108 



NATURE, FORMATION, AND ACTIVITIES 



A growing culture may acquire resistance 

 to an antibiotic by selection of cells initially 

 less sensitive than the average, or l\y a 

 sudden modification of the properties of 

 indi\-idual cells, namely, by mutation. Ac- 

 cording to Demerec (1950), resistance by 

 Staph, aureus to streptomycin originates by 

 gene mutation. Differences in the de\-elop- 

 meiit of resistance to penicillin and to strep- 

 tomycin were observed by plating cultures of 

 various organisms in the presence of increas- 

 ing concentrations of either antibiotic. The 

 penicillin-resistant strains produced in the 

 first step were fairly uniform in their degree 

 of resistance; on the other hand, strepto- 

 mycin-resistant strains showed great varia- 

 bility, some highly resistant forms being 

 obtained in one step. It was suggested that 

 penicillin mutations may be a result of a 

 number of genes, whereas in streptomycin 

 mutations some genes are more effective 

 than others. 



In the development of our knowledge of 



resistance of microorganisms to antibiotics, 

 several phenomena have been recognized: 



1. Microorganisms develop resistance to 

 antibiotics by different mechanisms; such 

 development of resistance takes place in 

 diJertnt ways, as in the case of penicillin 

 versus streptomycin type of resistance. 



2. Some antibiotics show cross-resistance, 

 as in the case of tetracyclines and chlor- 

 amphenicol, and among members of the 

 neomycin group; others do not, as in the 

 case of penicillin and streptomycin, where 

 an organism developing resistance to one 

 antibiotic remains sensitive to another. 



3. After continued use of a certain anti- 

 biotic, there is a gradual development of 

 resistance among the bacteria that formerly 

 were highly sensitive to this antibiotic. The 

 occurrence of staphylococci resistant to 

 penicillin has reached more than (30 per cent 

 in some hospitals. This is also true of other 

 bacteria and other antibiotics, as shown in 

 Table 37. 



Table 37 



Propensity of microorganisms to develop antibiotic-resistant 



strains (Dowling et al., 1955) 



