9/18/2016 Plate Count Methods for Enumeration of Bacteria Plate Counts Classical estimation method for counting bacterial Used to quantify bacterial populations cell numbers ->250 CFU ml for liquids Assumes every cell forms one colony and every colony originates from one cell ->2,500 CFU ml for solids colony forming unit-CFU Food sample is homogenized 1:10 in buffer followed by serial dilutions Limitations: not all bacteria grow under the same conditions not all bacteria grow as single cells samples require dilution or enrichment Serial Dilution Spread Plate vs Pour Plate 只兴兴兴夏 9 mL water 9 mL water mL water 39/18/2016 3 Plate Count Methods for Enumeration of Bacteria • Classical estimation method for counting bacterial cell numbers • Assumes every cell forms one colony and every colony originates from one cell colony forming unit - CFU • Limitations:  not all bacteria grow under the same conditions  not all bacteria grow as single cells  samples require dilution or enrichment 9 Plate Counts • Used to quantify bacterial populations - >250 CFU / ml for liquids - >2,500 CFU / ml for solids • Food sample is homogenized 1:10 in buffer followed by serial dilutions 10 Serial Dilution 11 Spread Plate vs Pour Plate 12