Enumeration of bacteriophages in water by different laboratories of the European Union in two interlaboratory comparison studies

As part of a European project on bacteriophages in bathing waters two interlaboratory comparison studies were carried out (May 1997 and March 1998). During these studies phage reference materials as well as naturally polluted standard samples were analysed in 16 European laboratories. Three groups of bacteriophages were tested using standardised methods: somatic coliphages, F-specific RNA-phages and phages of Bacteroides fragilis. Many of the participating laboratories applied one or more of the phage methods for the first time, after a one-week training session in a central laboratory. Nevertheless, the values of repeatability (r = 1.35-1.38 calculated on log10-scale) and reproducibility (R = 1.52-2.04 calculated on log10-scale) when analysing phage reference materials were close to the theoretical optimum for a Poisson distribution. When analysing the naturally polluted samples more variation in results within and between laboratories was found (r = 1.63-2.34; R = 3.10-5.72), in comparison with the results obtained with the pure phage reference materials.