You received a probiotic sample supposed to contain 20 billion CFU of probiotics per serving. You make four (4) serial dilutions (dilution 1, dilution 2, dilution 3 and dilution 4) of the sample by pipetting 0.5 ml into 19.5 ml of medium during each dilution step. Then you pipette a 0.2 ml sample from dilution 3 and 4 onto the surface of a MRS agar plate. After 48 hours incubation time, you count 89 bacterial colonies on the plate with the dilution 4 sample. What was the “CFU” of the probiotics sample you received? Is the number in agreement with the stated CFU number?
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