Drug delivery systems that can deliver therapeutic and/or diagnostic agents to the colon are disclosed. The systems include pectin beads crosslinked with zinc or any divalent cation of interest, which beads are then coated with Eudragit®-type polymers. The drug delivery systems are orally administrable, but can deliver the active agents to the colon, or, in some embodiments, to various other positions in the gastro-intestinal tract. The agents can be used to diagnose, treat, prevent, or investigate a variety of conditions, including infectious diseases, inflammatory diseases, cancers and the like. Certain agents, such as metallo-dependent enzymes, for example, .beta.-lactamase Ll from Stenotrophomonas maltophilia, as well as agents that inactivate macrolide, quinolone, fluoroquinolone or glycopeptide antibiotics, can reduce the quantity of residual antibiotics reaching the colon following antibiotic therapy.
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