Improved detection of F-specific RNA coliphages in fecal material by extraction and polyethylene glycol precipitation.

Male-specific RNA coliphages (F-RNA coliphages) have been proposed as a potential viral indicator of fecal contamination in water and foods because they are easy to culture and are a normal component of the mammalian gut flora. F-RNA coliphage plaque numbers are typically obtained by directly plating a 10-fold dilution of 1 g of fecal material, but the numbers of F-RNA coliphages shed by animals and humans may be too low for direct enumeration. Therefore, the sensitivity of detecting F-RNA coliphages in fecal material was improved by extracting and precipitating F-RNA coliphage from a 10-g fecal sample by use of polyethylene glycol (PEG). The highest recovery of F-RNA coliphage with 10% beef extract, pH 7.2, was obtained in the presence of 1 M NaCl and 10% PEG after 16 h of precipitation, but a pellet was not obtained after a short precipitation time of 2 h. There was no significant difference between eluant-to-fecal-material ratios of 4:1 and 9:1 or homogenization with a stomacher or pulsifier. F-RNA coliphage were detected in 64% (16 of 25 samples) of fecal samples from various sources when the sample size was 10 g but in 36% (9 of 25 samples) of samples when the sample size was 1 g. When F-RNA coliphage were detected in 1-g samples, they were also detected in 10-g samples. When F-RNA coliphage were detected in 10-g samples but not in 1-g samples, the levels were <100 PFU/g.