1 ml 1 ml 1 ml 1 ml 1 ml Original 9 ml broth inoculum in each tube Dilutions 1:10 1:100 1:10.000 1100.000 ml 1 ml m 1 ml Plating 1:10 1:100 1:1000 1:10.000 1:100.000 Calculation:Number of colonies on plate x reciprocal of dilution of sample number of bacteria/ml (For example,if 32 colonies were on a plate of /1000 dilution,then the count is 32 x 10.000 320.000/ml in sample Figure 6.15 Plate counts and serial dilutions.In serial dilutions,the original inoculum is diluted in a series of dilution tubes.In our example,each succeeding dilution tube will have only one-tenth the number of microbial cells as the preceding tube.Then samples of the dilution are used to inoculate Petri plates,on which colonies grow and can be counted.This count is then used to estimate the number of bacteria in the original sample. The most frequently used method of measuring bacterial populations is the plate count. 学Shanghai Jiao Tong University 陈峰，上海交通大学生命学院微生物学