EFFECT OF CONCENTRATED SOLUTIONS 195 



EXERCISE 19. EFFECT OF CONCENTRATED 

 SOLUTIONS UPON MICROORGANISMS 



Apparatus. 750 c.c. nutrient broth; gelatin; salt; 

 dextrose; saccharose; five 10 c.c. pipettes; 100 c.c. graduate. 



Cultures. Mycoderma; B. coli; M. varians; Sacch. 

 cerevisice; Penicillium; B. prodigiosus. 



Method. 1. Make up four tubes each of the following 

 concentrations : 



Electrolytes: sodium chloride, 5%, 10%, 15%, 20%, 25%. 



Non-electrolytes: dextrose and saccharose, 30%, 45%, 

 60%, 75%, respectively. 



Colloids: gelatin, 5%, 10%, 30%, 50%. 



2. With the exception of the gelatin the separate weigh- 

 ing out for each concentration can be avoided by using the 

 following method of mixing, with the stock solution con- 

 taining 50% or 75% of the substance under study: 



(a) Weigh out the correct quantity of material and place 

 it in a 100 cc. graduate. 



(6) Fill the graduate to the 100 c.c. mark with nutrient 

 broth. Place the hand over the mouth of the graduate and 

 shake until solution is complete. If necessary, fill to the 

 mark again with broth. For example: Dissolve 25 g. of 

 salt in about 90 c.c. of broth, fill the graduate to 100 c.c., 

 to obtain a broth of which 100 c.c. contain 25 g. of salt. 

 Mix this salt broth with common broth in the following 

 proportions, by means of pipettes: 



Salt Plain Salt content 



broth. broth. of mixture. 



2 c.c. + 8 c.c. 5% 



4 c.c. + 6 c.c. 10% 



6 c.c. + 4 c.c. 15% 



8 c.c. + 2 c.c. 20% 



10 c.c. + c.c. 25% 



Broth will give a precipitate after heating with salt, 

 consequently each salt broth mixture has to be filtered 

 separately after heating. What is this precipitated material? 



