The APHA standard method for the enumeration of somatic coliphages in water has low efficiency of plating

The standard method for the enumeration of somatic coliphages in water proposed by the American Public Health Association recommends the use of single layer plaque assays with 5 ml sample and the direct use of frozen and thawed host E. coli cultures. In assays of spiked river water the efficiency of plating (e.o.p.) of the standard method, using frozen/thawed host cultures, was significantly lower than comparable assays using unfrozen host cultures. Frozen/thawed cultures also produced significantly lower efficiency of plating in scaled up versions of the standard method using both 50 and 100 ml sample. The e.o.p. varied from 39.0–46.1% with frozen/thawed cultures and 67.7–74.4% with unfrozen cultures at OD 0.5, relative to overnight cultures. It is therefore recommended that overnight cultures are used in place of frozen/thawed cultures in the APHA standard method and scaled up versions of it.