Assignment of Listeria grayi and Listeria murrayi to a Single Species, Listeria grayi, with a Revised Description of Listeria grayi

This study reassesses the taxonomic status of two rare, nonpathogenic Listeria species—L. grayi and L. murrayi—using three molecular methods: DNA-DNA hybridization, multilocus enzyme electrophoresis, and rRNA gene restriction fragment analysis. The researchers examined five strains of each species and found high genomic relatedness between them. DNA-DNA hybridization showed 83–94% reassociation values between L. grayi and L. murrayi strains, with small thermal differences (ΔTm ≤ 0.9°C). Multilocus enzyme electrophoresis of 18 enzymes revealed that all ten strains formed a single major cluster, distinct from other Listeria species, with genetic distances well below the threshold for separate species designation. rRNA gene restriction patterns also demonstrated substantial homology between the two species. Based on these molecular findings and supporting earlier biochemical and phenotypic data, the authors conclude that L. grayi and L. murrayi represent a single homogeneous species and should be consolidated. Following nomenclatural priority rules, the unified species is designated L. grayi, with an emended description provided.

Key findings

• DNA-DNA hybridization between L. grayi and L. murrayi strains yielded 83–94% relatedness with ΔTm values of 0–0.9°C, exceeding the 70% threshold for species definition and confirming previous 1973 observations.
• Multilocus enzyme electrophoresis of 18 enzymes grouped all L. grayi and L. murrayi strains in a single distinct cluster with maximum genetic distance of 0.34, far below the 0.60–0.70 threshold for separate species.
• rRNA gene restriction patterns revealed four EcoRI groups and three HindIII groups, with both species' type strains sharing identical HindIII profile HG1, demonstrating substantial genomic homology.
• L. grayi and L. murrayi should be recognized as a single species named L. grayi, consolidating two decades of molecular evidence suggesting their genomic and phenotypic similarity.