1948] ORIGIN OF BACTERIAL RESISTANCE TO ANTIBIOTICS 73 



second-step mutants. These are difficult to control. If there is any suspicion 

 that a pathogen may be more resistant to penicillin than the usual strains, it is 

 advisable to determine the degree of resistance before starting treatment, and to 

 adjust the concentration accordingly. If adequate precautions are taken against 

 the development of second-step resistant bacteria, there should be no danger from 

 resistant pathogens in penicillin treatment. It is particularly important to avoid 

 the indiscriminate use of penicillin, however, especially for applications where it 

 can scarcely be of any help (for example, as a mouth wash), because there is 

 positive danger that such use may stimulate the development of resistant strains. 



In the clinical use of streptomycin, the situation can be controlled to a much 

 smaller extent. Since highly resistant bacteria are found among the first-step 

 mutants, treatment with high concentrations is not effective in eliminating the 

 whole population of bacteria present in an infection. What it does accomplish 

 is a reduction of the number of bacteria to a level with which the organism is 

 capable of dealing. If for some reason the organism cannot do this, the chances 

 for the development of a resistant strain are exceedingly good. Therefore, it 

 must be expected that pathogenic strains resistant to streptomycin will frequently 

 develop, in the course of time replacing sensitive strains in communities where 

 streptomycin is used and rendering this antibiotic ineffective. This eventuality 

 can be postponed by restricting the use of streptomycin to serious infections 

 which cannot be controlled in any other way. 



It may be appropriate to mention here the most effective way, theoretically, 

 of preventing the origin of resistant strains of bacteria. This is the use in clinical 

 treatment of a mixture of two antibiotics, when such are available, that affect the 

 same pathogen but are independent in their actions. The evidence of independ- 

 ence is that bacterial strains that have developed resistance to one antibiotic are 

 still sensitive to the other, and vice versa. If such a mixture of two antibiotics 

 is used, then only bacteria that are resistant to both can survive the treatment 

 and form first-step resistant strains. Such bacteria would be exceedingly rare. 

 For example, if first-step resistant bacteria for each of two antibiotics should be 

 found in a large population with a frequency of 1 X 10 -7 , then the expected 

 frequency of bacteria resistant to both these antibiotics would be 1 X 10~ u . 



SUMMARY 



A method is described that has been used to determine whether resistance to 

 streptomycin is induced by interaction of the compound with bacteria or origi- 

 nates by gene mutation. Data are presented indicating that mutations are 

 responsible for the origin of streptomycin resistance in Staphylococcus aureus. 

 These agree with previously published data regarding the origin of penicillin 

 resistance in the same organism. 



The stepwise increase of resistance to penicillin by selection is explained by 

 assuming that mutations in several equally potent genes are effective in inducing 

 resistance, and that the slight degree of resistance characteristic of the first step 

 is due to a mutation in one of these genes, the higher degrees of resistance of 

 subsequent steps to successive mutations in other genes. 



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