A new, two-tier system for biotyping Salmonella typhimurium gives a fined and more reliable differentiation of strains than the Kristensen scheme and is. capable of future extension by the addition of new types and new tests. Strains are allocated to a primary type (1-32) by their reactions in five primary tests with Bitter's xylose medium, meso-inositol, L-rhamnose, d-tartrate and mtartrate. Subtypes are distinguished within the primary types by reactions in ten secondary tests, which include observations for flagella and type-1 (haemagglutinating) fimbriae. Full biotypes are designated by letters indicating the subtype reactions appended to the primary-type numbers.

A series of 2030 strains of S. typhimurium collected from many different sources and countries during 53 years was classified into 19 of the 32 potential primary biotypes and into 144 full biotypes. Of the series, 14% (275) were non-fimbriate inositol-nonfermenting rhamnose-nonfermenting (FIRN) strain in primary biotypes 29-32. Most other strains were fimbriate and rhamnose fermenting.

Observations on several series of cultures isolated from different human or animal sources in the same epidemic showed that the biotype characters of a strain were generally stable during its growth in the natural environment and in the unselective media used for isolation and storage. Most non-fermenting strains gave rise to fermenting mutants on prolonged incubation in the substratecontaining- and therefore selective-test medium, and false-positive results from this cause were avoided by making the definitive readings of tests after a short, carefully chosen period of incubation.

A genealogical tree has been drawn to show how eighteen observed primary biotypes may have evolved from a presumed archetypal ancestor of biotype 1.


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