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In this article we will discuss about the bacteriophage typing of various serotypes.
1. Bacteriophage Typing of S. typhi:
This is done by determining the sensitivity of the culture to a series of variants of a single phage, Vi-phage II, which have been adapted to the different types of typhoid bacillus. In 1938, Craigie and Yen developed the first Vi-phage-typing method for Typhi which is still unique in many ways.
With the help of a series of selection and adaptation to spontaneous mutation in Vi-II phages they classified most of the strains into 11 phage types. So far about 106 different phage-types of S. typhi have been distinguished-and are designated by letter or number.
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This method has its own limitations. Some strains are un-typable because they have no Vi- antigen and others are degraded Vi-strains (DVS) which are sensitive to many of the battery of Vi- typing phages. Therefore, they do not conform to a specific typing pattern. Some strains are resistant to all the specific adaptations of Vi-II
The techniques of phage-typing are complicated and it is usual for cultures for typing to be sent to a reference laboratory i.e. Lady Hardinge Medical College (New Delhi), and Central Research Institute, Kasauli (India).
2. Bacteriophage Typing of S. typhimurium:
Felix (1956) and Callow (1959) developed a system for phage typing strains of Typhimurium which has been established internationally for the epidemiological study of isolated serotypes. This system originally distinguished 34 phage types, and now it provides a very fine degree of discrimination for the 232 definitive types with a battery of 36 phages which are currently recognized.
3. Bio-Typing:
Within a particular serotype the bacterial strains may be differentiated into biotypes based on their different fermentation reaction with selected substrates. In many serotypes there are few biochemical tests among which significant numbers of strains behave differently.
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Therefore, the number of identifiable biotypes is small. But it is a useful supplement to phage typing. It can subdivide a large group of un-typable strains or members of common phage types.
In 1975, Dudguid and co-workers developed a scheme for bio-typing to study the epidemiology of infections with Typhimurium based on the use of 15 biochemical characters. Thirty-two potential primary biotypes were defined by the combinations of positive and negative reactions shown in the 5 tests (D-xylose, w-inositol, L-rhamnose, s-tartarate and m-tartarate) most discriminating in this type (5. typhimurium).
These primary biotypes were designated by numbers (1-32) and the full biotypes by appending to these numbered letters which indicated results in 10 secondary tests. By now 24 primary and 184 full biotypes have been identified. Modified versions of the Typhimurium bio-typing scheme have been successfully applied to the epidemiology of other Salmonella serotypes.
