268 H. H. PLOUGH 



monas (1945). He and Zelle had indicated the genetic basis of virulence in 

 Salmonella (Zelle, 1942). 



ADVANTAGES OF SALMONELLA FOR GENETIC STUDIES 



I became acquainted with the Enterobacteriaceae and particularly with 

 the pathogenic forms in Zinsser's laboratory at the Columbia Medical School. 

 My own realization that Salmonella offered excellent material for studies in 

 microbial genetics was heightened when, as an Army bacteriologist in the 

 Philippines, I had to diagnose enteric infections. I found most of the Salmo- 

 nellas which Flexner first described from Manila still present in the islands. 

 More than 140 strains or species of Salmonella are recognized which are dis- 

 tinguishable by a common pattern of fermentation reactions (dextrose and 

 maltose- AG, lactose and sucrose-negative, citrate and H2S positive). Each 

 one has been shown by the serological studies of White (1929), Kaufmann 

 (1944), or Edwards and Bruner (1942) to have a very precise and readily 

 separable antigenic constitution. 



The antigens are determined by agglutination studies using serums from 

 different rabbits immunized to one or another of the major strains. They 

 fall into two distinct groups: the somatic (0) antigens associated with the 

 surface protein layers, and the flagellar (H) antigens determined by proteins 

 of the flagella. Each of these groups is known to be compound, with 

 some twenty separate O antigens — each strain may carry three or four 

 (O) antigens — and eight or ten different specific (H) antigens as well as cer- 

 tain alternative and non-specific phases of the latter. Thus each strain can 

 be shown to have a distinctive and readily determinable antigenic constitu- 

 tion (S. typhimurium is I, IV, V, XII — i, 1, 2, 3). The whole group naturally 

 falls into a tree-like pattern very like the evolutionary trees made for fami- 

 lies of animals or plants on the basis of structure. 



Tatum's (1946) discovery that mutagenic agents (including radiation and 

 nitrogen mustards) could induce mutants of colon bacteria having constant 

 growth factor requirements more limited than the parental organism, just 

 as with Neurospora, has re-emphasized the one gene-one enzyme hypothesis. 

 It has strengthened the idea of bacterial evolution developed by Lwoff (1943) 

 that the parasitic forms have been derived from the less exacting hetero- 

 trophic organisms by successive losses of synthetic abilities. Thus it gives 

 added meaning to the tree-like interrelationships suggested by the antigenic 

 analyses. 



Soon after the war our Amherst group entered on an intensive study of 

 induced biochemical and antigenic mutations in the food poisoning organ- 

 ism, Salmonella typhimurium. It was our hope that this organism would 

 prove more favorable for genetic studies than E. coli, not only for the analysis 

 of the mode of action of genes, but for evidence on the genetic nature of type 

 specificity, virulence, and their bearing on evolutionary relationships. 



