LABORATORY AND TEACHING METHODS 625 



and cooled agar. When Ihc material is crushed, it is well shaken up with agar and 

 the whole poured into a Petri dish. If the growth of one fungus appears, it means 

 that we have the parasite in captivity, or pure culture. If more than one fungus 

 IS obtained, they must all be transferred separately into agar slants in test-tubes 

 and tested by inoculation for their pathogenicity. The true pathogen is of course 

 the one which will reproduce all of the symptoms of the disease. 



Note. — To keep out bacterial infection put one drop of a s per cent, lactic acid 

 in each of the agar tubes used in making the cultures. 



Differential Methods of Isolation 



Pasteurization and Sterilization. — In order to compare the effect of these two 

 operations on organic material, take some milk and pasteurize part of it and sterilize 

 the other part by one sterilization. Conduct both operations in previously sterilized 

 flasks plugged with cotton after the milk is introduced (Fig. 223). 



Milk is pasteurized by heating it up to a temperature of 85°C. followed by a 

 rapid cooling. Milk is sterilized by heating up to ioo°C. for five minutes. Set 

 the flasks aside and compare. Note any changes that may take place. 



Differential Media. — (a) Selective. — Some media are specially suitable for cer- 

 tain species of bacteria and enable them to overgrow and finally choke out other 

 varieties. 



(i) Deterrent. — The converse of the above also. Certain media possess the 

 power of inhibiting the growth of a greater or less number of species. For instance, 

 media containing carbolic acid to the amount of i per cent, will inhibit the growth 

 of practically everything but the Bacillus coli communis. 



Differential Sterilization. — (a) Non-sporing Bacteria. — Similarly, advantage may 

 be taken of the varying thermal death points of bacteria. From a mixture of two 

 organisms whose thermal death points differ by, say, ^°C — e.g., Bacillus pyocyaneus, 

 thermal death point SS°C., and Bacillus mesentericus vulgatus, thermal death point 

 6o°C. — a pure cultivation of the latter may be obtained by heating the mixture in 

 a water bath to s8°C. and keeping it at that point for ten minutes. The mixture 

 is then planted on to fresh media and incubated, when the resulting growth will be 

 found to consist entirely of B. mesentericus. 



(b) Sporing Bacteria. — ^This method is found to be of even greater practical value 

 when applied to the differentiation of a spore-bearing organism from one which 

 does not form spores. In this case the mixture is heated in a water bath at 8o°C. 

 for fifteen to twenty minutes. At the end of this time the non-sporing bacteria are 

 dead, and cultivations made from the mixture will yield only a growth resulting 

 from the germination of the spores only. 



differential atmosphere cultivation 



Aerobic and Anaerobic. — For the separation of bacteria, it is possible to draw the 

 line between those that need oxygen for growth (aerobic) and those that will grow 

 without oxygen (anaerobic). By excluding oxygen, anaerobic forms alone develop. 



Inoculation into various animals or plants may be used as a means of separation. 



40 



