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

ABSTRACT

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.