Multiple-locus variable-number tandem-repeats analysis of Listeria monocytogenes using multicolour capillary electrophoresis and comparison with pulsed-field gel electrophoresis typing.

The multiple-locus variable-number tandem-repeats analysis (MLVA) method for genotyping has proven to be a fast and reliable typing tool in several bacterial species. MLVA is in our laboratory the routine typing method for Salmonella enterica subsp. enterica serovar Typhimurium and Escherichia coli O157. The gram-positive bacteria Listeria monocytogenes, while not isolated as frequent as S. Typhimurium and E. coli, causes severe illness with an overall mortality rate of 30%. Thus, it is important that any outbreak of this pathogen is detected early and a fast trace to the source can be performed. In view of this, we have used the information provided by two fully sequenced L. monocytogenes strains to develop a MLVA assay coupled with high-resolution capillary electrophoresis and compared it to pulsed-field gel electrophoresis (PFGE) in two sets of isolates, one Norwegian (79 isolates) and one Swedish (61 isolates) set. The MLVA assay could resolve all of the L. monocytogenes serotypes tested, and was slightly more discriminatory than PFGE for the Norwegian isolates (28 MLVA profiles and 24 PFGE profiles) and opposite for the Swedish isolates (42 MLVA profiles and 43 PFGE profiles).