514. 



BACTEKIOLOGY. 



each germ present should be sing'le and wholly free from contact with 

 other organisms. The massing of bacteria necessarily affords protec- 

 tion to the cells in the center of such groups. Because of this protec- 

 tion these few cells may survive, while all the others may be destroyed. 

 The results obtained would consequently be misleading. This error 

 can be avoided by filtering the suspension through sterile absorbent 

 cotton or glass-wool in the manner presently to be described. 



When it is desired to test the resistance of the veg^eta- 

 tive form of a given organism special care must be taken to 

 exclude the presence of spores. Many 

 bacteria like those of typhoid fever, 

 cholera, etc., do not form spores and 

 hence such precautions are not observed 

 when testing these organisms. The 

 anthrax bacillus, however, does form 

 spores, and hence in testing the resist- 

 ance of the growing bacillus the spore 

 form must be eliminated. This is ac- 

 complished by making so-called homo- 

 geneous cultures. These are obtained 

 by making transplantations to bouillon 

 every 6 or 8 hours. After several such 

 transplantations only actively growing 

 bacteria will be present. 



The best procedure for studying the 

 Fig. 74. Filter for bacterial action of moist heat on bacteria, is to 

 suspensions. place homogeucous suspensions in thin, 



straight capillary tubes. It is essential that the walls 

 of the capillary tube shall be as thin as possible. They 

 should not have a bulb of thick glass. When such sealed 

 capillary tubes are immersed they rapidly acquire the tem- 

 perature of the surrounding liquid. 



Preparation of a bacterial suspension.— As a rule, it is best to era- 

 ploy young cultures on agar. Sterile bouillon is introduced into the 

 test-tube by means of a drawn-out pipette (Fig. 61 e) and the growth 

 is thoroughly rubbed up. The suspension is now drawn up into the 

 pipette and is transferred to a sterile filter (Fig. 74), The filter-tube 



