Ten litres of sample is membrane-filtered. The membrane is transferred to coliform-selective agar, incubated37°C for 48 h, and yields an average64 colonies. Find original concentration.
The filtered volume is 10 L=10000 mL. Assuming each colony represents one colony-forming unit retained from the stated volume, concentration is $64/10000=.0064=6.4\times10^{-3}$ CFU/mL. Thus **C**. No dilution factor is supplied.
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