Indirect Selection and Drug Resistance 39 



Table IV 



Number of Resistants expected from the Multiplication of a Single 



Cell 

 /^ = Growth rate of resistant relative to growtli rate of sensitive 



MuUiplication factor 

 k 40 X 80 X 160 X 



with the drug concentrations necessary to ehminate all or 

 most of the sensitives. There may also be interactions 

 between sensitives and resistants in mixed populations, 

 increasing (protection: Cavalli-Sforza and Lederberg, 1956) 

 or decreasing (co-killing or suppression: Eagle, 1955) the 

 count of the resistant type. Such situations can usually be 

 revealed, and their consequences evaluated, by appropriate 

 reconstruction experiments. 



(3) Cells do not always divide regularly. For example, 

 chains may be formed with the strain used here if static 

 unaerated, but not aerated, cultures are employed. Where 

 static unaerated cultures are used, situations of the type 

 shown in Table V may be encountered. The irregularity 



Table V 



An Unaerated Culture containing 102 Resistants per ml. (Strepto- 

 mycin Agar Assay) tested by Indirect Selection 



Sample • 25 ml. ■ 125 ml. • 0625 ml. 



MuUiplication factor 20 x 40 x 80 X 



