This 1972 study assessed DNA relatedness among Klebsiella species and Enterobacter aerogenes strains using DNA-DNA reassociation techniques. Researchers used radioactively labeled DNA from reference strains of K. pneumoniae type 2 and E. aerogenes 1627-66, incubated them with unlabeled DNA from various strains, and measured reassociation on hydroxyapatite. Results showed that Klebsiella species form a highly cohesive genetic group with 80-91% DNA relatedness to K. pneumoniae type 2, with minimal sequence divergence. E. aerogenes strains also formed a tight group with 83-100% relatedness to the reference E. aerogenes strain, regardless of motility differences. However, only 56% relatedness existed between Klebsiella and E. aerogenes groups, with substantial sequence divergence (9% unpaired bases). E. cloacae showed even lower relatedness (40%) to both groups. The thermal stability analysis revealed that approximately one-third to one-half of the DNA between these groups had diverged significantly. The authors conclude that while current biochemical tests yield few phenotypic differences between K. pneumoniae and E. aerogenes, they represent significantly diverged genetic groups that warrant continued differentiation in clinical and taxonomic contexts.

Key findings

• Klebsiella species form a highly related genetic group with 80-91% DNA relatedness and minimal sequence divergence (0.4-1.2% unpaired bases)
• E. aerogenes strains show 83-100% DNA relatedness to each other, with motility status not affecting genetic relatedness
• Klebsiella and E. aerogenes are significantly diverged groups with only 56% DNA relatedness and substantial sequence divergence (9% unpaired bases), supporting their continued distinction despite phenotypic similarity
• E. cloacae shows lower relatedness (40%) to both Klebsiella and E. aerogenes, indicating it represents a more distantly related genus
• DNA-DNA reassociation provides a reliable molecular method to differentiate even atypical strains of Klebsiella and E. aerogenes that may be indistinguishable by biochemical tests