Composites: Composites use multiple materials in concert. One example of an antibacterial composite is the combination of copper compounds with an anion-exchange resin to kill bacteria in water for purification. The addition of tetrachlorocuprate(II) salts to an anion exchange resin and reduction with ascorbic acid yielded a composite resin containing Cu2O; exposure of Grampositive Enterococcus faecalis to the material reduced bacterial load by 10 5 while the resin did not affect Gram-negative Escherichia coli. Antimicrobial composite materials are useful for medical devices such as dental implants where infections may be difficult to treat or may cause secondary structural damage. For example, silver and zinc oxide nanoparticle-containing composite resins for dental use were tested and inhibited Streptococcus mutans and Lactobacillus species. Polylysine was incorporated into a dental composite to prevent cariesinduced demineralization and repair failure 374 , and noncovalent assemblies of N-Fmocpentafluorophenylalanine bound to a dental resin reduced bacterial growth of Streptococcus mutans at 0.25-1% concentrations and nearly abolished it at 2% concentration.