Chapter 8 



Antimicrobial and Antitumor 

 Activities of Antibiotics 



Antibiotics vary greatly in their biological 

 activities, especially in their antimicrobial 

 properties, toxicity to animals, and mode of 

 action upon sensitive organisms. Their prac- 

 tical potentialities, therefore, notably their 

 therapeutic effects in the treatment of human 

 and animal diseases and their growth- 

 promoting action, also vary greatly. Un- 

 fortunately, the specific chemical structure of 

 a compound alone is not necessarily indica- 

 tive of its biological behavior. 



It was at first believed that screening 

 methods in vitro were not satisfactory for 

 finding new chemotherapeutic substances. 

 Fortunately, this concept was found later 

 to be completely unjustified. Chain and 

 Flore}^ (1944) emphasized that those anti- 

 biotics which pass the biological activity and 

 toxicity tests can be expected to be effecti\'e 

 as general therapeutic agents. Further stud- 

 ies confirmed these observations, namely, 

 that biological activities of antibiotics in 

 vivo are in general parallel to their activities 

 in vitro. Usually the only factors controlling 

 their actual practical potentialities are tox- 

 icity, selective activity upon various disease- 

 producing organisms, and the extent of such 

 activity. 



Antimicrobial Spectra of Antibiotics 



Antibiotics are characterized primarily by 

 their selective action upon different micro- 

 organisms, or their antil)iotic spectra. It was 



once believed that this property was so 

 characteristic of each substance as actually 

 to identify the antibiotic. However, the ease 

 with which many antibiotics select resistant 

 mutants in bacterial populations, and the 

 frequent variations in the degree of sensi- 

 tivity among the different strains of the 

 same organism tend to diminish the signifi- 

 cance of this property for either the identifi- 

 cation of the antibiotic or its proper char- 

 acterization. 



The following types of antimicrobial 

 spectra can be recognized: 



1. Antibiotics active mainl}' against my- 

 cobacteria; examples: actithiazic acid and 

 nocardamin. 



2. Antibiotics active mainly against gram- 

 positive bacteria; examples: the macrolides 

 such as erythromycin and other antibiotics 

 such as thiostrepton. 



3. Antibiotics acti\'e against both gram- 

 positive and gram-negative bacteria; ex- 

 amples: streptomycin, neomycin, and the 

 tetracyclines. 



4. Antibiotics active against bacteria and 

 fungi; examples: mycothricin and aureo- 

 thricin. 



o. Antibiotics active mainl^y against fungi; 

 examples: most of the polyenes, such as 

 amphotericin B, and other sul)stances such 

 as cycloheximide. 



G. Antibiotics active against inti'acellular 

 parasites of the lymphogranuloma-psitta- 



