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

We compared four methods for the cultural recovery of legionellae from natu rally 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 mi of the original sample, and (D) filtration of 100 and 10 mi through cellulose nitrate filters and incubating the filter directly on the medium. Of the water samples tested in parallel, 103 samples tested pos itive 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 sig nificant except for methods B and C (p = 0.112), the results of which corre lated 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%, res pectively. We recommend the use of method A for the recovery of legionellae at concentrations of greater than or equal to 1 CFU/ml. If legionellae bel ow this concentration are to be detected Mle recommend method D as the easi est and most sensitive technique.