1948] ORIGIN OF BACTERIAL RESISTANCE TO ANTIBIOTICS 71 



POSSIBLE MECHANISM OF ORIGIN OF HIGH-DEGREE RESISTANCE 



The experimental evidence available at present indicates that resistance to 

 penicillin and resistance to streptomycin are independent of each other; that such 

 resistance is a heritable property induced by genetic changes comparable to 

 mutations; that first-step penicillin-resistant strains are fairly uniform in their 

 degree of resistance, a highly resistant strain being built up by selection through 

 several steps ; and that first-step streptomycin-resistant strains show a great deal 

 of variability in degree of resistance, highly resistant strains being produced either 

 in one step by selection from among first-step resistant mutants or in several 

 steps by repeated selection of strains having higher and higher degrees of 

 resistance. 



What mechanism is responsible for the stepwise build-up of resistance, and for 

 the difference between penicillin resistance and streptomycin resistance in this 

 regard? Evidence accumulated by several investigators in recent genetical re- 

 search with bacteria makes it appear reasonably certain that mutations in bac- 

 teria are caused by changes in genes. Granting this assumption, the complexity 

 of behavior observed in studies of resistance to antibiotics indicates that several 

 genes must be involved. Such an inference is not new. Several years ago 

 Demerec and Fano (1945) suggested that the complex situation observed in their 

 study of resistance of E. coli to 7 phages pointed to the presence of about 20 

 distinct mutant types in their material. Since many more than 7 phages affect 

 the strain of coli investigated, and since it is reasonable to assume that extension 

 of the study to these phages would reveal additional mutants, it is evident that 

 the genetic background of resistance to phages is very complex indeed, and that 

 it involves a considerable number of genes. 



If a like situation exists in respect to resistance to antibiotics, then it can be 

 assumed that many genes are instrumental in determining resistance to the two 

 antibiotics used in these experiments, and that the genes affecting resistance to 

 penicillin are different from those affecting resistance to streptomycin. If any 

 one of these genes should mutate, the bacterium in which such a mutation oc- 

 curred and the strain developed from that bacterium would be more resistant to 

 the respective antibiotic than was the original parent strain. Such a strain 

 would be what we have called a "first-step resistant strain." 



The fact that first-step penicillin-resistant strains are fairly uniform in degree 

 of resistance (Demerec, 19456, figure 2) is consistent with the assumption that all 

 genes affecting resistance to penicillin have a similar potency, so that the effect 

 of mutation is the same regardless of which of the genes happens to mutate. 

 According to this hypothesis, there is still present in a first-step resistant strain a 

 number of unmutated genes that affect resistance. Mutation of any of these 

 produces a second-step resistant strain, which possesses a higher degree of resist- 

 ance than the first-step strain. Similarly, by mutation of another gene in a 

 second-step resistant strain, a still higher degree of resistance is attained, charac- 

 teristic of the third-step resistant strain. From this, by further mutations, 

 highly resistant fourth- and fifth-step strains may be obtained. The curves in 

 figure 3 indicate that the increase in degree of resistance with each step is ex- 



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