ZINDER AND LEDERBERG 



other. If most or the bacteria are com- 

 petent to be transduced, the frequency 

 of a particular transduction will be the 

 probability that any of the particles 

 adsorbed will have a particular effect. 

 Double transductions will occur in the 

 same ratio to single exchanges as the 

 absolute frequency of the latter, and 

 this is too low (ca lO^^^) for double 

 exchanges to be detected in our ex- 

 periments. However, if transduction is 

 limited to a small proportion of com- 

 petent cells, dual transductions would 

 not have independent probabilities, 

 and further assumptions such as mutual 

 exclusion w^ould be required to ac- 

 count for the low frequency of ob- 

 served dual events. 



The following picture appears to be 

 most consistent with the observations 

 to date. An active filtrate contains a 

 population of numerous species of 

 granules, each corresponding to a 

 genetic effect although some may be 

 intrinsically inert. Each bacterium may 

 absorb a limited number of particles, 

 in the possible range from one to per- 

 haps one hundred. Each adsorbed 

 particle has a fixed, independent prob- 

 ability of exerting its particular trans- 

 ductive effect. The low frequency of 

 single, and particularly of double 

 transductions, is limited by the total 

 number of particles that may be ad- 

 sorbed as well, perhaps, as by the low 

 probability that an adsorbed particle 

 will complete its effect. 



Serial transduction. Dual transduc- 

 tion has never been observed in a sin- 

 gle experiment. That this is due to the 

 considerations described previously 

 rather than some intrinsic limitation is 

 shown by serial transfers. Once a cell 

 has been transduced it can be grown 

 out, reexposed to FA, and selected for 

 other changes. SW-351 (Aux, Gal—, 

 Xyl— ) has been serially transduced 

 from auxotrophy to prototrophy, from 

 galactose negative to positive, and 



233 



from xylose negative to positive. The 

 order in which these transfers were ac- 

 complished made no difference. There 

 was no loss of efficiency with the ite- 

 rated transductions as compared to the 

 single transduction of SW-351 for 

 any of the characters. 



Specificity of adsorption of FA. The 

 adsorption experiments had indicated 

 a correlation of adsorptive ability and 

 immunological specificity. Preliminary 

 experiments with some dozen Salmo- 

 nella serotypes confirmed and nar- 

 rowed this correlation to the presence 

 of somatic antigen XII. Broth cultures 

 of the serotypes to be tested were 

 sedimented and one ml of FA was 

 added. Adsorption proceeded for fif- 

 teen minutes, and then the reaction 

 tubes were heat shocked at 56 C for 

 one hour to sterilize the cells. Pre- 

 liminary experiments with known ad- 

 sorbing cells had shown that FA once 

 adsorbed was not eluted by this pro- 

 cedure. The mixtures were assayed on 

 LA-22 for free FA. Some fifty' differ- 

 ent serotypes have been tested in this 

 manner. Although some types with 

 XII are inert, none of the tv^pes with- 

 out XII adsorbed. This correlation is 

 maintained with the ''Sahtwnella coli" 

 types. The XII carrying strains that 

 adsorbed were: S. paratyphi B, S. 

 typhimiiriiim (25 strains), 5. Stanley, 

 S. heidelberg, S. Chester, S. san-diego, 

 S. aborti/s-ovis, S. typhi W, S. typhi V, 

 5. enteritidis, S. moscozv, S. blegdam, 

 S. eastboiirjie, S. sejidai, S. abony, E. 

 coli 3, E. coli 4, 5. kaapstad, S. salinatis, 

 S. pullonim, and S. gallinanim. The 

 following XII types did not adsorb: 

 S. paratyphi A and S. abortns-bovis, 

 presumably owing to the absence of 

 the XII2 component. The nonadsorb- 

 ing, non-XII types tested were: 5. 

 typhiimiriimi (rough variant), 5. 

 cholerae-siiis, S. Jiewport, S. london, 

 S. se7ifte?jberg, S. aberdeen, S. poona, 

 S. worthington, S. hvittingfoss, S. ken- 



