Genetic relationships of phage types and single nucleotide polymorphism typing of Salmonella enterica Serovar Typhimurium.

Salmonella enterica serovar Typhimurium is one of the leading causes of gastroenteritis in humans. Phage typing has been used for the epidemiological surveillance of S. Typhimurium for over 4 decades. However, knowledge of the evolutionary relationships between phage types is very limited. In this study, we used single nucleotide polymorphisms (SNPs) as molecular markers to determine the relationships between common S. Typhimurium phage types. Forty-four SNPs, including 24 identified in a previous study and 20 from 6 available whole-genome sequences, were used to analyze 215 S. Typhimurium isolates belonging to 45 phage types. Altogether, 215 isolates and 6 genome strains were differentiated into 33 SNP profiles and four distinctive phylogenetic clusters. Fourteen phage types, including DT9, one of the most common phage types in Australia, were differentiated into multiple SNP profiles. These SNP profiles were distributed into different phylogenetic clusters, indicating that they have arisen independently multiple times. This finding suggests that phage typing may not be useful for long-term epidemiological studies over long periods (years) and diverse localities (different countries or continents). SNP typing provided a discriminative power similar to that of phage typing. However, 12 SNP profiles contained more than one phage type, and more SNPs would be needed for further differentiation. SNP typing should be considered as a replacement for phage typing for the identification of S. Typhimurium strains.