46 



NATURE, FORMATION, AND ACTIVITIES 



the practical utilization of antibiotics in the 

 control of human, animal, or plant diseases, 

 and (4) the chemical nature of the anti- 

 biotics. Each of these systems has certain 

 distinct advantages and disadvantages. 



BIOLOCilCAL SYSTEM 



The biological system offers the advantage 

 of a knowledge of the specific nature of the 

 organisms producing the antibiotics. It has 

 even been argued that the ability of a certain 

 organism to form a particular antibiotic is 

 more characteristic of the species than such 

 properties as pigment production, sugar 

 utilization, or enzyme formation. The fact, 

 however, that the same antibiotic may be 

 produced by different organisms and, fur- 

 ther, that the same organism, or even strain, 

 may yield more than one antibiotic, reduces 

 the usefulness of this system of classification 

 designed primarily for characterization 

 piu"poses. The use of this system is further 

 complicated by the great variability of the 

 antibiotic-producing organisms, including 

 ordinary morphological, cultural, and bio- 

 chemical \'ariations and hereditary mutant 

 formations; the latter may involve either a 

 gain or a loss of the ability to produce a 

 certain antibiotic. Differences in chemical 

 composition or physical state (such as semi- 

 solid ve7-sus liquid) of media used for the 

 cultivation of antibiotic-producing organ- 

 isms, in the degree of aeration of the culture, 

 and in the length of the incubation period 

 may all result in chemical differences in the 

 antibiotics produced. The sensitivity or 

 resistance of the organisms to their own 

 antibiotics and the ability of certain organ- 

 isms to decompose the antibiotics after they 

 have been formed are other limiting factors 

 of such a system. Too little is yet known of 

 the mechanism of formation of the anti- 

 biotics in the mycelium and their liberation 

 into the medium to permit use of this reac- 

 tion for classification purposes. 



ANTIMICROBIAL ACTIVITIES OR ANTIBIOTIC 

 SPECTRA 



The antimicrobial activities or antibiotic 

 spectra offer another system for classifying 

 antibiotics. A knowledge of the range of 

 antimicrobial activities of an unknown anti- 

 biotic may permit its ready identification 

 with known substances. Naturally sensitive 

 and resistant organisms, as well as organisms 

 made resistant to known antibiotics, can be 

 used in the classification of antibiotics in 

 general. Unfortunately, different strains of 

 the same organism may \'ary considerably in 

 their sensitivity to a given antibiotic. Fur- 

 ther, the ease with which many organisms 

 may produce mutants resistant to an anti- 

 biotic reduces the usefulness of such a sys- 

 tem. Incidentally, this system has been 

 greatly abused for commercial purposes by 

 the designation of various antibiotics as 

 members of the "wade spectrum," "broad 

 spectrum," "narrow spectrum," and "stub- 

 born spectrum" groups. 



PRACTICAL UTILIZATION 



The practical utilization of antibiotics also 

 has much to disc^ualify it as a basis for 

 classification, since relatively few antibiotics 

 produced by actinomycetes, among the 

 many so far disco^^ered, have found practical 

 application. The fa\'or which a particular 

 antibiotic finds in the hands of the clinician 

 or the veterinarian has little to do with its 

 chemical nature and its })iological origin. 



CHEMICAL SYSTEM 



The chemical system appears to offer the 

 most logical basis for the classification of 

 antibiotics of actinomycetes. Unfortunately, 

 this system does not take into consideration 

 the range of antimicrobial activities of the 

 antibiotics; it does not explain the sensi- 

 tivity and resistance of different microorgan- 

 isms to what appear to be chemically closely 

 related antibiotics; and it does not indicate 



