Brucellosis 2003 proceedings - PHIDIAS

Keynote Lectures
COMPARISON OF THE GENOMIC SEQUENCES OF Brucella melitensis,
Brucella suis, AND Brucella abortus BIOVARS: STRUCTURE AND
PSEUDOGENES.
Shirley M. Halling. National Animal Disease Center, Agricultural Research Service, USDA, P. O. Box
70, Ames, Iowa, USA.
The whole genomic sequences of three classical species of Brucellae,
Brucella melitensis, Brucella suis and Brucella abortus have been determined, and
the sequence of Brucella ovis is nearly finished. These species preferentially infect
goats, swine, bovine, and sheep, respectively. Comparative genomics may aid in
answering questions related to species specificity, virulence, pathogenicity, as well
as phenotypic differences observed amongst the Brucellae. Genomic sequences of
B. melitensis, B. suis and B. abortus, excluding insertion/deletions (indels), are more
than 99% similar to each other. There are only 7,301 SNPs between B. suis and B.
melitensis among 3.2 Mbp. The SNPs are more or less spread randomly through out
the chromosome but some clustering was noted. Close taxonomic relationship of
Brucellae to the plant symbiont Sinorhizobium meliloti is supported by the extensive
gene synteny between their genomic sequences, especially the large chromosome of
Brucellae.
Determination of the genomic sequences of Brucellae confirmed the presence
of two circular chromosomes in B melitensis, B suis, and B. abortus. One
chromosome is about twice the size of the other. The largest chromosome is 2.1
Mbp and has a classical bacterial type origin of replication. The smaller chromosome
is 1.2 Mbp and has a plasmid based origin of replication. The pulsed field map of the
small chromosome of B. abortus 544 biovar 1 has a large inversion relative to others.
This inversion was found in the B. abortus 9-941 genomic sequence and, by PCR, in
B. abortus biovars, 2 and 4 but not B. abortus biovars 3, 5, 6, and 9.
There are two large regions encoding phage-associated proteins found among
the Brucellae genomic sequences. One is in B. suis and the other in B. melitensis.
The region in B. melitensis was also found in B. abortus. All three sequences have
the large putative transposable element, Tn1953, encoding a large number of sugar
and amino acid transport proteins. Some of these are polymorphic among the
classical species. The genomic sequence of B. abortus is more similar to that of B.
melitensis than B suis. However, the genomic sequence of B. abortus has several
large insertion/deletions (indels) that are in the genomic sequence of B. suis but not
B. melitensis.
Differences in genomic sequences of Brucellae were exploited to develop
PCR-based diagnostic tests long before any of the genomes were sequenced.
Currently used tests have varying ability to distinguish the classical species and
biovars. Last year at the Brucellosis Research Conference, Bricker described
variable nucleotide tandem repeats (VNTR) in Brucellae and established their
potential as a strain typing epidemiological tool for source trace back in outbreaks of
infection. A comparison of genomic sequences of the Brucellae lead to the
identification of other loci that have the potential to be used for epidemiological trace
back and biovar identification.
Pseudogenes were identified among the Brucellae, but any one of them was
rarely found in all three Brucellae genomic sequences. Pseudogenes were often the
result of premature stops caused by SNPs or frame shifts in regions where there
were strings of identical bases or small deletions.
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Brucellosis 2003 International Research Conference