A simple approach to obtain comparable Shigella sonnei MLVA results across laboratories

Multilocus variable-number tandem repeat analysis (MLVA) is a promising subtyping tool to complement pulsed-field gel electrophoresis for discriminating closely related strains of some monomorphic organisms, including Shigella sonnei, which is one of the major foodborne pathogens. However, MLVA results are usually difficult to compare directly between laboratories, impeding the application of MLVA as a subtyping tool for disease surveillance and investigation of common outbreaks across regions or countries. It has long been a big challenge in seeking an approach that can be implemented to obtain comparable MLVA results across laboratories. By implementing a panel of calibration strains in each participating laboratory for data normalization, the MLVA results of 20 test strains were comparable even though some analytical conditions were different among the laboratories. This approach is simple, protocol independent, and easy to implement in every laboratory, and a small calibration set is sufficient to generate mathematical equations for accurate copy number conversion.