現代の分子系統分類学

Modern molecular taxonomy uses a polyphasic approach that combines multiple methods to identify and classify bacteria. This integrated strategy employs conserved gene sequence analysis, multilocus sequence typing, genome fingerprinting, and ribotyping alongside recent DNA sequencing advances. By using several complementary techniques, researchers achieve more accurate and reliable bacterial identification than any single method alone.

Multilocus sequence typing (MLST) sequences approximately 450 base-pair fragments from several housekeeping genes across related species or strains. Each gene variant, or allele, receives a unique number, and a strain's allelic profile represents its combination of allele numbers. MLST is highly efficient, detecting even single-nucleotide differences, making it invaluable for distinguishing pathogenic strains like E. coli K-12 and O157:H7.

Ribotyping is a genome fingerprinting method that analyzes ribosomal RNA gene patterns to enable rapid species and strain identification. Genomic DNA is digested by restriction enzymes, separated via gel electrophoresis, and labeled with SSU rRNA probes, producing unique ribotypes for each strain. This technique is widely applied in diagnostics and analyses of food, water, and beverages.

Conserved gene sequence analysis uses highly conserved genes, such as recA and gyrB, to distinguish closely related bacterial species. These genes are essential for cellular functions and change slowly over evolutionary time, making them ideal markers for species-level differentiation. This method provides reliable taxonomic resolution for organisms with minimal genetic variation.

Whole-genome analysis provides unparalleled insights into microbial physiology and evolution by examining gene content, synteny, and GC content. Comparative analyses reveal relationships between strains, while phylogenetic analysis of shared orthologs determines average nucleotide identity, with species typically sharing less than 95%. Whole genomes also enable metabolic reconstruction and illuminate horizontal gene transfer's role in microbial genome dynamics.

Genome fingerprinting evaluates polymorphisms between strains using DNA fragments from genes or whole genomes. Key methods include ribotyping, which analyzes ribosomal RNA gene patterns; rep-PCR, which amplifies DNA fragments between repetitive elements; and AFLP, which digests DNA with restriction enzymes and selectively amplifies fragments. Each method generates strain-specific banding patterns for discrimination.

Recent advances in DNA sequencing technology enable the use of multiple genes and entire genomes for accurate bacterial identification and taxonomy. As sequencing costs decline and technologies improve, these approaches are increasingly used in clinical, environmental, and evolutionary studies. This shift from single-gene to multi-gene and whole-genome analysis provides unprecedented precision in microbial characterization.