Proper sample storage before RNA isolation is crucial for a reliable assessment of microbial activity in activated
sludge. In our studies, different amounts of activated sludge expressed in mg of total suspended solids (TSS) were
fixed with a constant volume of RNA later RNA Stabilization Reagent (QIAGEN, Germantown, MD, USA) and
stored for 3 or 6 months. The results obtained point out that for the best RNA recovery, 1 mg of activated sludge
TSS should be fixed with 33 μl of the RNA later. It was also observed that a longer storage period (6 vs. 3 months)
of the activated sludge in RNA later lowered RNA recovery by, on average, 25 ± 15% although bacterial diversity
in all samples remained unaltered.