280 H. H. PLOUGH 



Preliminary tests showed that these strains were antigenically similar and 

 gave the same agglutination titer with homologous serum as the parental 

 wild types — (I) IV, V, XII, for the O antigens. The parentheses indicate that 

 (I) is very weak or absent. The first case is typical. Specific serum from ani- 

 mals immunized by #519 was absorbed with a suspension of organisms of 

 #527, an unnamed strain known to have antigens IV, V only. After it was 

 passed through semisolid agar containing the absorbed serum now carrying 

 XII antibodies only, 519-PlO was retested and shown now to give agglutina- 

 tion at a very low titer (1/320 instead of 1/10,000) compared with the origi- 

 nal. Further testing has demonstrated that this strain retains the two major 

 antigens (IV and V), but has lost XII. Thus it has been transformed to IV, 

 V like strain #527. Further tests on differential media prove that the strain 

 is unchanged as an auxotrophic mutant, and still cannot grow unless the 

 medium contains histidine (519-10). 



In the other case 519 serum was absorbed by S. schleissheim (V, XII, 

 XXVII). The mutant after growing through the absorbed serum failed to 

 agglutinate in XII serum, and had a higher titer in XXVII than S. schleiss- 

 heim. Thus the changed mutant has lost XII and taken on antigen XXVII. 

 It still retains its histidine requirement. 



Thus we have two independent cases of the alteration of antigenic speci- 

 ficity by the Edwards method of passage through specific serum. Here again 

 the evidence indicated no relation between antigenic configuration and the 

 biochemical requirements. We are now exposing these antigenically altered 

 strains to further radiation with the idea of building up multiple auxotrophic 

 stocks combining the two major systems of mutations. These can then be 

 used for more conclusive tests of possible fusion and recombination. How- 

 ever, this demonstration that antigenic mutants can be induced by specific 

 serum adds to the possibility that mutual interaction of genes or gene 

 products between organisms in mixtures may give a more acceptable 

 explanation of the recorded cases of recombination in bacteria, than does one 

 based on genetic analogies with higher forms. 



SUMMARY 



An account has been given of the results of X-radiation of suspensions of 

 the two strains of Salmonella lyphimurium, and the isolation of strains with 

 specific nutrilite requirements (auxotrophic mutants). These strains are iso- 

 lated by the Davis-Lederberg method of growth for twenty-four hours in 

 enriched broth, followed by twenty-four hours in minimal broth containing 

 100 units per ml. of penicillin. The method screens out the unmutated organ- 

 isms according to a logarithmic survival curve, and preserves the mutant 

 bacteria. 



Successive tests show a relation between X-ray dosage and the percentage 

 of recovered auxotrophic mutants, and also between dosage and the number 

 of different mutants. 



