Molecular typing of Salmonella serotypes prevalent in animals in England: Assessment of methodology

Salmonella enterica serotypes Derby, Mbandaka, Montevideo, Livingstone, and Senftenberg were among the 10 most prevalent serotypes isolated from farm animals in England and Wales in 1999. These serotypes are of potential zoon otic relevance; however, there is currently no "gold standard" fingerprinti ng method for them. A collection of isolates representing the former seroty pes and serotype Gold Coast were analyzed using plasmid profiling, pulsed-f ield gel electrophoresis (PFGE), and ribotyping. The success of the molecul ar methods in identifying DNA polymorphisms was different for each serotype . Plasmid profiling was particularly useful for serotype Derby isolates, an d it also provided a good level of discrimination for serotype Senftenberg. For most serotypes, we observed a number of nontypeable plasmid-free strai ns, which represents a limitation of this technique. Fingerprinting of geno mic DNA by ribotyping and PFGE produced a significant variation in results, depending on the serotype of the strain. Both PstI/SphI ribotyping and Xba I-PFGE provided a similar degree of strain differentiation for serotype Der by and serotype Senftenberg, only marginally lower than that achieved by pl asmid profiling. Ribotyping was less sensitive than PFGE when applied to se rotype Mbandaka or serotype Montevideo. Serotype Gold Coast isolates were f ound to be nontypeable by XbaI-PFGE, and a significant proportion of them w ere found to be plasmid free. A similar situation applies to a number of se rotype Livingstone isolates which were nontypeable by plasmid profiling and /or PFGE. In summary, the serotype of the isolates has a considerable influ ence in deciding the best typing strategy; a single method cannot be relied upon for discriminating between strains, and a combination of typing metho ds allows further discrimination.