A simple membrane filtration-elution method for the enumeration of F-RNA, F-DNA and somatic coliphages in 100-ml water samples

Abstract

A method which combines membrane filtration (MF) with a new swirling elution (SE) procedure was developed for the enumeration of F-RNA, F-DNA and somatic coliphages in 100 ml water samples. Using replicates of a diluted sewage seed, test methods were compared with overlay plate counts. The method (MF-SE) involves 3 min sample filtration through a membrane filter, 6 min swirling of the filter in 2 ml of eluent, followed by separate assay of the filter (face down on a host lawn) and eluent (by overlay plating). The MF and eluent plate counts are then combined. By altering eluent composition (beef extract + NaCl and/or Tween 80), the following maximum recoveries were obtained, using tap water and 0.45 μm pore size filters: 72.9% for F-RNA coliphages (on host Salmonella typhimurium WG49), 81.5% for F-DNA coliphages (on E. coli RR), and 100.5% for somatic coliphages (on Escherichia coli 13706/60). With 0.22 μm filters, the equivalent recovery rates were 75.5, 92.2 and 99.8%, respectively. Using the optimum combination of filter pore size and eluent composition, the equivalent recovery rates were 75.2, 77.1 and 85.1% from river water, and 45.9, 95.7 and 100.2% from sea water. It is concluded that the MF-SE method gives good recovery rates for all three phages from 100 ml samples of tap water and low turbidity fresh waters, good recoveries of F-DNA and somatic coliphages from sea water, but poor recovery of F-RNA coliphages from sea water.