Comments on the routine testing of water samples for Legionella sp.

Water samples analysed for Legionella sp. by a commercial company were studied over a 6 month period. Samples yielding legionella were re-examined to determine the efficiency of heat or acid pre-treatment in isolating legionella compared to untreated specimens. In addition, more than one colony of each colony type were tested serologically to determine whether serotypes in addition to those originally identified were present. Significantly (P < 0.01) higher legionella colony numbers were found on the heat pre-treatment plates than on the acid pre-treatment plates. Legionellae were identified from only the untreated plate in 5.1% of positive water samples; in these instances the concentrations of legionellae tended to be low. Six out of 18 positive samples were found to contain more than one serogroup of Legionella pneumophila when more than one colony of each colony type was tested. It is recommended that the procedures used in legionella screening be rationalised to facilitate maximum detection without excessive workload.