Non-sterile, gridded membrane filters can be used for colony counting, particle testing and microscopy. Before usage, the filters first need to be sterilized (e.g. by autoclaving at 121°C). Cellulose nitrate filters (mixed cellulose ester) contain a mixture of cellulose nitrate and cellulose acetate, which assures effective retention combined with high flow rates and optimal colony growth. Cellulose acetate combines high flow rates and thermal stability with very low adsorption characteristics. Individually wrapped membranes have a 0.45 µm pore size.