Inactivation of Mycobacterium avium by copper and silver ions

Mycobacterium avium and Legionella pneumophila are opportunistic pathogens that are found in hot water systems of hospitals. Hospital water supplies have also been suggested as sources for M. avium in AIDS patients. Copper/silver ionization has been shown to be an effective method for controlling Legionella in hospital hot water systems. The effect of copper/silver ions on M. avium is unknown. The susceptibility of M. avium to copper/silver ions was tested by performing kill-curve experiments in 100 ml buffered water at the following concentrations of copper/silver ions: (1) 0.1/0.01; (2) 0.2/0.02; (3) 0.4/0.04; (4) 0.8/0.08 mg/l. Initial M. avium concentration was ca. 3×106 CFU/ml. Viability was assessed daily for 1 week by plate count on Middlebrook 7H10 agar, and by the BACTEC system. Culture plates and BACTEC bottles were incubated at 37°C for two weeks. Contact times (days) required for 99% and 99.9% inactivation of M. avium by copper/silver ion concentrations of 0.1/0.01; 0.2/0.02; 0.4/0.04; 0.8/0.08 were 3 d and 5 d, 3 d and 5 d, 2 d and 4 d, 1 d and 2 d, respectively. The data indicate that M. avium is susceptible to these metal ions. However, M. avium are more resistant to the bactericidal effects of copper/silver ions than Legionella, requiring 100 times longer exposure to achieve comparable killing in vitro. Nevertheless, this suggests that the copper/silver ionization system may rid hospital hot water systems of both of these potential pathogens.