80 Report of the Department of Bacteriology of the 



plate method and the examinations necessary for commercial 

 grading can be made by men who are not trained bacteriologists. 

 The morphology of the bacteria present may be determined as 

 well as the approximate number present. The microscope frequently 

 shows many bacteria present in samples of milk while agar plate 

 counts from the same samples are low. 



6. For these reasons, there is hope that the method here tested, 

 or some modification of it, can be made of practical use to the 

 milk dealer, butter-maker and cheese-maker as a means of grading 

 milk according to its bacterial condition. This should make it 

 easier for the farmer to secure a better price for a high-grade milk 

 than for a poorer grade. 



7. The adoption of the microscopical method of counting bacteria 

 in milk would involve a complete readjustment of present bacterio- 

 logical standards. For this reason it is not recommended that such 

 changes in standards be made until more comprehensive data have 

 been secured. 



INTRODUCTION. 



The number of bacteria in milk is ordinarily obtained by counting 

 the colonies which develop from an aliquot portion of a cubic centi- 

 meter of milk on nutrient agar or gelatin petri plates and multi- 

 plying according to the dilution used. Both the advantages and 

 faults of this method are well known. The need has long been 

 felt for some method which will give a better idea of the actual 

 number of germs present, enable a person to secure results in a 

 much shorter time, be simpler to use, require less expensive apparatus, 

 and give more accurate information as to the kinds of bacteria present. 

 There is considerable promise that the use of the microscope for 

 examining and counting the bacteria in dried, stained films of milk 

 will provide a method that will overcome many of the objections 

 to the plate method without introducing other serious difficulties. 



This bulletin gives the results of comparative studies between 

 the direct microscopic method and the plate method of counting 

 bacteria as applied to market milk. The objects of the investiga- 

 tion were to test the practical utility of the microscopic examina- 

 tion of milk and to determine whether it is a satisfactory substitute 

 for the plate method now in general use. The author is indebted 

 to Dr. H. A. Harding, former head of the bacteriological depart- 



