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90 COOPER AND JOLLY reversible chemical bonding with enzymes and other active molecules at cell surfaces. The lipid phase of the cell membrane appears to play an important role in the adsorption of silver ions by living cells. As an example of the comparative potency of silver and other metals, a purified prepara- tion of the enzyme trehalase was 75% in- hibited by a 1 mM concentration of Ag+ ions. Similar concentrations of zinc, cupric, and mercuric ions produced only 15%, 33%, and 33% inhibition, respectively [Guilloux et al., 1968J. Binding of -SH groups seems to be the prin- cipal mechanism by which silver ions inhibit enzyme activity. Silver and mercury are gen- erally considered among the most specific rea- gents for these groups, and are effective in very low concentrations. Hg++ and Ag+ instantan- eously inhibit activity of purified yeast alcohol dehydrogenase without removing zinc atoms from the enzyme molecule. Reaction with -SH groups to form silver mercaptides, rather than displacement of other metal atoms, is appar- ently the basis of enzyme inhibition by Ag, although the site at which the effect takes place cannot be precisely defined [Snodgrass et al., 1960]. Other investigators have shown that silver ions react quantitatively with purified enzymes having -SH groups. Inactivation of one mole- cule of yeast invertase (or one active site) was achieved by reaction with 2 Ag+ ions at all pH values investigated [Myrbiick, 1963J. About 8 Ag+ ions per molecule caused 50% inhibition of urease activity [Gorin and Chin, 1965]. The bonding is highly reversible. Dilute silver salts effectively inhibit the growth of bacteria, but are only slowly lethal. A considerable excess of silver is apparently required for sufficient penetration into the cell to cause irreversible denaturation. Water supply engineers have ex- pressed concern that bacteriostasis by silver may be mistaken for true bacteriocidal action [Chambers et al., 1962]. This situation is analogous to the better studied case of mercury inhibition. Mercury salts stop bacterial growth almost instantly, but are relatively poor germicides [Lawrence and Block, 1968J. Their bacteriostatic effect can be rever~ed by neutralizing or immobilizing the mercury. As with silver, the inhibitory effect of mercury seems due to reversible chemical bonding with -SH groups at the cell surface [Meyer, 1964]. Silver readily forms harmless insoluble com- plexes with a host of biological materials. This goes far to explain the differential effects of silver salts on different classes of organisms: why very low concentrations of soluble AgNO. are lethal to fresh water fish, for instance, whereas much larger amounts relative to body weight can be safely introduced into the human digestive tract or even the bloodstream. Because much of the Ag in natural water is in particulate or sequestered form rather than in solution, acceptable levels in drinking water can be set well above those lethal to small fish in the laboratory. The U. S. Public Health 8ervice allows 5 X 10-8 g mt1 in drinking water. This standard is based on the amount of silver that the human body is believed capable of tolerating over a 27-year period without de- veloping symptoms of argyria, a disfigurement due to silver accumulation. The biological action of silver compounds is strongly affected by temperature, oxygen con- centration, presence or absence of other cations, and pH. The poor state of knowledge concern- ing antagonistic and synergistic effects makes it difficult to evaluate many experiments reported in the literature where these effects were not properly considered. In particular, it is likely that observed effects of silver will be greater in soft, pure water than in hard water or water containing appreciable concentrations of other metal ions. Silver ions are strongly adsorbed by organic colloids, plastics, glass, and other materials. Some plastic containers adsorb nearly all the silver from a solution in the 10-10 g ml-1 range, and more than 20% of a 10-8 g ml-1 solution in a 10-day period (H. C. Millar and J. E. Fletcher, personal communication, 1968). Similar adsorp- tion by vegetation, detritus, and sediments in contact with water in streams, ponds, lakes, and marshes is likely. ,. I ! " LETHALITY OF SILVER COMPOUNDS Mammals and birds. Silver, even in highly soluble form, is only moderately harmful to mammals. Because of caustic action on the in- testinal tract, 10 g of AgNO. is usually fatal to man, but 3 g can be taken safely. After oral