAG chain. This study reveals the structural basis for antibody recognition of PNAG on surfaces of microbial pathogens and in biofilms.