IM^OnurTTON- OF AX'IMIUo'lTCS 



227 



prod lie i 11, l;; tlic siinic antibiotic may \aiy, tlie 

 morpli()l()j;icai critci'ia alone cannot he used 

 as a basis for identit'vintj; antibiotics, or the 

 formation of a particular antibiotic I'oi' iden- 

 tificat ion of species. 



4. A ciiauf^'e in the nutrition of the or<j;aii- 

 ism may result in a cliaugo in the nature of 

 the antibiotic produced. Tliis is true, for ex- 

 ample, of the formation of the various actino- 

 mycins. 



5. As a rule, aiitibiotic-formin<>; organisms 

 are resistant to the antibiotics they produce, 

 as shown in Chapt(>r 14. This phenomenon is 

 taken advantage of in the isolation of fresh 

 cultures capable of forming a given antibi- 

 otic, and in the selection, from a given cul- 

 ture, of more potent strains. Not all anti- 

 biotic-forming organisms, however, behave 

 in the same manner. 



Methods for Isolating and Testing Antibiotic- 

 producing Organisms 



In a search for antibiotic-producing cul- 

 tures of actinomycetes, certain steps are 

 usually f ollow'ed : 



1. A sample of soil is plated out on suit- 

 able agar media. Usually, simple synthetic 

 or poor organic media are selected to en- 

 courage the maximum de^Tlopment of colo- 

 nies but pre^'ent these from making heavy 

 growth, thus avoiding overcrowding. 



2. After a given period of incubation, 

 which may varj^ from 3 to 15 days, the colo- 

 nies of the actinomycetes developing on the 

 plates are picked and transferred to fresh 

 agar slants. If care is taken in making the 

 transfer, pure cultures are usually obtained. 

 Otherwise, the cultures may have to be puri- 

 fied by replating and reisolating. 



3. The cultures thus obtained are tested, 

 by the agar-streak method, for their abilit.y 

 to inhibit the growth of microorganisms. 

 Various bacteria, comprising gram-positive 

 and gram-negative forms, and fungi are com- 

 monly used as test organisms. In some cases, 



especially' in search for antixiral or ant it u- 

 moi- agents, more complicated procedures arc 

 employed. Such cultures of actinomycetes 

 as are found to j^jossess the desirable anti- 

 microbial pro[)erties are selected for further 

 study. 



4. The selected cultures are gi'own in 

 various li({uid media, in stationary or under 

 submerged conditions, for varying periods 

 (usually 3 to 10 days), and the antibiotic 

 spectra of the broths determined. 



5. Suitable media and proper growth con- 

 ditions are established for each individual 

 culture. The nature of the medium for the 

 production of the antibiotic is of great im- 

 portance; each culture may require special 

 media for optimiun production of the anti- 

 biotic. 



6. An efi'ort is then made to isolate from 

 the medium the active substance produced 

 by the culture, concentrate it, and purify it. 



7. The isolated antibiotic is studied for its 

 chemical, physical, and antimicrobial prop- 

 erties. A comparison is also made of its anti- 

 biotic spectrum, which should correspond to 

 that of the culture broth from which it was 

 isolated. A lack of proper correlation ma}^ be 

 due either to the presence of more than one 

 antibiotic in the broth, or to a chemical mod- 

 ification of the antibiotic in the process of 

 purification. 



8. The isolated antibiotic is tested for tox- 

 icity and activity in experimental animals. 



9. Before actual production of the antibi- 

 otic is undertaken, the culture is irradiated or 

 treated by other suitable procedures, and the 

 isolated colonies retested, since a freshly iso- 

 lated culture may not be a ^'ery potent one. 

 A search for the natural occurrence of more 

 potent strains is often made. 



10. The clinical evaluation of the isolated 

 antibiotic is the final step in the group of 

 procedures. This permits one to come to a 

 conclusion concerning the therapeutic po- 

 tentialities of the freshly isolated antibiotic. 



