STUDIES IN MICROBIOLOGY (2) 



Bacterial Mutation; Resistance to Antibiotics; 

 Radiation Effects; Action of Lysozyme; 

 Bacterial Anatomy* 



(Reading: H. J. Muller, "Radiation and Human Mutation," Sci. Am. 193, No. 5, 

 Nov. 1955, Reprint No. 29. Further readings are suggested at the end of this 

 exercise.) 



A bacterial population, even though it may 

 have descended from a single cell, contains many 

 cells which differ from the original bacterium 

 and from most of the cells about them. These 

 variants, or mutants, arise spontaneously as the 

 result of aberrations in the molecules concerned 

 with transmitting inheritance from parent to 

 daughter cells, the deoxyribose nucleic acids 

 (DNA). The aberrations responsible for muta- 

 tion are believed to involve the substitution 

 of one or more nucleotides for others originally 

 present in the DNA sequences. Such errors in 

 the replication of DNA probably occur while 

 these molecules are being multiplied prior to 

 cell division; but whenever they occur, they are 

 propagated thereafter from generation to gen- 

 eration. In this way what begins as a small 

 molecular change can end in forming a new 

 population, a new strain of bacteria. 



*Directions for setting up these experiments will 

 be found in Appendix A. 



Even though such changes are rare, there are 

 many of them in a large population. If one in 

 ten thousand bacteria is a mutant, a bacterial 

 population of ten million is likely to have one 

 thousand such mutants. This therefore consti- 

 tutes a tremendous potentiality for variation, 

 present in all bacterial cultures. 



Most mutations are disadvantageous, and 

 thus most mutant strains tend to die out rather 

 than to propagate and expand. A change in 

 environmental conditions, however, may favor 

 a previously unsuccessful mutant. Indeed, a 

 drastic change may kill all the other bacteria 

 and allow one mutant form alone to survive, 

 and — since it is now relieved from competition — 

 to flourish. This is exactly what happens when, 

 after you have taken heavy doses of an anti- 

 biotic, you may find that the antibiotic no longer 

 works. 



The experiment to be performed today should 

 show you that it is not difficult to develop 

 strains of bacteria that are resistant to penicillin 



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