a b s t r a c t
Viruses may play a critical role in the microbial dynamics of activated sludge systems;
however the difficulty of their quantification makes long term and large scale studies
costly, timely and challenging. Thus a flow cytometric protocol was optimised and
employed to determine virus abundance in activated sludge samples. The best flow
cytometry signature and highest virus count was obtained by separating the indigenous
floc-associated viruses using Tween 80 and sodium pyrophosphate, diluting the sample
with TriseEDTA and staining with SYBR Green II. Using the optimised protocol viral concentrations
from 25 activated sludge plants were determined, with average concentrations
of 2.35 109 mL1 observed. Direct counts by transmission electron microscopy were
highly correlated with flow cytometric counts (p ¼