A laboratory technician mixes 1 mL of a stock culture with 99 mL of water. She then performs five 1/10 serial dilutions with 1 mL of the diluted stock culture. Before she goes home for the evening, she plates 0.1 mL of the final serial dilution and spreads the sample on a TSA plate containing 50 mL of solid agar. If she arrives to work the next day to find 295 colonies on the plate, what is the concentration of bacteria in the stock culture? Solution The technician had made dilution of 107 . she have taken the last dilution of 0.1ml from where she gets 295 colonies, so in that dilution 1ml would have 295*10=2950 bacterial cell. Therefore the concentration of bacteria in the stock culture is 2950 *107=2.95 * 1010 babterial cell/ml..