Interaction of silver(I) ions with the respiratory chain of Escherichia coli:: An electrochemical and scanning electrochemical microscopy study of the antimicrobial mechanism of micromolar Ag

Electrochemical techniques were used to study the behavior of Escherichia coli on the addition of <= 10 mu M AgNO3. Respiration in the presence of glucose was measured using a Clark ultramicroelectrode to determine the oxygen concentration as a function of time. The rate of respiration increased initially upon the addition of silver(l) because of the uncoupling of the respiratory chain, followed by cessation of respiration. The toxicity of 1 mu M AgNO3, as determined by the time until respiration ceased, increased in the absence of glucose and in the presence of K+. The uptake of 1 mu M Ag+, measured from the stripping peak height of Ag electrodeposited on a 25 mu m Pt ultramicroelectrode, showed a fastest uptake in the presence of glucose, although glucose was not required for uptake. Efflux of Ag+ from cells was also detected in the presence of glucose. Scanning electrochemical microscopy (SECM) was used to follow the uptake of 1 mu M Ag+ by living and dead E. coli immobilized on a glass slide coated with poly-L-lysine. Ferricyanide was used as an alternative electron acceptor to oxygen to probe the site of inhibition of Ag+ in the respiratory chain.