1022-7954/05/4101- © 2005 Pleiades Publishing, Inc.
Russian Journal of Genetics, Vol. 41, No. 1, 2005, pp. 44–52. Translated from Genetika, Vol. 41, No. 1, 2005, pp. 53–62.
Original Russian Text Copyright © 2005 by Osin, Nefedov, Yeroshenko, Smirnova.
Cholera is an acute diarrheal disease, which has
been known for over two thousand years. Six initial
pandemics of cholera, resulting in death of million
human beings, were caused by
classical biotype. Beginning from 1961, the latest, sev-
enth pandemic caused by
of the El Tor bio-
type  has continued for over 40 years in many coun-
tries of southeastern Asia, Africa, and South America.
Recent outbreaks of cholera in Daghestan (1994), the
Far East (1999), and Tatarstan (2001) suggest that this
infection is dangerous in Russia even at present .
What is the origin of the El Tor cholera? Vibrios of
this biotype were ﬁrst isolated in 1905 at the El Tor quar-
antine station of Sinai from the ﬂuid of intestines of pil-
grims who supposedly died of dysentery. For a long time,
a debate over the signiﬁcance of these vibrios in causing
cholera epidemics has been in progress. Some authors
believed that isolated vibrios were virulent, whereas, in
accordance with the notions accepted by other research-
ers until 1958, vibrios of this biotype were avirulent.
Meanwhile, the outbreak of cholera, which occurred in
1937 at the Celebes Island (Indonesia), led to 50.0–
60.0% lethality. However, even after these events, the
view that El Tor vibrios are unable to cause outbreaks of
cholera has persisted, since diseases caused by these
vibrios did not spread beyond the major focus of infec-
tion [1, 2].
Rapid spread of El Tor cholera in many countries was
registered only in 1961. The seventh pandemic began.
The appearance of the pandemic potential in El Tor
vibrios was necessarily connected with structural and
functional changes in their genomes. At present, it is
obvious that a signiﬁcant contribution to the evolution of
the pathogenicity of a cholera-causing agent that has two
chromosomes was made by horizontal gene transfer.
This transfer is mediated by mobile genetic elements
(MGEs) containing virulence and pandemic genes.
A temperate phage CTX
inserted into the chromosome
carries the cassette of genes
the biosynthesis of three toxins Zot, Ace, and CT (chol-
era toxin)—cholera enterotoxin responsible for the
development of the main symptom, diarrhea. The perma-
nent satellite prophage RS1 possesses gene
antirepressor, one function of which is enhancement of
gene transcription [3–5]. The genome of cholera
agent contains, apart from the prophages, vibrio patho-
genicity island (VPI) and pandemic island. VPI carries
the following genes: genes
that are involved in
the production of toxin-coregulated adhesion pili
(TCP), the key colonization factor; gene
tion of pathogenesis) whose product, the Mop protein,
plays an important role in the pathogenicity of cholera
[3, 4]; gene
responsible for the production of alde-
hyde dehydrogenase; and the major regulatory
gene. A novel, previously unknown vibrio pathogenic-
ity island (VPI-2) comprises the
neuraminidase that enhances the effect of CT .
The pandemic potential of El Tor vibrios causing the
seventh cholera pandemic seems to be related to the
presence in their genomes of 22 foreign genes, which
Comparative Genomic Analysis
El Tor Preseventh
and Seventh Pandemic Strains Isolated in Various Periods
A. V. Osin, K. S. Nefedov, G. A. Yeroshenko, and N. I. Smirnova
Mikrobe Russian Anti-Plague Research Institute, Saratov, 410005 Russia; fax: (8452) 51-52-12; e-mail: firstname.lastname@example.org
Received May 31, 2004
—Genetic organization of 52
El Tor biotype preseventh and seventh pandemic strains
isolated in various periods was studied by PCR assay and DNA–DNA hybridization. It was established that the
genome of most ancient of analyzed strains isolated from a diarrhea patient in 1910 was devoid of CTX and
RS1 prophages, vibrio pathogenicity islands (VPI and VPI-2), and pandemic islands (VSP-1 and VSP-2) that
contain key virulence genes. The appearance of pathogenic properties in cholera vibrios for the ﬁrst time caus-
ing a local outbreak of cholera in 1937 is connected with the acquisition of VPI and CTX that carried genes
respectively, which are responsible for the colonization of the small intestine and encode the
production of cholera toxin. The appearance of the seventh pandemic agent for cholera was shown to correlate
with the acquisition by its precursor of two additional blocks of genes VSP-1 and VSP-2. This ﬁnding strongly
supports the involvement of these genes in formation of the pandemic potential in the strains. Molecular typing
methods allowed elucidation of differences in the genetic organization between prepandemic and pandemic
strains. The detected variability of the genome of contemporary virulent strains may be a reason for the occur-
rence of etiological agent of cholera with new properties.