16 DIFCOMANUAL 



GENERAL CONDITIONS 



Pertaining to the Cultivation of Microorganisms 



The development of microorganisms upon culture media is dependent upon a 

 number of very important factors : 



(a) The proper food elements must be available. 



(b) Oxygen must be available as required. 



(c) A certain degree of moisture is necessary. 



(d) The medium must be of the proper reaction. 



(e) Proper temperature relations must prevail, 

 (f ) The medium must be sterile. 



(g) Contamination must be prevented. 



A satisfactory microbiological culture medium must contain available sources 

 of carbon, nitrogen, inorganic salts and, in certain cases, vitamins or other growth- 

 promoting substances. These were originally supplied in the form of the meat in- 

 fusions which were, and still are in certain cases, widely used in culture media. 

 Beef extract frequently replaces meat infusions, but the preparation of this sub- 

 stance subjects it to the loss of its heat labile nutritive factors in much the same 

 way as infusions are affected. The addition of peptone provides a readily available 

 source of nitrogen and carbon. 



Peptone is used in culture media to supply an available form of nitrogen since 

 native proteins are not generally attacked by bacteria. Most organisms are capable 

 of utilizing the amino acids and other simpler nitrogenous compounds present in 

 peptone. Continued investigations in our laboratories indicate that for the isola- 

 tion and propagation of many organisms the complicated infusion media can be 

 replaced by simpler media prepared by using the proper peptones in place of the 

 meat infusions heretofore employed. 



Certain bacteria require additions of other food substances such as serum, 

 blood, or ascites to the culture medium upon which they are to be propagated. 

 Carbohydrates may also be desirable at times, and certain salts such as those of 

 calcium, manganese, magnesium, sodium, and potassium seem to be required. 

 Dyes may be added to media as indicators of metabolic activity or because of their 

 selective inhibitory powers. Growth promoting substances of a vitamin-like nature 

 are essential or assist greatly in the development of certain types of bacteria. 



The consistency of a liquid medium may be modified by the addition of agar, 

 gelatin or albumin in order to change it into a solid or semisolid state. Solid 

 media, which were originally devised for the isolation of organisms in pure cul- 

 ture, are now universally used for almost all general cultural work. The semi- 

 solid media are used chiefly for carrying stock cultures or for propagating the 

 anaerobes. 



One of the principal landmarks in bacteriology was the preparation of a 

 satisfactory medium by Hesse's introduction of agar as a solidifying agent for 

 bacteriological culture media, Hueppe: Die Methoden der Bakterienforschung, 

 250: 1891. Previous to that time infusions of plant and animal tissues, solutions 

 of organic compounds, and gelatin media only were employed. Until the intro- 

 duction of gelatin media by Koch in 1881, the only method for obtaining pure 

 cultures was the very unsatisfactory dilution procedure devised by Lister. The 



