284 PHYSIOLOGY OF THE FUNGI 



to organic compounds produced by microorganisms which inhibit the 

 functioning of other microorganisms. General usage of the term anti- 

 biotic is, however, wider than this and appHes the term to those organic 

 compounds of fairly simple structure produced by organisms which inhibit 

 microorganisms. These substances are referred to more specifically as 

 antibacterial, antifungal, or antiviral substances. 



There are no universal antibacterial or antifungal substances. Anti- 

 biotics are specific in action. Penicillin, for example, is active against 



A 



Fig. 54. Method of assay for antibiotics. A, control culture of Penicillium notatum 

 on agar medium; radial series of plugs cut at 6 days. B, agar-plug assay plate show- 

 ing zones of inhibition of Staphylococcus developed after agar blocks removed from A 

 have been incubated for 16 hr. at 37°C. (Courtesy of Raper, Alexander, and Coghill, 

 Jour. Bad. 48: 644, 1944. Published by permission of The Williams & Wilkins 

 Company.) 



many Gram-positive bacteria and only a relatively few Gram-negative 

 organisms. 



The occurrence of antibiotics is probably far more widespread than 

 suspected at present. The reason for this lies in the way in which anti- 

 biotics are discovered. Antibiotics are detected by their inhibiting action 

 on living organisms. A susceptible test organism is essential for the 

 detection of an antibiotic. For obvious reasons, human pathogenic 

 bacteria are most used for screening tests. If one desires to obtain anti- 

 fungal substances active against pathogenic fungi, these fungi should be 

 used as test organisms. 



The same principle underlies all methods for detecting antibiotic action. 

 The test organisms are brought into contact with the products elaborated 

 by the organism suspected of producing an antibiotic. This may be done 

 by growing two organisms on the same Petri dish. A clear zone between 



