TYPES OF COLONIES IN LITMUS GELATIN. 273 



time. The difficulty with the whole problem of qualitative 

 analysis lies in the presence of liquefying bacteria. It some- 

 times happens that the milk contains a number of rapidly 

 liquefying bacteria which liquefy the gelatin so as to destroy 

 the plates within a couple of days. Such plates cannot be 

 studied and the differentiation of the bacteria is very difficult, 

 or frequently impossible. It is for this reason that the two 

 agar plates are prepared. The differentiation of the bacteria 

 upon the agar plates is very unsatisfactory, but it is better 

 than nothing, and if the liquefiers should prove to be so 

 abundant as to make the study of the gelatin plates impos- 

 sible, the agar plates can be used. The presence of a few 

 liquefiers, however, does not necessarily interfere seriously 

 with the study of the gelatin plates. The liquefiers may be 

 quite abundant, but if they are not very rapidly growing 

 forms they may not so far liquefy the gelatin within four 

 days as to prevent the tolerably satisfactory study of the rest 

 of the plate. It is only the presence of large numbers of 

 liquefiers or of a few extremely rapidly growing forms that 

 interfere with the analysis to be made, and experience has 

 shown that this does not very frequently happen. In 

 making a long series of analyses the large majority are satis- 

 factory, but there will be a small percentage which cannot be 

 differentiated because of the abundance of rapid liquefiers. 

 In these cases all that can be done is to depend upon the agar 

 plates which have been simultaneously made. These agar 

 plates will enable us to determine the total number of bac- 

 teria and differentiate tolerably well the lactic forms from 

 other species, but a more complete differentiation is not 

 possible upon agar. 



The Types of Colonies found in Litmus Gelatin. This is 

 not the place to attempt any detailed description of the nor- 

 mal milk bacteria. Such a description must be found in more 

 technical works on bacteriology. A few general facts in re- 

 gard to the nature of the most common bacteria in normal 

 18 



