Report of the ad hoc committee for the re-evaluation of the species definition in bacteriology

An international ad hoc committee convened in Belgium in February 2002 to re-evaluate the definition of bacterial species in light of advances in molecular methodologies. Since the last major recommendations in 1987, new techniques have emerged including improved 16S rDNA sequencing, DNA typing methods, multilocus sequence typing (MLST) of housekeeping genes, whole genome sequencing, and physical characterization methods. The committee concluded that the current pragmatic species definition—based on DNA-DNA similarity and thermal stability (ΔTm)—remains sound and universally applicable. However, they recommend integrating new techniques while maintaining compatibility with existing classification. Key recommendations include encouraging researchers to validate alternative genomic methods against DNA-DNA reassociation data, sequencing at least five conserved housekeeping genes for phylogenetic analysis, using quantitative DNA profiling methods, and including nearly complete 16S rDNA sequences in all species descriptions. The committee emphasizes that phenotype, genotype, and ecological role should collectively inform species delineation, and recommends standardized reporting formats and methods to facilitate comparisons across laboratories and taxa.

Key findings

• The committee affirmed that DNA-DNA similarity and thermal stability remain the gold standard for bacterial species delineation despite methodological limitations.
• Multiple promising techniques were endorsed for supplementing species definition, including MLST of housekeeping genes (minimum five genes) and DNA profiling methods such as AFLP and ribotyping.
• All new species descriptions should include nearly complete 16S rDNA sequences (≥1300 nucleotides) and be validated against reference strains of closely related species.
• Phenotype combined with genotype should guide the determination of genomic data cut-off points for species delineation, and species should be identifiable by readily available methods.
• The committee recommended developing standardized electronic formats and curated databases for taxonomic information exchange to support bacterial systematics.