120 



NATURE, FORMATION, AND ACTIVITIES 



Crossings of parental strains with rearrange- 

 ment of genetic materials are usually per- 

 formed with nutritionally deficient mutants 

 of sti'ains that normally can grow in syn- 

 thetic media with a sugar as the sole carbon 

 source. Penicillin is usually employed to 

 concentrate and recover the induced mu- 

 tants. 



2. A better understanding of biological 

 synthesis, especially of large molecules (nota- 

 bly proteins, nucleic acids, and cell-\vall 

 material), can be attributed to the introduc- 

 tion of antibiotics. The effect of chloram- 

 phenicol on the building of the protein 

 molecule and on amino acid incorporation 

 established the fact that this antibiotic 

 uncouples the synthesis of nucleic acid fi'om 

 that of protein (Lacks and Gros, lUoD). This 

 is also true of the effect of penicillin upon 

 protoplast formation in Escherichia coli. 



3. The widespread use of antibiotics has 

 stimulated organic chemical I'esearch, and 

 several new or very rare compounds \\i\\e 

 been discovered. Streptose, the first branched- 

 chain sugar to be identified in a microbiologi- 

 cal product, and streptidine, a base re- 

 lated to inositol, were found in streptomycin. 

 Only three naturally occurring polyacetyl- 

 enes were known before 1950, but study of 

 the polyene antibiotics has increased the 

 number to at least 2o. Dichloroacetic acid 

 and nitrobenzene, although well known to 

 the organic chemist, were found in a natural 

 product for the first time in chloramphenicol. 



4. The very fact that there is relatively 

 little cross-resistance among the various 

 antibiotics suggests the prol)ability that 

 different mechanisms of antimicrobial ac- 

 tivity are involved. This effect of antibiotics 

 upon microbes causing infectious diseases 

 offers the clinician a number of possibilities 

 in the selection of chemotherapeutic agents, 

 alone or in combination with other anti- 

 biotics or chemical substances. The whole 

 principle involved in the preparation of 

 polio vaccine is based on the preservation 



of the polioviruses, by means of antibiotics, 

 against destruction through bacterial con- 

 tamination. 



5. The gi'owth-promoting efl'ects of anti- 

 biotics on higher animals appear to be dis- 

 tinctly different from those exerted by es- 

 sential growth factors or \'itamins. They are 

 not so specific as are true vitamins. They 

 involve processes heretofore scarcely recog- 

 nized in nature. This stimulating effect of 

 antibiotics has been ascribed to a disturb- 

 ance of the intestinal microbial populations 

 of the animals. A direct effect upon animal 

 growth has also been postulated. Since anti- 

 l:)iotics may affect adversely the bacterial 

 population of the rumen which assists the 

 animal in the digestion of its cellulosic food 

 materials, care must be exercised to use 

 antil)iotics only at a certain stage of the de- 

 velopment of the animal. 



6. Human and animal semen can be pre- 

 served from bacterial attack by means of 

 antibiotics. The same is true of the preserva- 

 tion of foods, especially poultry and certain 

 vegetal)les. Since antibiotics do not inhibit 

 all forms of microbial life, more than one 

 antibiotic may be recjuired for proper preser- 

 vation. Before the food is eaten, however, 

 the antibiotics must be destroyed by boiling, 

 for their constant con.sumption in the food 

 would tend to ha^•e certain dangerous effects 

 upon the human body. 



7. Antibiotics ha\'e introduced a new con- 

 cept of microbial life in natural environ- 

 ments. They have added greatly to oiu* 

 understanding of various aspects of biology, 

 under natural conditions. The actinomycetes 

 ha\'e pro\'ed to be the richest source of anti- 

 biotic producers. In 1989, on the eve of the 

 advent of antibiotics, these organisms were 

 considered as a rather insignificant group of 

 microloes, largely inhalnting soils, composts, 

 and lakes. Today, because of their ability to 

 form antibiotics, their biochemical activities 

 and their role in natiu-e have become prob- 

 lems of paramount significance. 



