A bacterial culture was diluted 1000-fold, and 0.1 mL of this sample was plated. If 100 colonies formed, what was the or
Viable count estimation via plate counting requires accounting for dilution magnitude and volume plated to infer original concentration. Only 30 to 300 colonies per plate statistically reliable. General formula colony forming units per mL equals colonies observed divided by product dilution factor fraction and volume plated in milliliters. If culture diluted 1000 fold meaning dilution factor 1 times ten to minus three, and 0.1 mL aliquot plated yielding 100 colonies, denominator equals ten to minus three times 0.1 equals ten to minus four. One hundred over ten to minus four equals one million per mL in diluted tube. Multiplying back by dilution factor 1000 gives original at one million? Actually careful: colonies already from diluted sample, original calculated as colonies over volume times inverse dilution equals 100 over 0.1 times 1000 equals 1 times ten to six. This illustrates how small pipetting error amplifies hundredfold. Mastery of this calculation central for food safety standards, water coliform counts, antibiotic minimal inhibitory concentration assays where accurate CFU essential for regulatory compliance, dose response, and reporting results as CFU per mL or per gram.
Ref: Prescott's Microbiology, 11th ed., Chapter 7: Viable count - CFU/mL = colonies/(dilution×volume).