82 Report of the Department of Bacteriology of the 



determinations of sewage bacteria, and later Winslow and Will- 

 comb 7 made a comparative study of this and the plate method. 



The first application of the microscopic method for counting 

 bacteria in milk was made by Slack. 8 His method consisted in 

 centrifuging 2 cubic centimeters of milk in a special rubber-stop- 

 pered tube and counting the bacteria in a stained film prepared 

 from the slime thus obtained. Such a method is indirect and the 

 use of the centrifuge introduces many unknown factors. 



A more direct method of counting tissue cells in dried, stained 

 smears prepared from milk was devised by Prescott and Breed 9 

 in 1910. This led to a publication the following year by the latter 

 author 10 on the usefulness of the same method for counting the 

 number of bacteria present. This method was employed in the 

 series of investigations reported in this bulletin, the technique of 

 which is described on page 83. 



Skar, 11 in 1912, reported a method of counting bacteria in milk, 

 apparently devised independently, which differs from the one sug- 

 gested by Breed principally in the manner of staining and in allowing 

 the fat drops to remain in the smears. He stained the bacteria by 

 putting 0.4 cubic centimeter of carbolated methylene blue into 9.6 

 cubic centimeters of milk. This mixture was heated from 5 to 10 

 minutes not above 70° C, and then the smears were made by 

 withdrawing 0.02 cubic centimeter by means of a capillary pipette. 

 This milk was spread over an area of 480 square millimeters. 

 The smears were air dried and studied without further treatment. 



The latest attempt to apply the microscopic method of counting 

 bacteria in milk has been made by Rosam 12 who places a drop of 

 milk, and a drop of methylene blue to which has been added a little 

 pyridin, in a spoon and heats the mixture to steaming. Then a 

 loopful of the mixture is placed on a clean glass slide by means of 



7 Winslow, C-E. A. and Willcomb, G. E. Tests of a method for the direct micro- 



scopic enumeration of bacteria. Idem, 273-283. 



8 Slack, Francis H. The microscopic estimate of bacteria in milk. Techn. Quart., 



19:1-4. 1906: Die mikroskopische Schatzung der Bakterien in der Milch. Centbl. 

 Baku, Abt. II, 16 : 537-538. 1906. 



9 Prescott, S. C. and Breed, R. S. The determination of the number of body cells 



in milk by a direct method. Journ. Infect. Dis., 7:632-640. 1910. 



10 Breed, R. S. The determination of the number of bacteria in milk by direct micro- 

 scopical examination. Centbl. Baku, Abt. II, 30:337-340. 1911. 



11 Skar, Olav. Eine schnelle und genaue Methode fur Zahlung von Bakterien und 

 Leukocyten. Milchw. Zentbl., 41 : 454-461, 705-712. 1912. 



12 Rosam, A. Eine einfache mikroskopische Beurteilung des Gehalts der Milch 

 an Mikroorganismen. Milchw. Zentbl., 42: 333-334. 1913. 



