372 



METABOLISM OF MICROORGANISMS 



acid accumulates. Associated with citric acid production is sparse spore 

 formation. The mycelium presents a beaded or braided appearance, and 

 this appearance supports the idea that accumulation of citric acid is an 

 abnormal type of metabolism. 



One of the theoretical problems connected with citric acid production 

 is how to harmonize the high yield with the conventional system of inter- 

 mediary metabolism that operates in yeast and animals (p. 331). This 

 system would require three 2-carbon pieces, or one and one-half moles 

 of glucose per mole of citric acid. On a percentage basis only 71 per 

 cent of citric acid could be obtained. Many theories have been proposed 

 to account for higher yields. The current and best explanation is the 

 uptake of carbon dioxide by pyruvic acid to form oxalacetic acid (Wood- 

 Werkman reaction) and condensation of this acid with acetic acid to form 

 citric acid: 



CO2 + CH3 • CO • COOH-^ HOOC • CHo • CO • COOH 



HOOC • CH2 • CO • COOH + CH3 • COOH-^ HOOC- CHo • C(OH) • COOH 



I 

 CH2 • COOH 



Uptake of isotopic carbon dioxide has been shown to take place, but 

 whether this is adequate to account for the high yields is still not certain. 

 The mechanism of citric acid formation is under active investigation in 

 both animal and mold studies, and many of the questions now unanswered 

 should be cleared up in the near future. 



Penicillin. Besides the major products mentioned in Table 14-1, 

 molds produce hundreds of other compounds in amounts from a fraction 

 of a per cent to 10 per cent. The best known of these products is peni- 

 cillin. Approximately 20 tons of penicillin are produced every month 

 in the United States alone. Yields of 1 g. of penicillin per liter of medium 

 are usual, and about 70 per cent of the penicillin in the broth is recovered 

 as finished product. A typical medium is: lactose, 3 per cent; corn 

 steep solids (the concentrate of the water extract obtained in the indus- 

 trial manufacture of starch, gluten, and other corn products), 3 per cent; 

 calcium carbonate, 0.5 per cent; sodium sulfate, 0.1 per cent; phenyl- 

 acetic acid, 0.3 per cent. The medium is sterilized, inoculated with a 

 high-yielding strain of Penidllium chrysogenum, and aerated and stirred 

 vigorously during the fermentation period, 3-4 days. The penicillin is 

 extracted from the acidified broth with amyl acetate, transferred to a 

 buffer, purified, and finally crystallized as the sodium, potassium, or 

 procaine salt. Since penicillin is an acid, many different salts can be 

 made, but the above three are those in commercial use. More than a 

 dozen companies are producing penicillin in this country, and the market 

 value of the yearly product has been more than 100 million dollars for 

 several years. See Fig. 14-3. 



