196 GENERAL MICROBIOLOGY 



Note 1. The stock solution of dextrose is best prepared by adding 

 to 75 g. dextrose, 50 c.c. of broth, heating the mixture in the steam 

 until dissolved and then making up to 100 c.c. with broth. The sugar 

 solutions may have to be filtered also. 



Note 2. As it is a difficult procedure to make up as high concen- 

 trations of gelatin as 30% and 50% with any degree of ease and 

 accuracy, gelatin prepared according to the following procedure will 

 serve to illustrate the point of the exercise. 



With a blue pencil, mark the 10 c.c. level on each of sixteen tubes. 

 To make up 5% gelatin, place 0.5 gm. gelatin in each of four tubes 

 and make up to the 10 c.c. mark with broth. Proceed similarly with 

 the remaining concentrations. After heating once, mix well with a 

 sterile platinum loop. 



Gelatin is practically the only colloid that can be obtained in solu- 

 tions concentrated enough for this experiment (up to 70%). Great 

 care must be taken to avoid the condensation of moisture on the sides 

 of the test tubes or flask, because this moisture will reduce the con- 

 centration of the surface gelatin and thus cause incorrect data. 



3. Sterilize the tubes by the intermittent method. 



4. Inoculate heavily one tube of each concentration of 

 the salt with Mycoderma, one with B. coli and one with M. 

 varians. Inoculate tubes of each concentration of dextrose, 

 saccharose and gelatin with Penicilliwn, Sacch. cerevisice, 

 and B. prodigiosus, leaving a tube of each concentration 

 uninoculated for control. 



Note. The inoculation must be heavy, because experience teaches 

 that a small inoculum is sometimes not sufficient to secure growth. 



5. Note and tabulate the growth after seven days. 



6. Reinoculate the lowest concentration of each set 

 that does not show growth from the highest of the same 

 set that does grow, e.g., if Penicillium grows at 45% 

 dextrose but not at 60%, inoculate 60% from 45%. If 

 it now grows, what is indicated? Is there not plenty of 

 water and food material present? Explain your results. 



Does the natural habitat or food requirements of each 

 organism explain in any. way the action occurring? 



7. What is meant by osmotic pressure? Electrolyte? 

 Colloid? What is known of the relative osmotic pressure 



