Inhibitory effects of silver ions on Legionella pneumophila grown on agar, intracellular in Acanthamoeba castellanii and in artificial biofilms

Aims: We undertook a series of experiments to investigate the susceptibility of Legionella pneumophila grown under extracellular and intracellular conditions and other water-related bacteria to silver ions.

Methods and results: In this study, the antimicrobial effect of silver ions to intra- and extra-cellular grown Legionella bacteria was investigated. The minimal inhibitory concentration (MIC) after 24 h exposure, leading to a 5 log reduction, was c. 64 μg l(-1) AgNO(3) for extracellular grown Legionella and other tested Gram-positive and Gram-negative bacteria. In contrast, the MIC for intracellularly grown Legionella was up to 4096 μg l(-1) AgNO(3) after 24 h. Furthermore, the heterotrophic bacteria grown within a biofilm model were killed at a concentration of 4-16 μg l(-1) AgNO(3). In contrast, biofilm-associated Legionella were less sensitive (MIC 128-512 μg l(-1) AgNO(3)).

Conclusion: Intracellularly and biofilm-grown legionellae are less sensitive against silver compared with agar-grown bacteria.

Significance and impact of the study: The reduced sensitivity of Legionella grown in amoebae might explain why the effect of silver decontamination requires an extended exposure in field trials.