Question

A student swabbed 5 utensils and placed the swab into a 2-ml diluent. Then, two additional 10^-1 dilutions were made. One ml and 0.1 ml aliquots were plated in duplicate from the last dilution tube. All plates yielded TNTC. What was the estimated cell count per utensil?

Answer 1

It is given that all plates yielded TNTC, i.e, too numerous to count. Number of colonies above 250 to 300 is considered as TNTC as too many colonies in close proximity makes it difficult to tell whether the colonies are separated or not. Hence, estimated cell count per utensil cannot be estimated at the given dilution scheme. Number of colonies in a plate ranging from 25 to 250 or 300 is considered countable and are ideal for approximating the CFU in a culture sample. Less than 25 colonies are too less to count to give statistically significant results.