Standardisation of multilocus variable-number tandem-repeat analysis (MLVA) for subtyping of Salmonella enterica serovar Enteritidis.

Salmonella enterica serovar (S.) Enteritidis is an important cause of food-borne infection in Europe and the United States. Further subtyping of isolates is necessary to support epidemiological data for the detection of outbreaks and identification of the vehicle of infection. Multilocus variable-number tandem-repeat analysis (MLVA) is reportedly more discriminatory and produces data that are easier to share via databases than other molecular subtyping methods. However, lack of standardisation of the methodology and interpretive criteria for data analysis has meant that comparison of data between laboratories can be problematic. On the basis of MLVA profiles of 298 S. Enteritidis isolates received at the Health Protection Agency’s Salmonella Reference Unit and sequence analysis of selected isolates, we propose a MLVA scheme for S. Enteritidis based on five loci (SENTR4, SENTR5, SENTR6, SENTR7 and SE-3) that have been selected from previously published S. Enteritidis MLVA schemes. A panel of reference strains has been developed that can be used by laboratories to normalise their raw fragment data to actual fragment sizes. We also provide recommendations for analysing and interpreting MLVA data. We urge laboratories to consider implementing these guidelines, thereby allowing direct comparison of data between laboratories irrespective of the platform used for fragment analysis, to facilitate international surveillance and investigation of international outbreaks.