228 NATURE OF ANTIBIOTIC ACTION 



The concentration of the active substance and the composition of the 

 medium are highly important in modifying the activity of the sub- 

 stance. Some antibiotic substances, like penicillic acid, lose considerable 

 bacteriostatic activity when incubated with sterile broth or with sterile 

 peptone water at fH 7 and 37° C. for i to 3 days (700) j a similar ef- 

 fect was observed with certain simple amines and amino acids. The con- 

 centration of the substances reacting with penicillic acid is diminished 

 on autoclaving the peptone broth in the presence of 2 per cent glucose. 

 The neutralizing or anti-inhibiting agent interacts with the antibiotic 

 substance and neutralizes its antibacterial effect either in the absence or 

 in the presence of the organism. 



Since not all antibiotics of microbial origin have been isolated in a 

 crystalline state, confusion often resulted from the use of crude prepa- 

 rations. Concentrated and partly purified actinomycetin had no appre- 

 ciable lytic action upon living cells j however, the presence of a small 

 amount of a highly bactericidal 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 (1002). 



ANTIBACTERIAL ACTION 



Chain and Florey (122) divided all antibiotic substances into two 

 groups: 



1. Antibiotics which react with protoplasmic constituents and kill 

 both bacterial and animal cells, comparable to the action of "antisep- 

 tics." These antibiotics can be further subdivided into (a) those that are 

 active against both gram-positive and gram-negative bacteria, and (b) 

 those that exert a selective antibiotic action, usually against gram-posi- 

 tive organisms, such as gramicidin and actinomycin. The selectivity is 

 not absolute, since gramicidin acquires strong bactericidal activity 

 against gram-negative bacteria in the presence of protamines, due to the 

 fact that protamines remove phospholipids, which inhibit the antibac- 

 terial action of gramicidin. 



2. Antibiotics which react with substances having a specific signifi- 

 cance in the bacterial cell only. Some of these substances are largely 



