CHAPTER X. 



CULTURE MEDIA AND THEIR STERILIZATION. 



A SUCCESSFUL investigation of pathogenic bacteria, permitting of 

 definite, trustworthy conclusions, is possible only if they can be 

 obtained in pure culture. By this term is meant the isolation of one 

 species of bacterium to the exclusion of all other living organisms. A 

 pure culture, accordingly, represents an otherwise sterile culture 

 medium containing only one species of microorganisms. To isolate 

 pathogenic bacteria suitable sterile culture media are needed. Some 

 of these substances, such as blood serum, milk, potatoes, occur in 

 nature and are simply sterilized by suitable methods without, as a 

 rule, adding anything to them; others are artificially compounded from 

 various substances and chemicals. In the preparation of artificial 

 culture media, the few necessary ingredients must be so selected and 

 mixed in proper proportions that they supply all the elements neces- 

 sary for the growth and multiplication of bacteria. At the same time 

 the medium must be either neutral or faintly alkaline. Pathogenic 

 bacteria generally grow best on a faintly alkaline medium; a few also 

 grow on a very slightly acid soil, but the latter is rather exceptional. 

 Substances which are changed in a characteristic manner by certain 

 bacteria are frequently added. Sugar, for instance, is introduced to 

 show whether the bacterium which is being grown possesses the 

 faculty of splitting up sugar and forming carbon dioxide. Litmus in 

 an alkaline medium indicates whether the growing bacterium forms 

 acids and finally changes the alkaline reaction to acid. 



Natural sterilized culture media, such as milk, coagulated blood 

 serum, potatoes, etc., are not transparent. Very often, however, it is 

 desirable to work with perfectly transparent culture media. These can 

 be obtained by adding to suitable clear solutions either gelatin or a sub- 

 stance called agar-agar, derived from a Japanese sea- weed. Both these 

 substances, when dissolved in a watery fluid by heat, form transparent 

 masses with it. Gelatin culture media which melt at about 25 C. 

 cannot be kept in the incubator without losing their solid consistency. 

 Generally, as stated, culture media must be sterilized. Sometimes, 

 however, it is desirable to use natural media, such as blood, blood 

 serum, ascitic fluid, etc., without subjecting them to heat. In such 

 cases the fluids must be obtained in a perfectly aseptic manner, so 

 that they are and remain sterile. 



Sterile blood serum may be obtained from the living animal. 

 Since the method in the case of culture media is identical with that 



