Let's break down the information provided: - **Sample diluted in a 1:10 dilution series:** This means the original sample is diluted 10 times in the first tube. - **0.1 mL of the second dilution tube is plated:** This means after the first 1:10 dilution, another 1:10 dilution is made, and then 0.1 mL from the second tube is plated. So, overall, this is a 1:100 dilution (1:10 x 1:10). - **Number of colonies counted:** 191 Let's calculate the number of bacteria per milliliter in the original sample: 1. **Total dilution factor:** - First dilution: 1/10 - Second dilution: 1/10 - Volume plated: 0.1 mL (instead of 1 mL, so multiply by 10 to get per mL) Total dilution = 1/10 × 1/10 × 0.1 = 1/100 × 0.1 = 1/1000 2. **Number of bacteria per mL in original sample:** Number of colonies × dilution factor = 191 × 1000 = **191,000 bacteria per mL** **Final Answer:** The original water sample contained **191,000 bacteria per milliliter**.