Inactivation of Legionella pneumophila and Pseudomonas aeruginosa: Evaluation of the bactericidal ability of silver cations

In this study, silver cations dissolved as silver nitrate at various concentrations were exposed to Legionella pneumophila, Pseudomonas aeruginosa, and Escherichia coli to quantitatively estimate the bactericidal ability of silver. Observed data were analyzed using a newly developed model (Cs×T) that introduced a specific amount of chemisorbed silver onto a bacterial cell (Cs), which represented the chemisorption properties of silver on the bacterial cell body. Silver cations were rapidly chemisorbed onto bacterial cells after injection into samples, and Cs values (initial concentration of silver was 0.1 mg Ag/l) were calculated as 1.810×10–6 (L. pneumophila), 1.102×10–6 (P. aeruginosa), and 1.638×10–6 μg Ag/celli (E. coli) after incubation for 8 h. During that time, the three tested bacteria were completely inactivated under the detection limit (>7.2 log reduction). Based on the calculated Cs values, bacterial tolerance against silver was estimated by using the equation (Cs×T) multiplying the Cs values with exposure time (T). The Cs×T values well represented the bactericidal abilities of silver against the tested bacteria. The demanded Cs×T values to accomplish a 1 log inactivation (90% reduction) of L. pneumophila, P. aeruginosa, and E. coli (the initial numbers of bacteria were 1.5×107 CFU/ml, approximately) were estimated as 2.44×10–6, 0.63×10–6, and 0.46×10–6 μg h/celli of silver. The values were significantly reduced to 1.54×10–6, 0.31×10–6, and 0.25×10–6 μg h/celli, respectively, with simultaneous injection of silver and copper. This study shows the successful quantitative estimation of the bactericidal ability of silver by applying the newly developed model (Cs×T). Among the tested bacteria, L. pneumophila showed the strongest tolerance to exposure of the same concentration of silver.