386 BACTERIOPHAGES 



colicins V and E and only to them. This indicates that sensi- 

 tive cells possess different sites of action for the various types of 

 colicin. A classification of colicins by Fredericq (1948) is based 

 on such resistance patterns. The factors for resistance to the 

 different types of colicin segregate independently in bacterial 

 crosses (Fredericq and Betz-Bareau, 1952). 



Most remarkable is the observation of Fredericq (1946) and 

 Bordet (1947) that resistance to certain phages and colicins is cor- 

 related. For example, Sh. sonnei E90 is sensitive to various coli- 

 cins (A, B, C, etc.) and to various phages (called I, II, III, etc.). 

 Clones isolated for their resistance to colicin E are also resistant 

 to phage II, but remain sensitive to the other phages and colicins. 

 If mutants are isolated for their resistance against phage II, they 

 are found to be resistant also against colicin E. In the same way, 

 mutants resistant to colicin K are resistant to phage III and 

 vice versa (Fredericq, 1953). Moreover, in bacterial crosses, re- 

 sistance to colicin E and phage II behaves as a single character 

 (Fredericq and Betz-Bareau, 1 952) . It is clear therefore that the 

 action of colicins and phages on sensitive cells is dependent upon 

 specific receptors which may be common to both agents. 



b. Mode of Action 



Exposure of susceptible bacteria to colicin results in the death 

 of cells which, however, are not lysed. The action of colicin is 

 thus bactericidal but not bacteriolytic. The kinetics of coli- 

 cin action seems to indicate that the fixation of a small number 

 of molecules is sufficient to bring about the death of a bacterium. 



When sensitive bacteria are mixed with a suitable excess of 

 colicin ML, and samples of the mixture are titrated at intervals 

 by dilution and plating to determine the number of colony-form- 

 ing cells, the number surviving decreases exponentially with time 

 of sampling. The rate of killing is proportional to the initial 

 colicin concentration. These facts suggest that a single particle 

 of colicin is able to kill a bacterium. Moreover, if the colicin is 

 not in excess, the survival curve reaches a plateau that measures 



