Numerical Taxonomy and Deoxyribonucleic Acid Reassociation in the Taxonomy of Some Gram-Negative Fermentative Bacteria

Abstract

One hundred and five strains of gram-negative, fermentative bacteria, representing members of the Enterobacteriaceae together with strains of Vibrio and Aeromonas, were subjected to numerical taxonomy and deoxyribonucleic acid (DNA)/DNA reassociation studies. The numerical taxonomy sorted the strains into major clusters corresponding to Edwardsiella tarda, Escherichia coli, Citrobacter freundii, the tribe Klebsielleae, and Vibrio alginolyticus. Smaller clusters could be identified as Serratia marcescens, Hafnia alvei, and Shigella species. Seven reference strains were used in the DNA/DNA reassociation studies.Klebsiella pneumoniae at 60 C shared reassociation values of 80 to 100% with other strains of Klebsiella, 37 to 64% with Enterobacter spp., and lower values with Hafnia, Citrobacter, Serratia, and Escherichia coli. Phenetically, K. pneumoniae and Enterobacter aerogenes shared high relatedness, although they are only 41 to 64% related in the reassociation studies.Hafnia, Salmonella typhimurium, and Edwardsiella tarda showed little relatedness to any of the other strains included in this study. Both reference strains of Escherichia coli shared genetic resemblances greater than 65% at 60 C with Shigella and at least 70% with other strains of Escherichia coli. Overall, the agreement between numerical taxonomy and the DNA/DNA reassociation studies was satisfactory, with correlation coefficients ranging from 0.5 to 0.9. The choice of reference strains may well affect the correlation between the two techniques, and it is suggested that hypothetical median organisms be determined from the phenetic groupings and that actual strains that approximate to the hypothetical median organisms be used as reference strains for DNA reassociation.