20 A BACTERIOLOGICAL STUDY OF EGGS. 



INCUBATION. 



Plate cultures should be incubated for from 3 to 5 days, according 

 to the temperature and media employed. Materials containing anti- 

 septics may require a longer time to permit colonies to develop than 

 do those not containing such substances. 



Duplicate plain agar plates should be made, one set being incu- 

 bated at 37 C., or body temperature; the other set at room tempera- 

 ture, or about 20 C. Gelatin plates are incubated at 20 C. and 

 observed daily for the presence of liquefiers. 



COUNTING COLONIES. 



At the end of the necessary incubation period the colonies should 

 be counted, if necessary, by the use of a low-power lens and ruled 

 plate or by the naked eye when such colonies are well isolated and 

 fully developed. Plates containing about 50 colonies should be 

 selected for this purpose if possible. The number of colonies on a 

 given plate should be multiplied by the dilution used in making such 

 plates in order to determine the total number of bacteria originally 

 present in the material examined. 



B. COLI DETERMINATIONS. 



From the fermentation tubes showing gas in the two highest 

 dilutions, plate cultures are made on a special differential medium to 

 distinguish between acid and alkaline colonies and to separate the 

 gas-producing types of organisms. When obtained in pure culture 

 the necessary study is made by inoculating such organisms into 

 various media to determine their exact character. The same detailed 

 method should be employed in the study of any organism under con- 

 sideration. 



STREPTOCOCCI DETERMINATIONS. 



The presence of streptococci may be determined by staining smear 

 preparations from each dilution of the bile cultures and examining 

 them under the microscope for spherical organisms, arranged in 

 typical chain formation. 



OTHER ORGANISMS. 



The nature of other organisms found in various food materials are 

 considered in relation to the B. coli and streptococci. 



The use of laboratory animals, such as white mice, guinea pigs, 

 rabbits, etc., for experimental purposes in relation to infected and 

 decomposed substances is highly desirable in many instances. The 

 original material itself, or cultures and extracts made therefrom, may 

 be used for this purpose. Such animals may be either fed with the 



