Quantification of Legionella pneumophila by real-time quantitative PCR from samples with humic acid and ferric ion

► We evaluated the influences of Fe or Fe/humic acid (HA) on four DNA isolation methods. ► Sepharose 4B gel column (Q) removed HA effectively and obtained the highest DNA yield for all samples. ► The qPCR inhibition caused by Fe or Fe/HA was found in all isolated DNA, which was relieved by dilution treatment. ► The method Q, in combination with 10-fold dilution, acquired the highest cell recovery determined by qPCR. ► The Q with post 10-fold dilution is suggested prior to qPCR for quantifying legionellae from water containing Fe or Fe/HA.