388 ANTIBIOTIC SUBSTANCES 



third day; and the growth is green by the fifth. The pH of the 

 medium which is about 3.5 to 4.5 at the beginning does not rise until 

 about the fourth or fifth day. The production of penicillin increases 

 as the pH rises to 7. As the medium becomes alkaline the antibiotic 

 content is lowered so it is imperative that the penicillin be harvested 

 from about the seventh to the eleventh day, depending on when its 

 content is at a maximum. 



As already indicated in an earlier section, the investigators at the 

 Northern Regional Research Laboratory have had considerable ex- 

 perience with submerged fermentations, e.g., gluconic acid produc- 

 tion. Therefore, studies were carried out by these workers to see if 

 penicillin could be made by this method. It was found that P. no- 

 tatum 832 was better suited for this type of production than the 

 strain used for the surface culture method. It is necessary to aerate 

 and agitate the culture medium with absolutely sterile air in order 

 to obtain submerged growth of the mold. In the vat or rotary drum 

 fermenters, the mold develops as small pellets, although it may grow 

 in a filamentous form if the agitation has been quite vigorous. The 

 obvious advantage of the submerged growth method over the surface 

 growth method is the saving in time and labor. The penicillin can 

 be harvested in from three to seven days. 



A third method of producing the antibiotic that has been proposed 

 is the growth of the mold on bran. After sterilization and inocula- 

 tion, the bran may be spread thinly in trays or placed in rotary 

 drums. According to Coghill,* this method presents several difficul- 

 ties, of which two are the difficulty of sterilizing the bran and the 

 difficulty of dissipating the heat produced by the mold in the fer- 

 mentation, both weaknesses being due to the poor heat transfer 

 property of the substrate. 



The fourth method of producing penicillin that has been proposed 

 is the circulation of the culture medium through a column of wood 

 shavings (as in the manufacture of vinegar) or pebbles. 



With all these methods, one of the important causes of failure to 

 develop a good yield of penicillin is bacterial contamination. If 

 contamination occurs either through inadequate sterilization of the 

 culture medium or by the introduction of unsterile air, particularly 

 in the submerged growth method, the entire batch may be ruined. 

 Hence, pure cultures mitst be used and aseptic precautions rigidly 

 observed. Certain organisms are said to possess penicillinase, an 

 enzyme capable of destroying the antibiotic. In most other fermen- 

 tations, contaminations merely lower the yields, but in the penicillin 

 fermentation the growth of undesirable bacteria may completely de- 



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