452 STATE BOARD OF AGRICULTURE. 



probable? Nothing- contradicts the statement that the period of most 

 rapid growth and of the most rapid feniientation coincide although it is 

 beyond actual demonstration at a time when the products are not 

 chemically determinable. This statement becomes logically necessary 

 if we consider fermentation as the source of energy for the life func- 

 tions of the microbial cell. The period of incubation should be defined 

 as the period during which the fermentation products are within the 

 limits of analytical error. 



Tlie single cell as unit. — A brief explanation should be given why the 

 single cell is chosen as unit instead of the weight of the acting cells. 

 Both these standards haye their disadyantages. Rubner^ has demon- 

 strated that there is a large difference between different cells; he has 

 also pointed out that many yeast cells may still ferment, but not de- 

 velop on plates, the plate method being practically the only one com- 

 ing into consideration in this paper." In spite of the great difficulties 

 in determining the Ayeight of bacteria in a culture, Rubner in his ex- 

 periments on metabolism of bacteria preferred it to the counting of 

 cells. The reasons which caused Rubner to weigh the bacteria, have 

 little bearing in this work. Since only young cultures are considered, 

 the plate method is exact enough for counting and probably more exact 

 then Rubner's method which could not be applied to milk cultures. 

 The difference between the individual cells cannot be denied but the 

 variation in a well-bred culture is very small and on the other hand, 

 the weight is not a very reliable unit inasmuch as cells may or may 

 not contain considerable amounts of glj'cogen, fat, etc. Therefore, for 

 the purposes of this paper the counting of bacteria seems fully as, or 

 more desirable than the weighing method. 



As to Duclaux, he probably did not think of the possibility of com- 

 puting the fermenting power per cell. His method was quite different. 

 He added so many bacteria to the culture that there would probably 

 be no increase of cells, consequently he had a constant weight of bacterial 

 mass acting upon the sugar. This method, though it served his pur- 

 pose, is less accurate and can be applied only in a few instances, where 

 bacteria can be filtered out quantitatively. 



Method of Counting. — The common method of counting bacteria by 

 l)lating is doubtless not an accurate detennination of the number of 

 single cells. Very commonly, especially with Bacterium lactis acidi, 

 two cells will remain attached for some time after the fission has taken 

 place as may be easily demonstrated by the microscope and will tlius 

 give but one colony on the plate. Besides, old cells are apt to lose their 

 vitality and may not be able to multiply in agar or gelatin though 

 they still may cause fermentation. In order to determine the relation 

 between the actual microscope and the jjlate method, comparative 

 counts were made in litmus lactose agar and by the direct method of 

 MacNeal,^ Latzer and Kerr. The latter consists in spreading the con- 

 tent of a standard loop between two cover-glasses, staining and counting 

 a number of microscopic fields. Two different strains of lactic bacteria 

 were used for these experiments. 



L 'Rubner, Archiv. f. Hygiene, Bd. 48, p. 260 and Bd. 49, p. 355. 

 ^Ahout microscopic counts see next paragraph. 

 "Journal of Jnfectious Diseases, vol. 6 (1909) p. 146. 



