ISOLATION AND IDENTIFICATION OF ANTIBIOTICS 



43 



Her (1956), who found cross-resistance be- 

 tween certain antibiotics, such as neomycin, 

 and a mild silver protein. 



In the case of streptomycin, tlependent 

 nuitants are available which will not grow 

 except in the presence of streptomycin or 

 certain of its derivatives. 



Development of Resistance 



If a large number of cells of a given 

 microbe are subjected to an antibiotic, most 

 of the (^ells will be inhibited, but a few will 

 grow. These cells will be resistant to either 

 low or high concentrations of the antibiotic. 

 A population of Escherichia coli, for example, 

 will contain a few cells that are very resistant 

 to streptomycin. Only a few cells in the 

 particular population of E. coli are slightly 

 more resistant to neomycin than the bulk 

 of the cells; the slightly more resistant popu- 

 lation can again be put in contact with 

 neomycin and still more resistant mutants 

 will be selected. The ease with which mu- 

 tants of a given organism resistant to an 

 antil)iotic can be obtained is a useful cri- 

 terion for the characterization of the sub- 

 stance. However, one must keep in mind 

 that development of resistance to one given 

 antibiotic may be fast for one microorganism 

 and slow for another. Polyenic antifungal 

 antibiotics are noteworthy examples of sub- 

 stances to which resistance develops slowly 

 and never achieves high levels. The practical 

 problems in chemotherapy arising from the 

 development of resistance are discussed in 

 Chapter 10. 



Microbio static and Microbicidal Action 



If a microbial population is subjected 

 under identical conditions to concentrations 

 of antibiotics high enough to prevent growth, 

 and if after a period of contact the anti- 

 biotics are removed and the cells placed in a 

 suitable nutrient medium, one can easily 

 demonstrate that some antibiotics will have 

 killed the test organism whereas others will 



simply have prevented its growth. Leche- 

 valier (19()0) has shown differences in the 

 fungicidal action of polyenes which could 

 be useful in the characterization of these 

 polyenes. 



Use of Inhibitors 



The antimicrobial action of certain anti- 

 biotics is inhibited by specific substances. 

 For example, the action of streptomycin is 

 inhibited by cysteine, hydroxylamine, and 

 nucleic acids; that of neomycin, only by 

 nucleic acids. The action of chlortetracycline 

 on E. coli has been found to be competitively 

 inhibited by riboflavin. The action of oxy- 

 tetracycline against both gram-positive and 

 gram-negati\'e bacteria is re\'ersed by mag- 

 nesium ions, whereas in the case of novobio- 

 cin, magnesium sulfate reverses the inhibi- 

 tion of gram-negati\'e bacteria but not that 

 of gram-positive bacteria (Brock, U)")!)). 



Mode of Action 



Each antibiotic inhibits one metal)olic 

 reaction or a series of such reactions in the 

 microbial cell. These reactions can be used 

 as criteria for the characterization of the 

 antibiotic. The mode of action of antibiotics 

 in general is poorly understood (see Chapter 

 9), but some antibiotics have become bio- 

 chemical tools for the study of microbial 

 metabolism. Vor example, chloramphenicol 

 is an inhibitor of protein synthesis; nystatin 

 inhibits endogenous respiration and gly- 

 colysis in fungal cells; and antimycin A is an 

 inhibitor of electron transport in the cyto- 

 chrome C system. 



The effect of the antibiotic on the micro- 

 bial cell may at times alter the morphology 

 of the sensitive organisms. For example, low 

 concentrations of fradicin induce the cells of 

 Candida albicans to l)ecome filamentous, 

 whereas other antifungal antibiotics, such as 

 nystatin and candicidin, do not induce any 

 change in the form of the yeast. 



