Pulsed-field Gel Electrophoresis Subtyping for Salmonella Serotype Discrimination

Introduction: Traditional serotyping has been widely used for Salmonella identification, indeed more than 2,500 Salmonella serovars have been identified through this method. Nevertheless, its limited discriminatory ability has led to the development of different genotypic-based techniques. Pulsed-field gel electrophoresis (PFGE) is considered the “gold standard” Salmonella subtyping method; and due to its great discriminatory ability it plays a key role in the determination of clonal relatedness between isolates and in identifying genetic diversity and distribution of pathogens in different environments.

Purpose: This study aimed to analyze the relationship among Salmonella serotypes obtained by the traditional serotyping method and PFGE patterns from beef carcasses and feedlot fecal isolates, to assess genetic diversity and determine the spread of different clones in and between carcasses in an attempt to improve interventions at different points in the food chain.

Methods: A total of 95 Salmonella isolates, 81 from beef carcasses (hide, pre-evisceration, pre-cooler and cooler) and 14 from feedlot were used in this study. PFGE was performed according to the "One-Day Standardized Protocol for Molecular Subtyping of Salmonella serotypes" of PulseNet. Cluster analysis was performed by using Bionumerics software v. 6.6. The isolates that were found clustered with a different serotype than the one obtained in the first traditional serotyping were sent for a second serotyping to the National Veterinary Services Laboratories in Ames, Iowa.

Results: The set of 95 Salmonella isolates were typable by PFGE; 26 unique PFGE patterns were identified with 42.3% (n = 11) of them corresponding to a single serovar. High concordance (81.8%) was found between the PFGE predicted serotype and the serotype based on the Kauffman-White serotyping scheme.

Significance: The findings of this study suggest PFGE is a suitable method for serotyping discrimination and reflect the need of improving interventions at different points in the food chain to avoid further Salmonella spread among carcasses.