DIFCOMANUAL 17 



solid media used after the introduction of gelatin were only partially effective 

 since many of the organisms under investigation would not develop satisfactorily 

 at temperatures below the melting point of the gelatin, while others liquefied the 

 gelatin. Bacteriology as a science began with the development of methods for the 

 cultivation of bacteria, and the use of agar was a step of greatest importance. 



Most bacteria are capable of growth under ordinary conditions of oxygen 

 tension. Certain types, however, are capable of deriving their oxygen from their 

 food substances. The aerobic organisms require the free admission of air, while 

 the anaerobes grow only in the exclusion of atmospheric oxygen. Between these 

 two groups are the microaerophiles which develop best under partial anaerobic 

 conditions and the facultative aerobes and anaerobes which develop over a wide 

 Eh range. 



Anaerobic conditions for growth of microorganisms are obtained in a number 

 of ways : 



(a) Addition of small amounts of agar to liquid media. 



(b) Addition of fresh tissue to the medium. 



(c) Culturing in the presence of aerobic organisms. 



(d) Addition of a reducing substance to the medium. 



(e) Displacement of the air by carbon dioxide. 

 (f ) Absorption of the oxygen by chemicals. 



(g) Removal of oxygen by direct oxidation of readily oxidizable substances 

 such as burning a candle, heating of palladiumized asbestos, copper, hydro- 

 gen, phosphorus or other readily oxidizable metals. 



(h) Incubation in the presence of germinating grain or pieces of potato. 



( i ) Inoculation into the deeper layers of solid media, or under a layer of oil 



in liquid media. 

 ( j ) Combinations of these methods. 



Methods of readily obtaining anaerobic conditions in the laboratory are dis- 

 cussed in detail on page 118. Swancara^ describes a method of removing oxygen 

 from individual tubes of culture media and also a method of supplying partial 

 carbon dioxide tension. For anaerobic conditions a cotton plug is placed just 

 above the culture medium in tubes and a gelatin capsule containing pyrogallic 

 acid in a Durham fermentation tube is placed on top of this plug. A solution of 

 sodium hydroxide is placed in the Durham fermentation tube and the culture 

 tube then sealed with a rubber stopper or screw cap. The tube is then ready for 

 incubation, the oxygen being removed by the action of the sodium hydroxide on 

 the pyrogallic acid. He also describes a method of obtaining partial carbon 

 dioxide tension in individual tubes by similarly placing a broken household match 

 on a cotton plug just above the medium. Remove the percussion tip from the 

 match prior to placing in the tube. Seal the culture tube with a rubber stopper 

 or screw cap and ignite the match head by applying heat to the outside of the 

 tube. 



Proper moisture conditions must prevail in the culture media employed for the 

 propagation of microorganisms. A moist medium and a moist atmosphere are 

 necessary for the continued luxuriant growth of the vegetative cells. 



The pH or reaction of the culture medium, expressing its hydrogen ion concen- 

 tration, is extremely important for the growth of microorganisms. The majority of 

 the microorganisms prefer culture media which are approximately neutral, while 

 others may require a medium which is distinctly acid. The pH or reaction of the 

 culture medium is determined by colorimetric or electrometric measurement of 

 its hydrogen ion concentration. It should be noted that additions of acid or alkali 

 which are insufficient to prevent the growth of bacteria in a medium may inhibit 



