DETERMINING BIOLOGICAL ACTIVITIES OF BACTERIA 169 



DIASTATIC ENZYMES. The presence of diastatic ferments may be 

 determined by mixing broth cultures of the bacteria with thin starch 

 paste. It is necessary that both the cultures and the starch paste be 

 absolutely free from sugar. After remaining in the incubator for five or 

 six hours, the fluid is filtered and the filtrate tested by methods used for 

 determining the presence of sugars. 



INVERTING FERMENTS. Inverting ferments are determined by a pro- 

 cedure similar to the above in principle. Dilute solutions of cane sugar 

 are mixed with old cultures or 

 culture filtrates of the respective 

 bacteria and the mixture allowed 

 to stand. It is then filtered, 

 and the filtrate tested for glucose, 

 preferably by Fehling's solution. 



ANIMAL EXPERIMENTATION 



In the study of pathogenic 

 microorganisms, animal experi- 

 mentation is essential in many 

 instances. The virulence of any 

 given organism for a definite ani- 

 mal species and the nature of the 

 lesions produced are character- 

 istics often of great value in 

 differentiation. Isolation, more- 

 over, of many bacteria is greatly 

 facilitated by the inoculation of 

 susceptible animals and recovery 



FIG. 48. TYPES OP GELATIN LIQUEFAC- 

 TION BY BACTERIA. 

 of the pathogenic organism from 



the heart's blood or from the lesions produced in various organs. That 

 investigations into the phenomena of immunity would be absolutely 

 impossible without the use of animal inoculation is, of course, self- 

 evident, for by this method only can the action of bacteria in relation to 

 living tissues, cells, and body-fluids be observed . 



The animals most commonly employed for such observations are 

 guinea-pigs, white mice, white rats, and rabbits. The method of 

 inoculation may be either subcutaneous, intrapleural, intraperi- 

 toneal, intravenous, or subdural, etc. It must be borne in mind 

 always that the mode of inoculation may influence the course of an 



