412 BACTERIOPHAGES 



separately or in combination, can occur. It has already been 

 pointed out that 58 distinct Vi-types of the typhoid bacillus 

 can now be defined with this phage, and that many more types 

 may exist. What is perhaps still more remarkable is that this 

 phenomenon of wide host-range plasticity seems to be relatively 

 common in Vi phages. The writer (unpublished) has iso- 

 lated a Vi phage, serologically distinct from Vi-phage II, yet 

 showing a spectrum of host-range modification virtually in- 

 distinguishable from that of Vi-phage II. Moreover, the 

 phenotypic and mutational changes exhibit an exactly similar 

 pattern in the two Vi phages. No non-Vi phages have been 

 shown to possess a similar degree of host-range adaptability. 

 There are clearly many intriguing problems still to be solved in 

 this field. 



Baron, Formal, and Spilman (1953, 1955) have reported that 

 transduction can be carried out in Salmonella typhi with Vl-phage 

 II. 



5. The Practical Application of Vi-Phage Typing 



This is not the place to embark on a detailed account of the 

 epidemiological value of Vi-phage typing, and only one ex- 

 ample will be given. In June, 1948, three children contracted 

 typhoid fever as the result of drinking water from the Walling- 

 ton river near Winchester in England. The infecting type was 

 El. Investigation resulted in the isolation of the same Vi-type 

 from the places on the river visited by the children. The 

 organism was traced up the river to a tributary stream, thence to 

 a sewage outlet, through the sewage system, and finally to a single 

 house where a chronic typhoid carrier excreting phage type El 

 was found to live. The inquiry ended one year after the children 

 had been infected, and over two miles upstream from the point at 

 which this had occurred. It was controlled throughout by 

 phage typing (see Lendon and MacKenzie, 1951). Typhoid 

 bacilli belonging to various Vi-types are common in river water, 

 and without the precision of phage typing this investigation 

 w(Aild have been difl^cult to conclude satisfactorily. Many 



