Genetic Aspects of Drug Resistance 55 



of the plated bacteria. The colonies on the streptomycin 

 plates, however, were smaller than those on the plates lacking 

 streptomycin. After an additional 24 hours of incubation 

 their size increased, and their shape became irregular because 

 of the growth of secondary colonies. Microscopic examination 

 revealed that each was composed of a thin base of background 

 growth with several fast-growing secondary colonies on top 

 of it (Fig. 1). Bacteria taken from the secondary colonies 

 exhibited higher resistance to streptomycin than bacteria 

 taken from the light background region. Thus it is evident 

 that, even at low concentrations of the drug, resistance does 

 not develop adaptively but originates through an abrupt and 

 discrete change, and that the drug acts as a selective agent by 

 favouring the growth of resistant variants. 



As a rule, a bacterium that becomes resistant to one drug 

 still remains sensitive to others, which is consistent with the 

 assumption that mutations in different genes are responsible 

 for resistance to different drugs, and with the prevailing view 

 of geneticists that a gene mutation affects primarily one bio- 

 chemical reaction. One would expect the spontaneous 

 occurrence of multiply resistant mutants to be very rare, 

 since in general the probability of double mutation is equal 

 to the product of the individual mutation rates of the two 

 genes involved. Experimental tests of that expectation were 

 made by Szybalski and Bryson (1953), working with resist- 

 ance to isoniazid and sodium ^-aminosalicylate (PAS) in 

 Bacillus megaterium. The high mutation rates in this organism 

 and the single-step development of resistance to those drugs, 

 without the production of intermediately resistant variants, 

 made the experimental determination of rate of mutation to 

 double resistance technically feasible. The rate of mutation 

 to isoniazid resistance was found to be Q{±1 -5) X 10-^, that to 

 PAS resistance 1( ±0-3) X 10-6, ^nd that to double resistance 

 8(dzl-8)xlO-io. Thus the observed rate of mutation to 

 double resistance, although statistically higher than predicted, 

 falls well within one order of magnitude of expectation. It is 

 very likely that a better agreement between the observed and 



