ANTIBACTERIAL ACTION 199 



substance and neutralizes its antibacterial effect either in the absence or 

 in the presence of the organism. 



Since only few antibiotic substances of microbial origin have been 

 isolated in a crystalline state, confusion often resulted from the use of 

 crude preparations. Welsch (971, 972) found that concentrated and 

 partly purified actinomycetin had no appreciable lytic action upon liv- 

 ing cells. However, the presence of a small amount of a highly bac- 

 tericidal substance, which was especially active against gram-positive 

 bacteria, resulted in the lysis of living bacteria by actinomycetin. This 

 action was thus a result of the activity of at least two different agents 

 present in one preparation. 



ANTIBACTERIAL ACTION 



Two antibiotic agents have recently received special consideration, 

 tyrothricin and penicillin. They will be considered here in further 

 detail. 



Tyrothricin 



The phenomenon of antibiotic action by a specific substance can best 

 be illustrated by the action of tyrothricin upon bacterial cells. Five dis- 

 tinct stages have been described (190) : 



1. Inhibition of growth. Certain gram-positive bacteria are inhibited by 



as little as I microgram or less of the substance per 10 milliliters of 

 nutrient broth or agar, thus giving an activity of i : 10,000,000 or 

 more. 



2. Bactericidal action consists in the killing of the bacterial cells, either in 



a washed state and suspended in saline, or in a growing state in broth 

 culture. 



3. Lytic activity comprises the rate of lysis of a suspension of bacterial 



cells. Streptococci, for example, are readily lysed by gramicidin, 

 whereas staphylococci are acted upon more slowly and less com- 

 pletely. 



4. Inhibition of enzyme activity includes dehydrogenases or enzymes or 



respiration. Gram-positive cocci, incubated at 37° C, lose their abil- 

 ' ity to reduce methylene blue in the presence of glucose, upon addi- 

 tion of gramicidin. Since inactivation of the dehydrogenase takes 



