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The modification of zeolite A (NaA) by impregnation with different concentrations of silver ions (Ag+) or silver nanoparticles (AgNPs) for antibacterial activity was investigated. The Ag+-NaA composites were prepared by ion exchange against different concentrations of silver nitrate solution (25-200 mg/L) while AgNPs were subsequently prepared by reducing the impregnated Ag+ with sodium borohydride. The addition of Ag+ and AgNPs had no discernible effect on the NaA structure. The AgNPs in the AgNP-NaA composites exhibited a typical broad absorption peak at 394 nm that was absent in the Ag+-NaA composites. The Ag+-NaA and AgNP-NaA composites both showed a dose- and time-dependent antimicrobial activity against the Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli bacteria, but were substantially more effective against E. coli than S. aureus. Moreover, Ag+-NaA composites had a substantially higher antibacterial efficiency against E. coli than the AgNP-NaA composites, whereas the highest concentration at 3 h, the AgNP-NaA composites were weakly more effective than the Ag+-NaA composites against S. aureus. [ABSTRACT FROM AUTHOR]