COLONIES ALLOWABLE ON AGAR PLATES 323 



in others it is clearly due to the fact that by-products of bacterial 

 growth inhibit the growth of other colonies; and occasionally 

 colonies fuse or overgrow each other and so reduce the count. 

 On the other hand colonies growing side by side sometimes 

 stimulate each other, a phenomenon which has been noted 

 in this work on plates containing large numbers of B. bulgaricus 

 with an occasional mold or bacterial colony of a different type. 

 The molds and many bacteria so stimulate the B. bulgaricus 

 that these organisms form visible colonies in the region of the 

 larger colonies, faiUng to develop in all other parts of the plate. 

 The same condition has been noted in plating material con- 

 taining large numbers of long chained streptococci. This 

 phenomenon naturally produces marked irregularities in count 

 when it occurs. 



Because of these and other difficulties certain plates in any 

 series made from a given sample are more satisfactory for use 

 in computing a total count than are others. The matter of select- 

 ing plates to be used in computing a count becomes therefore a 

 matter requiring considerable judgment. 



EXPERIMENTAL DATA 



a. Analyses made in the Station Laboratory 



The object of this study has been to determine the hmits in 

 the number of colonies on plates which are satisfactory for 

 making bacterial counts. The data used have been obtained 

 by plating market milk samples on standard agar in triplicate 

 and in three different dilutions, incubating for five days at 21°C., 

 following with an incubation for two days at 37°C. The plates 

 were counted at the end of five days and again after the two 

 days incubation at 37°C. The five day and seven day counts 

 are tabulated separately and show the conditions for each period 

 of incubation. 



In deciding which plate counts to select as probably nearest 

 correct it became necessary to discard all of the counts on a 

 few samples where no satisfactory average could be made be- 

 cause of spreaders or because the milk contained more bacteria 



