The present invention relates to peptides, particularly human monoclonal antibodies, that bind specifically to poly-N-acetyl glucosamine (PNAG), such as Staphylococcal PNAG, in acetylated, partially acetylated and/or fully deacetylated form. The invention further provides methods for using these peptides in the diagnosis, prophylaxis and therapy of infections by bacteria that express PNAG such as but not limited to Staphylococci and E. coli. Some antibodies of the invention enhance opsonophagocytic killing and in vivo protection against bacteria that express PNAG such as but not limited to Staphylococci and E. coli. Compositions of these peptides, including pharmaceutical compositions, are also provided, as are functionally equivalent variants of such peptides.