Pulsed-field fingerprinting of listeriae: identification of genomic divisions for Listeria monocytogenes and their correlation with serovar.

Clamped homogeneous electric field (CHEF) electrophoresis was optimized for genomic analyses of Listeria monocytogenes. Various human, animal, food, and environmental isolates, as well as strains representing other Listeria species, were separately digested with rarely cutting endonucleases. Of 176 L. monocytogenes strains analyzed, the enzymes AscI and ApaI established 63 and 72 unique restriction endonuclease digestion profiles (REDP), respectively. The 22 non-L. monocytogenes strains exhibited 18 AscI and 19 ApaI unique REDP. Statistical analyses of REDP information using the Dice coincidence index and principal component analysis revealed two distinct genomic divisions of L. monocytogenes that also correlated with the flagellar (H) antigen type: division I contained serovar 1/2a, 1/2c, 3a, and 3c stains and division II contained serovar 1/2b, 3b, 4b, 4d, and 4e strains. Division I isolates digested with ApaI were further grouped into cluster IA (serovar 1/2c and 3c) and cluster IB (serovar 1/2a and 3a) strains. Likewise, division II isolates digested with ApaI were further grouped into cluster IIA (serovar 1/2b and 3b) and cluster IIB (serovar 4b, 4d, and 4e) strains. These data indicate that genotypic data generated by CHEF can be directly related to phenotypic data generated by serotyping for establishing the overall relatedness of isolates. Moreover, these data further substantiate that CHEF analysis is a reproducible and highly discriminating method for characterizing L. monocytogenes strains at the molecular level.