MECHANISM OF ANTIBIOTIC ACTION 223 



cyanin, and the culture filtrate of P. notatum (due to the presence of 

 notatin) are similar in some respects but not in others to the chemical 

 compound in their action 3 they are found to be generally bacteriostatic 

 over a wide range of test organisms, no sharp division being obtained 

 upon the basis of the gram stain. Streptothricin is unique in its action j 

 the gram-positive spore-former B. suh tills is most sensitive, but the 

 other spore-former B. mycoides is not affected at all. The gram-negative 

 E. coll is more sensitive to streptothricin than either M. lysodeiktlcus 

 or S. lutea. Streptomycin is almost as active against B. mycoides as 

 against B. subtllls but is less active against fungi. The antibiotics of mi- 

 crobial origin are generally found to be stronger bacteriostatic agents 

 than the chemicals tested. A high bacteriostatic effect is not necessarily 

 accompanied by a correspondingly high bactericidal action. Gliotoxin, 

 one of the most active bacteriostatic substances among those tested, pos- 

 sesses lower bactericidal properties than other preparations. Strepto- 

 thricin and streptomycin, on the other hand, are highly bacteriostatic 

 and bactericidal against certain gram-negative bacteria. 



The specific morphological differences among the bacteria, based 

 upon the gram stain, as shown by their sensitivity to antibiotic sub- 

 stances, are thus found to be relative rather than absolute. Most gram- 

 positive bacteria are more sensitive to the majority of antibiotics than 

 are gram-negative bacteria. But other antibiotics, such as streptothricin, 

 streptomycin, and clavacin, act quite differently and show marked 

 variations within each group. 



MECHANISM OF ANTIBIOTIC ACTION 



In an attempt to interpret the antibacterial properties of antibiotic 

 substances, one may benefit from a comparison of the action of these 

 substances and that of other antibacterial agents. Recent studies of the 

 mechanism of antibacterial action of chemotherapeutic agents led to 

 rather definite concepts concerning the nature of this action. The action 

 was believed to consist in depriving the bacteria of the use of enzymes 

 or metabolites by various types of interference. When the organisms 

 ai-e thus inhibited their nutritional requirements are more exacting than 

 in their normal state. E. coll and S. hemolytlcuSj when inhibited by acri- 





L i B R A ft Y 



