METABOLIC PRODUCTS 285 



the colonies indicates inhibition (Fig. 45). A second method consists 

 in growing an organism on agar and cutting radially a series of agar plugs 

 and placing these agar disks, which contain the antibiotic, on agar plates 

 sown uniformly with the test organism (Raper et al., 1944). This method 

 is illustrated in Fig. 54. Other methods of detecting antibiotics have 

 been summarized by Waksman (1947). 



Fig. 55. The antibiotic effect of Streptomyces sp. on two plant pathogenic fungi, 

 Monilinia fructicola, on the left, and Helminthosporiuin sativum, on the right. 



The production of antibiotic substances by fungi is common. In a 

 screening test of over 400 species, which included over 300 wood-inhabit- 

 ing fungi and 22 dermatophytes, somewhat over 200 species produced 

 substances active against Staphylococcus aureus and Escherichia coli 

 (Robbins et al., 1945). A large number of Basidiomycetes and other 

 fungi have been tested for the presence of antibiotics by Wilkins and 

 Harris (1944). The actinomycetes are the source of many useful anti- 

 biotics including streptomycin, Chloromycetin, aureomycin, terramycin, 

 and other unidentified compounds (Waksman, 1947). With the excep- 

 tion of Phytophthora erythroseptica none of the Phycomycetes appear to 

 have been reported as producing antibacterial substances. For a survey 

 of Fungi Imperfecti in the role of producing antibacterial substances 

 (against Staphylococcus aureus) and antifungal substances (against 

 Botrytis allii), see Brian and Hemming (1947). The inhibiting effect of 

 Streptomyces sp. on two plant pathogenic fungi is shown in Fig. 55. 



Many soil organisms produce antibiotics. Whether these organisms 

 produce antibiotics in sufficient amounts to inhibit plant pathogens under 

 natural conditions in the soil is not certain. It is known, however, that 



