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Journal of Clinical Microbiology, May 2003, p. 1946-1950, Vol. 41, No. 5
0095-1137/03/$08.00+0 DOI: 10.1128/JCM.41.5.1946-1950.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Genomic Diversity of Clinical and Environmental Vibrio cholerae Strains Isolated in Brazil between 1991 and 2001 as Revealed by Fluorescent Amplified Fragment Length Polymorphism Analysis
Fabiano L. Thompson,1,2* Cristiane C. Thompson,1 Ana C. P. Vicente,3 Grace N. D. Theophilo,4 Ernesto Hofer,4 and Jean Swings1,2
Laboratory for Microbiology,1
BCCM/LMG Bacteria Collection, Ghent University, Ghent 9000, Belgium,2
Department of Genetics,3
Department of Bacteriology, Oswaldo Cruz Institute, Rio de Janeiro, Brazil4
Received 3 December 2002/
Returned for modification 8 January 2003/
Accepted 31 January 2003

ABSTRACT
Vibrio cholerae is a ubiquitous and abundant organism in aquatic
environments, particularly in coastal areas, estuaries, and
rivers. This organism was the cause of a considerable number
of deaths in Brazil during the last decade. In this study we
applied the genomic fingerprinting technique fluorescent amplified
fragment length polymorphism (FAFLP) to analyze 106
V. cholerae O1 and non-O1 and non-O139 strains isolated from clinical specimens
and the environment between 1991 and 2001. Numerical analysis
of the FAFLP patterns disclosed seven main groups of genomes,
all of them originated from a variety of different places in
different years, suggesting that
V. cholerae is a very diverse
species. O1 and non-O1 and non-O139 strains were distinguishable
by FAFLP, although clinical and environmental strains clustered
together in a few cases. The persistence of some strains of
highly related genomes during several years and in completely
different geographical regions suggests that these strains are
highly successful in adapting to changing environmental conditions.

INTRODUCTION
Between 1991 and 1998 at least 1,296 deaths due to cholera occurred
in Brazil, mostly in the northern and northeastern regions of
the country (
25). The lack of a water network supply and sanitation
as well as poverty were concluded to be the main reasons for
this high rate of mortality. The study of the
Vibrio cholerae population structure and cholera molecular epidemiology may
help us to understand the spread of this disease. Molecular
typing techniques, i.e., fluorescent amplified fragment length
polymorphism (FAFLP), repetitive extragenic palindrome-PCR,
multilocus enzyme electrophoresis (MLEE), multilocus sequence
typing (MLST), randomly amplified polymorphic DNA, and ribotyping,
have been extensively applied in the study of
V. cholerae (
5).
Although ribotyping has been perhaps the most used technique
in the study of the diversity, epidemiology, and evolution of
V. cholerae (
24), Lan and Reeves (
14) have recently highlighted
the fact that most of the variation in ribotyping is due to
recombination events which take place at a very high rate (10
-5 per cell per generation). In these cases, ribotyping the similarity
between isolates does not reflect their origin from the same
ancestor. It has been suggested that a robust and reproducible
alternative is the use of FAFLP fingerprinting technique (
11,
15). FAFLP analysis is performed basically in four steps (
5,
10). (i) The digestion of total genomic DNA is accomplished
with two restriction enzymes, a 4-base cutter (e.g.,
TaqI) which
digests the DNA frequently and a 6-base cutter (e.g.,
ApaI)
which digests the DNA less frequently. Subsequent ligation of
the restriction halfsite-specific adaptors to all restriction
fragments is performed by using a DNA ligase. (ii) Selective
amplification of these fragments is done with two PCR primers
that have corresponding adaptor and restriction site sequences
as their target sites. Selective bases that flank the restriction
site sequences are included in the primers to reduce the number
of fragments amplified. The final steps are (iii) electrophoretic
separation of the PCR products on polyacrylamide gels with selective
detection of fragments which contain the fluorescently labeled
primer and (iv) computer-assisted numerical analysis of the
band patterns. FAFLP indexes variation in the whole genome and
thus is considered to give useful information on the short-
and long-term evolution of bacterial strains. In the present
study we applied FAFLP analysis to uncover the genomic diversity
of clinical and environmental
V. cholerae O1 and non-O1 and
non-O139 isolates from Brazil during a 11-year sampling period,
corresponding to the beginning of the seventh pandemic in Latin
America.

MATERIALS AND METHODS
Bacterial strains, growth condition, and DNA extraction.
A total of 106
V. cholerae strains, i.e., 10 reference strains
and 96 isolates originated from different Brazilian states were
examined in this study (Fig.
1). Eighty-seven strains originated
from clinical specimens, and 20 strains were isolated from the
environment. Strains were grown aerobically on tryptone soy
agar (Oxoid) supplemented with 2% (wt/vol) NaCl for 24 h at
28°C. DNA was extracted following the method described by
Pitcher et al. (
16a). All strains included in this study are
deposited in the BCCM/LMG bacterial collection at Ghent University
(Ghent, Belgium) and in the Oswaldo Cruz Institute Collection
(Rio de Janeiro, Brazil).
FAFLP template preparation and PCRs.
Template preparation was carried out essentially as described
previously (
11,
22). Briefly, 1 µg of high-molecular-mass
DNA was digested with
TaqI and
ApaI (Amersham Pharmacia Biotech,
Uppsala, Sweden) followed by ligation of restriction half-site-specific
adapters to all restriction fragments with T4 ligase (Amersham
Pharmacia Biotech). An aliquot of 2.5 µl of template was
mixed with 0.8 µl of A01-6FAM primer (5'-GACTGCGTACAGGCCC
A-3';
1 µM), 0.8 µl of T01-ABI (5'-CGATGAGTCCTGACCGA
A-3';
5 µM), and 12 µl of amplification core mix (Applied
Biosystems). The A01-6FAM primer is fluorescently labeled, and
the selective bases (adenosine) at the 3' end are underlined.
The amplification reactions were performed in a GeneAmp PCR System 9600 thermocycler (Applied Biosystems) as described previously (22).
Capillary electrophoresis and numerical analysis.
Six microliters of the selective PCR product was mixed with 24 µl of deionized formamide, 1 µl of GeneScan-500 TAMRA size standard, and 1 µl of GeneScan-2500 TAMRA size standard and heated at 95°C for 3 min. Subsequently, the mix was chilled on ice for a few minutes. Separation of the selective PCR products was generated in an ABI 310 genetic analyzer (Applied Biosystems). The time and voltage of sample injection were 16 s and 15 kV, respectively. Each run was performed at 60°C for 28 min at 10 mA and 15 kV. Normalization of the patterns was performed by the GeneScan 3.1 software (Applied Biosystems). Normalized tables of peaks, containing fragments of 50 to 536 bp, were transferred into BioNumerics 2.5 software (Applied Maths, Sint-Martens-Latem, Belgium) for numerical analyses. Pattern similarity was calculated by using the Pearson coefficient, and dendrograms were obtained by unweighted pair group method of arithmetic averages (21). Cophenetic correlation and a cluster cutoff algorithm based on the Point-Biserial correlation (12) were applied in order to evaluate the robustness of clusters. Additionally, principal component analysis (PCA), a nonhierarchical technique, was performed on binarized FAFLP data in order to obtain the relatedness among strains in a three-dimensional space.
DNA hybridization.
DNA-DNA hybridization was performed under stringent conditions by using the microplate technique with photobiotin-labeled DNA at a temperature of 40°C for 3 h as described previously (26). DNA similarity values are means of reciprocal and nonreciprocal reactions, each of which are performed in quadruplicates.

RESULTS AND DISCUSSION
The results depicted in Fig.
1 clearly denote that
V. cholerae is a diverse species. The strains examined consist of seven
main groups of genomes, not correlated to particular dates or
places. This grouping was supported by cophenetic correlation
and a cluster cutoff algorithm. Cluster 1 (
n = 70) was the largest
group, including the type strain of
V. cholerae, LMG 21698
T,
and other established reference strains such as LMG 19996 (
22),
LMG 21699, El Tor R-18308, and Amazon variant strains (
3). Cluster
1 was composed of O1 and non-O1 and non-O139 strains isolated
from patients and the environment. Overall isolates from the
different serogroups formed subgroups, whereas some clinical
and environmental strains (e.g., R-18301 and R-18338; R-18251
and R18258) were highly related. Some clonal groups of strains
(i.e., strains having about 95% pattern similarity) were correlated
by either the year or the region of isolation. For instance,
strains R-18309, R-18316, R-18305, and R-18304 were all isolated
in 1993 but in states at least 1,500 km away from each other.
Strains R-18331, R-18332, R-18333, R-18355, R-18356, R-18359,
and R-18362 were all isolated in Amazonas state in 1991 to 1992
and were related to the majority of the cases of cholera in
that region (
3). These strains have been referred to as a new
variant of
V. cholerae O1, the Amazon variant. According to
many phenotypic and genotypic techniques, this variant is completely
distinct from other
V. cholerae O1 strains (
3).
Cluster 2 (n = 8) consisted of non-O1 and non-O139 strains, with all strains but one (R-18267) from patients mostly in northeastern Brazil between 1992 and 1999. Strains R-18288 and R-18289, isolated at very distant places in 1998, were clearly a single clone. Cluster 3 (n = 10) harbored clinical and environmental non-O1 strains, including the reference strain R-14847 analyzed in a previous study (22). A conspicuous subgroup of highly related strains, i.e., R-18246, R-18248, R-18252, and R-18340, which were all isolated in northeastern Brazil was disclosed. Cluster 4 (n = 7) consisted mostly of environmental non-O1 and non-O-139 strains isolated in 1992 while cluster 5 (n = 2) was composed of two less-related strains. Cluster 6 (n = 4) harbored four reference O1 strains from clinical specimens isolated over a span of 65 years. Cluster 7 (n = 3) consisted of three environmental non-O1 and non-O139 strains. A few strains were unclustered; this was the case for the former Vibrio albensis type strain LMG 4406 and for R-18240.
V. cholerae O1 and non-O1 were distinguishable but yet closely related by FAFLP analysis (Fig. 1 and 2). Although certain overlap between O1 and non-O1 strains was observed, most O1 and non-O1 strains were clustering apart (Fig. 2). The high genome plasticity of V. cholerae O1 and non-O1 as revealed by our FAFLP analysis is in agreement with previous detailed investigations. When MLEE analysis of 15 housekeeping enzyme loci was applied to a collection of 107 clinical and environmental V. cholerae strains, 99 electrophoretic types were disclosed and no significant clustering between serogroups, biotype, and country of isolation were observed, suggesting that V. cholerae strains share a common genetic background regardless of phenotype and/or geographical distribution (6). Subsequently, the same workers applied MLST of six housekeeping enzyme loci on a subset of 31 V. cholerae serogroup O139 strains (7). Among this subset they found four groups of strains, clearly proving that V. cholerae serogroup O139 consists of multiple epidemic lineages.
Because the Amazon O1 population was so apart from the other
V. cholerae strains as revealed by PCA (Fig.
2), we decided
to perform DNA-DNA hybridization experiments to check whether
this population would not be in fact another species.
V. cholerae strains, including the Amazon variant, were highly related,
i.e.,

94% similarity, despite their long span of isolation (Table
1). This similarity level corresponded to FAFLP pair-wise pattern
similarities of about 75%, indicating the higher discriminatory
power of the latter technique. Our careful experiments not only
proved that the Amazon variant belongs to the species
V. cholerae but also showed that
Vibriomimicus LMG 7896
T is highly related
to
V. cholerae (Table
1). The species
V. mimicus was proposed
by Davis et al. in 1981 (
4) to encompass biochemically atypical
V. cholerae strains. These authors reported high DNA-DNA similarity
(i.e., 67%) among
V. cholerae and
V. mimicus. Nevertheless,
these two species can be clearly differentiated on the basis
of AFLP (
22) and MLEE (
23).
Another interesting aspect of this study is the diversity of
non-O1 and non-O139 strains within the epidemics of cholera
in Brazil. These strains, which were isolated mostly from patients
with watery diarrhea, showed a remarkably high genome diversity
as they appeared in different FAFLP clusters and they were also
related to O1 strains. Serogroup O1 strains were supposed to
be the only strains responsible for epidemic cholera, but in
1993 serogroup O139 was found to be the cause of epidemics of
cholera in Bangladesh and India (
8). Although currently most
outbreaks throughout the world are caused by serogroup O1 or
O139, in Brazil, cholera cases due to non-O1 and non-O139 strains
have been observed. A typical example of such a successful lineage
is the group represented by the strains R-18246, R-18248, R-18252,
and R-18340, which were isolated in northeastern Brazil during
1992 to 1999 (Fig.
1, cluster 3).
While the precise role of non-O1 and non-O139 strains in the cholera outbreaks in Brazil is unknown at present, our results suggest that strains of different serogroups are highly related and may be important in the evolution of V. cholerae and cholera. Recent studies on the distribution of virulence genes in V. cholerae strains isolated in Brazil and worldwide have demonstrated that both clinical and environmental strains of V. cholerae of different serogroups may harbor virulence genes, i.e., ctxAB and tcpA genes, and are thus potentially pathogenic (1, 2, 17). Lateral gene transfer of both virulence and antibiotic resistance genes is an important and ancient process that has been taking place in the evolution of gram-negative bacterial pathogens (8, 13, 18, 19). Bearing this process in mind and taking into consideration the high abundance of vibrios (i.e.,
108 cells · liter-1 or up to 4% of Bacteria) in aquatic environments (9), an important aspect to consider in the study of the evolution of cholera would be the role of other Vibrio species (currently more than 55) as potential reservoirs of virulence genes (16, 20). MLST of housekeeping and virulence genes is under way in order to better understand the population biology of vibrios, including V. cholerae.

ACKNOWLEDGMENTS
F.L.T. has a Ph.D. scholarship (no. 2008361/98-6) from the Conselho
Nacional de Desenvolvimento Científico e Tecnológico
(CNPq), Brasilia, Brazil. J.S. acknowledges grants from the
Fund for Scientific Research (FWO), Brussels, Belgium.
The authors are indebted to R. Coopman for skillful technical assistance during genotypic analyses and to E. Falsen from CCUG (Göteborg, Sweden) for providing reference strains.

FOOTNOTES
* Corresponding author. Mailing address: Laboratory of Microbiology, Ghent University, K. L. Ledeganckstraat 35, B-9000 Ghent, Belgium. Phone: 32-9-264 5116. Fax: 32-9-264 5092. E-mail:
Fabiano.Thompson{at}rug.ac.be.


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Journal of Clinical Microbiology, May 2003, p. 1946-1950, Vol. 41, No. 5
0095-1137/03/$08.00+0 DOI: 10.1128/JCM.41.5.1946-1950.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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