84 



NATURE, FORMATION, AND ACTIVITIES 



Table 34 



A^iDiiber and percentage of cuUform bacilli resistant to Jour antibiotics 

 isolated in two periods (Vaccaro et al., 1956) 



depends on the eoncentration of the aiiti- 

 })iotic.s which are put in contact with micro- 

 l)ial cells and on the amount and nature of 

 nutrients and salts present. Environmental 

 conditions, such as pH, also play an im- 

 portant role in the killing process. 



Many antibiotics have strong bactericidal 

 action. The addition of O.o mg of actino- 

 mycin to a 10-ml suspension of E. coli re- 

 duced the number of viable cells from 0,400,- 

 000 to 493,000; 1 mg of the antibiotic 

 brought about a i-eduction to 4,800; and 2 

 mg resulted in complete destruction of all 

 the living cells. 



The microbiostatic action of very similar 

 antibiotics can differ. For example, the 

 bacteriostatic action of both mannosido- 

 streptomycin and dihydromannosidostrepto- 

 mycin is significantly less than that of 

 streptomycin and dihydrostreptomycin for 

 all organisms except Salmonella typhosa and 

 Salmonclki schuttmuUeri. These results are 

 similar to those pre\'iously reported (Rake 

 et al., 1947). 



Similarly, Lechevalier (1960) repoited 

 differences in the fungicidal properties of 

 polyenic antifungal antibiotics. 



A strong lytic action, similar in some cases 

 to the action of enzymes, has also been 

 indicated for some of the antibiotics. This 



effect may be a result of the lysis of the 

 cells. Autol.ysis is usually defined as the 

 destruction of some of the cell constituents 

 by enz^anes originating within the cell. The 

 lytic effect is bi-ought about, however, by 

 but few of the antibiotics and does not affect 

 most of the bacteria. The greatest bacteri- 

 cidal action of penicillin, for example, occurs 

 during maximal cell division, when the cells 

 capable of producing lytic agents undergo 

 Ij^sis rapidly. It must therefore be concluded 

 that lysis of the cells follows the killing ef- 

 fect of penicillin. 



Interaction among Antibiotics 



It has been noted that when antibiotics 

 are used in mixtures, one antibiotic will 

 sometimes repress or boost the antimicrobial 

 action of another. 



Jawetz, in a chapter of a monograph on 

 neomycin (Waksman et al., 1958), has 

 pointed out that antibiotics can be divided 

 into two groups on the basis of their be- 

 havior when used in mixtures. Group 1 

 comprises penicillin, streptom^'cin, bacitra- 

 cin, and neomj^cin, and Group 2 is composed 

 of the tetracyclines, chloramphenicol, and 

 erythromycin. Members of Group 1 are 

 never antagonistic to one another but not 

 infre(iuently are synergistic. Members of 



