This study examined the genetic relationships among Carnobacterium species and related lactic acid bacteria using DNA-DNA hybridization and ribosomal RNA homology analysis. Researchers tested four Carnobacterium species (C. piscicola, C. divergens, C. mobile, and C. gallinarum) originally isolated from meat, fish, and poultry products, along with various Lactobacillus species and other lactic acid bacteria. DNA-DNA homology values between Carnobacterium species and other tested bacteria were low (less than 22%), suggesting limited genetic relatedness at the DNA level. However, ribosomal RNA analysis revealed that all four Carnobacterium species clustered together in the same RNA homology group, with temperature melting differences (ΔTm) of 1-3°C among them. This RNA-based clustering was distinct from other bacteria tested. The findings support Collins et al.'s proposal for recognizing Carnobacterium as a separate genus, distinct from Lactobacillus despite some shared characteristics like meso-diaminopimelic acid (mDAP) in their cell walls. The Carnobacterium species share a coherent phylogenetic group characterized by low G+C content (35-37%) and specific metabolic traits.

Key findings

• Carnobacterium species showed low DNA-DNA homology (less than 15%) with each other and other lactic acid bacteria, suggesting genetic distance at the genomic level.
• All four Carnobacterium species formed a single ribosomal RNA homology cluster with ΔTm values of 1-3°C, indicating phylogenetic coherence despite low overall genomic similarity.
• The genetic findings support recognition of Carnobacterium as a distinct genus separate from Lactobacillus, characterized by mDAP cell wall type, low G+C content, and specific metabolic features.
• Carnobacterium species were distantly related to other mDAP-containing bacteria, with only Lactobacillus yamanashiensis showing closer RNA similarity (ΔTm 7.5°C).