Comparison of membrane filtration methods for the recovery of legionellae from naturally contaminated domestic drinking water supplies

Abstract

We compared four methods for the cultural recovery of legionellae from naturally contaminated water samples: (A) inoculating 1 and 0.1 ml directly on the selective solid growth medium, (B) filtration through cellulose nitrate membrane filters and inoculating the resuspended residue corresponding to 100 and 10 ml of the original sample on the medium, (C) filtration through polycarbonate filters and inoculating the resuspended residue corresponding to 100 and 10 ml of the original sample, and (D) filtration of 100 and 10 ml through cellulose nitrate filters and incubating the filter directly on the medium. Of the water samples tested in parallel, 103 samples tested positive for legionellae in at least two of these methods. The mean number of CFU/ml recovered by methods A, B, C, and D werte 5.45, 1.85, 1.70, and 2.26, respectively. The differences between the methods proved to be highly significant except for methods B and C ( p = 0.112), the results of which correlated closely with each other ( r = + 0.96). In comparison with method A the recovery rates of methods B, C, and D were found to be 60, 57, and 69 %, respectively. We recommend the use of method A for the recovery of legionellae at concentrations of ≥ 1 CFU/ml. If legionellae below this concentration are to be detected we recommend method D as the easiest and most sensitive technique.