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Journal of Clinical Microbiology, July 2000, p. 2768-2771, Vol. 38, No. 7
Departamento de Microbiología y
Genética, Edificio Departamental,1 and
Departamento de Medicina Preventiva y Microbiología
Médica, Facultad de Medicina,2 Universidad
de Salamanca, 37007 Salamanca, Spain
Received 24 January 2000/Returned for modification 26 March
2000/Accepted 30 April 2000
A whole-cell hybridization assay with fluorescent oligonucleotide
probes derived from the 16S rRNA sequence of Brucella
abortus in combination with flow cytometry has been developed.
With the three fluorescent probes selected, a positive signal was
observed with all the representative strains of the species and biovars of Brucella and with a total of nine different
Brucella clinical isolates. Using the B9 probe in the
hybridization assay, it was possible to discriminate between
Brucella suis biovars 2, 3, 4, and 5 and almost all the
other Brucella spp. On the basis of differences in
fluorescence intensities, no discrimination was established between
Brucella spp. and other phylogenetically related
microorganisms. No positive fluorescence signals were detected with any
of the bacteria showing serological cross-reactions with
Brucella spp. and with a total of 17 clinical isolates not
belonging to the genus Brucella. These results suggest that
the 16S rRNA whole-cell hybridization technique could be a valuable
diagnostic tool for the detection and identification of
Brucella spp.
Brucellae are gram-negative
intracellular bacteria that are pathogenic for humans and many animal
species. On the basis of differences in pathogenicity and host
preferences, the genus Brucella has six recognized species:
Brucella melitensis, B. abortus, B. suis, B. canis, and B. neotomae
(8). At the DNA level, these Brucella spp.
show a high degree of homology, thus forming a
highly homogeneous and possibly monospecific group (36,
37). On the basis of the 16S rRNA sequence, Brucella
has been found to be a member of the alpha-2 division of the class
Proteobacteria and closely related to the
Rhizobiaceae family and other related genera not included in
this family (40).
A diagnosis of brucellosis in humans and animals is often difficult to
establish. For the identification of Brucella spp., mainly
bacteriological and immunological detection techniques have been used.
However, since isolation of the etiological agent from clinical samples
by conventional methods is not always possible (2),
serological tests play a major role in the routine diagnosis of
brucellosis (38). Although the sensitivity of these
diagnostic tests for the detection of specific antibodies against
Brucella spp. can range from 65 to 95%, their specificities
can be low in areas where brucellosis is endemic, mainly due to the
high prevalence of antibodies in the healthy population (6).
Moreover, other gram-negative bacteria may cross-react with smooth
Brucella spp., and vaccinated animals can also give
false-positive results in such tests (9, 18). Thus, the
development of new procedures for the detection and differentiation of
brucellae is currently of great practical importance.
Nucleic acid-based detection methods are very promising tools for the
diagnosis of brucellosis. The 16S rRNA gene sequence has been
extensively used to elucidate the phylogenetic relationships among
bacteria at intra- and intergeneric levels, and it is also an excellent
target for diagnostic methods (3, 5). PCR assays using
specific primer pairs derived from the 16S rRNA gene sequence of
B. abortus have been developed for the detection and
identification of Brucella spp. (10, 21, 30, 32,
33). Whole-cell hybridization with fluorescently labeled 16S
rRNA-targeted oligonucleotide probes (3-5) has been applied
for the detection and identification of different bacteria (19,
24, 26, 31, 34). In contrast to PCR, by rendering the probes
complementary to more abundant 16S rRNA rather than to the genes
encoding this RNA, the sensitivity obtained is more than sufficient for
use in diagnostic tests (31). However, to date this method
has never been used for both the detection and identification of
Brucella spp. or the diagnosis of brucellosis.
Bacterial strains and growth conditions.
The bacterial
strains used in this study and their sources are listed in Tables
1, 2, and
3. Reference Brucella strains and clinical isolates were grown on tryptic soy agar supplemented with
0.1% (wt/vol) yeast extract at 37°C for 48 h.
Agrobacterium, Alcaligenes,
Phyllobacterium, Mycoplana, and
Ochrobactrum strains were grown on the same medium at
26°C. Rhizobium and Sinorhizobium strains
were cultured in tryptone-yeast medium at 30°C. The remaining bacteria, including those showing serological cross-reactions with
Brucella spp. (9), were grown on nutrient agar at
37°C for 24 h. Clinical samples were isolated from hospitalized
patients and were identified by standard methods.
0095-1137/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Fluorescent Whole-Cell Hybridization with 16S
rRNA-Targeted Oligonucleotide Probes To Identify Brucella
spp. by Flow Cytometry
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ABSTRACT
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TABLE 1.
Fluorescence intensities obtained in whole-cell
hybridization with reference microorganisms and clinical isolates
unrelated to Brucella spp.
TABLE 2.
Fluorescence intensities obtained in whole-cell
hybridization with reference strains and clinical isolates of
Brucella spp.
TABLE 3.
Fluorescence intensities obtained in whole-cell
hybridization with bacteria phylogenetically or serologically
related to Brucella spp.
Probe design.
Multiple sequence alignments of the 16S
rRNA sequences of B. abortus (EMBL accession number
X13695), Rochalimaea quintana (M11927),
Ochrobactrum anthropi (D12794), Phyllobacterium rubiacearum (D12790), Mycoplana dimorpha (D12786),
Agrobacterium tumefaciens (D13294), Agrobacterium
rubi (I67228), Rhodopseudomonas palustris (D25312), and
Bradyrhizobium japonicum (D12781) were carried out, and on
the basis of foreseeable specificity for Brucella spp.,
three probes were designed. The oligonucleotide probes selected were
synthesized and 5' labeled with fluorescein isothiocyanate by Isogen
Bioscience BV, Maarsen, The Netherlands. All probes, together with
their sequences and target positions, are listed in Table
4.
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Cell fixation and fluorescent whole-cell hybridization.
Bacterial cells grown according to their specific growth conditions
were harvested in the logarithmic phase of growth (absorption at 650 nm
[A650] of 0.3; see "Results and
discussion" below), washed twice with phosphate-buffered saline (PBS)
(130 mM sodium chloride, 10 mM sodium phosphate buffer [pH 7.2]), and
fixed with 4% paraformaldehyde as described previously (3,
4). Fixed cells were stored at
20°C for up to 8 weeks
(3). For the fluorescent whole-cell hybridization,
approximately 5 × 105 fixed cells µl
1
were hybridized in 100 µl of hybridization buffer containing 0.9 M
sodium chloride, 20 mM Tris-HCl (pH 7.2), 0.1% sodium dodecyl sulfate,
and 7 ng of fluorescent probe µl
1 (see "Results and
discussion" below) at 46°C for 2 h (39). Subsequently, cells were pelleted by centrifugation for 2 min at
8,000 × g and resuspended in 100 µl of hybridization
buffer containing no probe. After a 20-min wash at 46°C, cells were
mixed with 500 µl of PBS (pH 8.4), immediately placed on ice, and
analyzed within 3 h.
Flow cytometry. Analyses were performed with a FACScan flow cytometer (Becton Dickinson, San Jose, Calif.). The forward-angle light scatter, right-angle light scatter, and green fluorescence parameters were detected. Each measurement was made for 10,000 events. Data were collected in the list mode (individual measurements per cell were collected and stored). Subsequent analysis was done using Cell Quest software (Becton Dickinson). The fluorescence intensity conferred by each probe was determined as the mean of the green fluorescence values of single cells. In each measurement, the fluorescence of the cells was corrected by subtracting the background fluorescence of the cells hybridized with the same probe without fluorescence (negative control). The individual results obtained with each probe are expressed as mean fluorescence values of triplicate determinations, with the standard deviations (SD) always being less than 12% of the mean values.
Statistical methods. Statistical analyses were performed with analysis of variance and Fisher's exact test. A P value of <0.05 was considered statistically significant.
Results and discussion.
Preliminary studies were carried out
in order to optimize some parameters which might affect the results
obtained in the whole-cell hybridization procedure (19).
Thus, to select the optimum fluorescent probe concentration,
hybridizations using 5 × 105 fixed B. abortus 2308 and Escherichia coli ATCC 25922 (negative control) cells and concentrations of 1, 5, 10, 15, and 20 ng of each
fluorescently labeled oligonucleotide probe µl of hybridization buffer
1 were carried out. Similar fluorescence
intensities of hybridized cells were observed with each probe at
concentrations ranging between 5 and 10 ng µl
1. At
higher concentrations, an increase in fluorescence signals was observed
in both B. abortus and E. coli cells (data
not shown), probably due to nonspecific staining (39).
Accordingly, all further hybridization experiments were performed
using each probe at a concentration of 7 ng µl
1 under
the experimental conditions described above. Following this, attempts
were made to determine the effect that the growth phase of cultured
cells used in the hybridization assay had on the fluorescence signal
obtained with each probe. The results are shown in Fig.
1. With the three probes used in the
hybridization with B. abortus 2308 and E. coli
25922 (control) cells, the strongest fluorescence intensity signal was
achieved with cells of both microorganisms fixed in the logarithmic
phase of growth (A650 ranging between 0.25 and
0.35). These results are in agreement with those described previously
(12, 39), in which it was demonstrated that the signals
conferred by fluorescently labeled rRNA-targeted probes on
whole fixed cells are correlated with cellular rRNA contents and
therefore reflect the cells' metabolic activities and their growth
rate. Accordingly, the fixation of all the bacterial cells used in this
study was performed on growing cells at an A650
of 0.3.
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ACKNOWLEDGMENTS |
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This work was supported by grant no. SA47/98 from the Junta de Castilla y León, Spain.
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FOOTNOTES |
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* Corresponding author. Mailing address: Departamento de Microbiología y Genetica, Edificio Departamental, Universidad de Salamanca, Avda. Campo Charro s/n, 37007 Salamanca, Spain. Phone: 34-923-294532. Fax: 34-923-224876. E-mail: lrlago{at}gugu.usal.es.
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