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Journal of Bacteriology, November 1999, p. 6747-6755, Vol. 181, No. 21
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Bacterial Phylogenetic Clusters Revealed by Genome Structure

Shu-Lin Liu,1,* Anthony B. Schryvers,1 Kenneth E. Sanderson,2 and Randal N. Johnston3

Department of Microbiology and Infectious Diseases,1 Department of Biological Sciences,2 and Department of Biochemistry and Molecular Biology,3 University of Calgary, Calgary, Alberta T2N 4N1, Canada

Received 13 April 1999/Accepted 19 August 1999

Current bacterial taxonomy is mostly based on phenotypic criteria, which may yield misleading interpretations in classification and identification. As a result, bacteria not closely related may be grouped together as a genus or species. For pathogenic bacteria, incorrect classification or misidentification could be disastrous. There is therefore an urgent need for appropriate methodologies to classify bacteria according to phylogeny and corresponding new approaches that permit their rapid and accurate identification. For this purpose, we have devised a strategy enabling us to resolve phylogenetic clusters of bacteria by comparing their genome structures. These structures were revealed by cleaving genomic DNA with the endonuclease I-CeuI, which cuts within the 23S ribosomal DNA (rDNA) sequences, and by mapping the resulting large DNA fragments with pulsed-field gel electrophoresis. We tested this experimental system on two representative bacterial genera: Salmonella and Pasteurella. Among Salmonella spp., I-CeuI mapping revealed virtually indistinguishable genome structures, demonstrating a high degree of structural conservation. Consistent with this, 16S rDNA sequences are also highly conserved among the Salmonella spp. In marked contrast, the Pasteurella strains have very different genome structures among and even within individual species. The divergence of Pasteurella was also reflected in 16S rDNA sequences and far exceeded that seen between Escherichia and Salmonella. Based on this diversity, the Pasteurella haemolytica strains we analyzed could be divided into 14 phylogenetic groups and the Pasteurella multocida strains could be divided into 9 groups. If criteria for defining bacterial species or genera similar to those used for Salmonella and Escherichia coli were applied, the striking phylogenetic diversity would allow bacteria in the currently recognized species of P. multocida and P. haemolytica to be divided into different species, genera, or even higher ranks. On the other hand, strains of Pasteurella ureae and Pasteurella pneumotropica are very similar to those of P. multocida in both genome structure and 16S rDNA sequence and should be regarded as strains within this species. We conclude that large-scale genome structure can be a sensitive indicator of phylogenetic relationships and that, therefore, I-CeuI-based genomic mapping is an efficient tool for probing the phylogenetic status of bacteria.

* Corresponding author. Mailing address: Department of Microbiology and Infectious Diseases, University of Calgary, 3330 Hospital Dr., NW, Calgary, AB T2N 4N1, Canada. Phone: 403-220-3799. Fax: 403-283-8727. E-mail: slliu{at}ucalgary.ca.

Journal of Bacteriology, November 1999, p. 6747-6755, Vol. 181, No. 21
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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