16 SEX IN MICROORGANISMS 



First, cell-free filtrates gave no prototrophs and direct contact of the 

 cells themselves seemed essential, as shown by growing the two types 

 separated by an ultra-fine sintered glass bacterial filter (Davis, 1950). 

 Second, the successful recovery of most of the possible recombination 

 types in later crosses involving a much greater variety of characters, 

 including resistance to antibiotics and viruses and sugar fermentation 

 characters, would necessitate simultaneous transformation in different 

 directions for different characters, and in both directions in different 

 cells. 



Thus the results of experiments of the type described above are 

 satisfactorily explained only as resulting from a sexual mating process, 

 followed by reassortment or segregation of genetic material. Repeti- 

 tion and extension of these experiments in a considerable number of 

 laboratories during the past five years have amply confirmed the 

 reality of the essential phenomenon and the validity of this conclu- 

 sion. 



These experiments have added considerable information about 

 environmental factors affecting the recombination process, and sup- 

 port the concept that direct cell contact is necessary for the sexual 

 process. Some of the strongest support comes from the demonstration 

 by Nelson (1951) that recombination behaves as a bimolecular 

 reaction, as if factors such as relative and absolute concentrations of 

 the two types of cells, which would affect the frequency of contact 

 of appropriate cells, similarly affect frequency of recombination. The 

 experiments of Davis (1950), showing the need for cell contact, and 

 in a more positive sense the production of a genetically diploid cell 

 (Lederberg, 1949) likewise support the postulated occurrence of a 

 cell to cell sexual process in E. coU K-12. 



The intimate details of mating are still obscure. Owing to its 

 infrequency we have been discouraged (until very recently) from 

 any serious attempts to detect its morphological basis, and were 

 obliged to be content with genetic inferences. E. Klieneberger-Nobel, 

 of the Lister Institute, London, England, has made a most painstaking 

 study of mating cultures of E. coli K-12 (unpublished work, quoted 

 by her kind permission). Although she occasionally observed what 

 appeared to be stages in the abortive development of L-forms 

 (Klieneberger-Nobel, 1951), she was unable to correlate them in any 

 way with recombination. The only conclusion that is warranted is 

 that recombination in E. coli does not involve spectacular formations. 



