THE MICROBIOTICS — MCDONNELL 447 



to that employed in usual bacteriologic procedures, was empiric and 

 unsuited for exacting work. A modification was developed, using 

 porcelain cups set on agar plates. Heatley defined the unit of peni- 

 cillin antibacterial activity for the cup method as that amount of 

 penicillin which when dissolved in 1 cc. of water gives a zone of 

 inhibition 24 mm. in diameter, or the amount by serial dilution which 

 when dissolved in 50 cc. of broth completely inhibits the growth of 

 a test strain of jS. aureus. 



The unit of penicillin was accepted by most workers and was named 

 in honor of Florey. However, the latter modestly disclaimed the trib- 

 ute and proposed instead the now generally accepted "Oxford unit," 

 after its place of discovery. This term is still in use in this country, 

 but because its measure is so small, about 0.6 microgram of penicillin, 

 and because so many are represented in usual therapeutic procedure 

 and industrial production, the British have invented the term "mega- 

 unit" to indicate 1 million Oxford units. 



In this country and in England over a score of research labora- 

 tories are collaborating in a study of penicillin assays under United 

 States Pharmacopoeial and Food and Drug Administration sponsor- 

 ship, in order to develop standardized and improved methods. 



EXTRACTION 



The first method of production employed was the bottle process, 

 where bottles, flasks, or trays were used, requiring thousands of con- 

 tainers to yield any appreciable quantities of the broth culture. This 

 was the sure method and the one employed by all producers in their 

 early programs. Other methods developed later in this country were 

 the trickle method, where inoculated broth flows over stones or chips 

 as an adaptation of the quick vinegar process, and the bran process, 

 where sterile bran is inoculated by cultures, and the penicillin extracted 

 later. 



Penicillium notatum grows under both surface and submerged con- 

 ditions, although different strains are used for the differing processes. 

 The submerged or deep-tank method which is responsible for most 

 of present-day production allows the penicillium-inoculated sterile 

 broth to be aerated for the period of the fermentation cycle, in 1,000- 

 gallon tanks or larger. 



The solution to the problem of extraction, which had impeded prog- 

 ress for over a decade, was discovered soon after the unit was developed. 

 It involved the passing of impure acid penicillin broth (pH 2 to 3) 

 from a watery solution into an organic solvent such as ether, chloro- 

 form, or amyl acetate, and the subsequent passage of the purified agent 

 again into water, shaken with alkali. This principle, by appropriate 

 cycling and with the aid of further steps in purification with acti- 

 vated carbon, followed by the snap-freezing and desiccation procedure 



