Abstract
Four serologically distinct types of Vi-phages (Type I, II, III, and IV) specific for V-form strains of B. typhosus have been described. 1 2 Type II Vi-phage was found to exhibit a highly selective lytic activity for the strain of B. typhosus from which it was propagated. Utilizing this selective behavior of the Type II Vi-phage a method has been evolved whereby V-form strains of B. typhosus could be divided into the typical V-forms and the Imperfect V-forms. The typical V-forms were further subdivided into several distinct types (A, B1, B2, C, D1, D2, E, F1, F2, G, H, J). 2 Studies on 706 strains of B. typhosus isolated in Canada, England, Norway, Sweden and Denmark have demonstrated the validity of such a scheme of typing. 3 In order to test further the applicability of this scheme of bacteriophage typing and to determine the incidence of different types of B. typhosus occurring in this locality, a study was made on 79 strains isolated in Peiping. The results together with an account of isolation of 2 new type strains are given below.
Seventy-nine strains of B. typhosus isolated from active cases and carriers during 1937-1939 were inoculated in semisolid agar media and kept in the refrigerator (0°-6°C). Subculturing was carried out every 3 months. In the present study these stock cultures were plated out on nutrient agar plates (1.5% agar, pH 7.6) and single colonies were picked and employed for typing. These cultures thus derived were then stocked in the nutrient agar stubs, instead of semisolid agar. The original method 2 of typing with Type II Vi-phage preparations∗ was strictly followed.
Out of a total of 79 strains studied 74 strains of V forms were isolated.
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