TECHNIQUE 



89 



or blood heat. The devices to regulate the temperature should 

 be such that the variation from the two standards given shall not 

 be more than 2°, that is, not more than 1° in either direction. 



There is no standard time of incubation. For work in the 

 study of water sanitation as carried out in Germany, England 

 and also in the United States, gelatin plates are incubated for 2 

 days at a temperature of 20° C. It is suggested that the period 

 be extended to 3 days in order to get more accurate results. 





i i: 





Fig. 24. — Types of streak culture, i, Filiform {B. coli); 2, echiuulate (B. acidi 

 lactici); 3, beaded {Sir. pyogenes); 4, effuse (B. vulgaris); 5, arborescent {Bacillus 

 mycoides). — {McFarland after Frost.) 



From I to 2 days is the usual time of exposure for the higher 

 temperature (37° C). 



8. Practical Application of the Quantitative Estimations by the 



Plating Methods 



The relative importance of the quantitative bacteriological de- 

 terminations by the method of direct counting and by the plating 

 method has been explained. Both methods must be made 

 standard in every food and drug laboratory. Quantitative esti- 

 mations by the plating method should take precedence with all 

 substances containing largely hving organisms such as water 

 supplies of all kinds, milk, raw meats, and shellfish, etc., and all 

 substances in which infection is suspected, even though such sub- 



