ESTIMATING NUMBERS OF BACTERIA. 261 



result of the six regarded as the average number of bacteria 

 per one one-hundredth cubic centimeter. But, apart from 

 this irregularity in the samples, there are at least three other 

 facts which make the analysis only approximate. First, if 

 in the diluted milk there chance to be several bacteria 

 clinging together, as is quite probable even after thorough 

 shaking, these, when placed within petri dishes, would de- 

 velop into a single colony and would be counted as one. 

 This will naturally give a number in the analysis somewhat 

 too low. A second and more serious difficulty is the fact 

 that not all bacteria present in the milk will grow in the 

 culture medium as above prepared. While a large proportion 

 of the bacteria will develop on plates and make their appear- 

 ance in analysis, there are some that do not grow at all, and, 

 therefore, do not appear in the analysis. Third, it is impos- 

 sible to pour out all of the contents of the tube into the petri 

 dish, for some will inevitably stick to the tube. To obtain the 

 absolute number of living bacteria present in a cubic centi- 

 meter of milk is quite impossible by any means at our 

 command. 



The number obtained by the method described .will al- 

 ways be an underestimate. But while it must always be 

 recognized as approximate, the results in different cases may 

 be compared with each other. If two samples of milk show, 

 one ten thousand and the other ten million, it is quite certain 

 that these numbers express approximately the relative num- 

 ber of bacteria in the two samples, though neither expresses 

 the number accurately. * 



RESULTS OF SUCH ANALYSES. 



The results of analyses of milk, as obtained by the above 

 method, show the widest possible variations. If the milk 

 is fresh and exceptionally clean the number may be as low 

 as one hundred per c.c., or sometimes even lower. Fresh 

 milk from an ordinary dairy, however, will contain from 300 



