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Applied and Environmental Microbiology, April 2003, p. 2377-2382, Vol. 69, No. 4
0099-2240/03/$08.00+0 DOI: 10.1128/AEM.69.4.2377-2382.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Direct Profiling of Environmental Microbial Populations by Thermal Dissociation Analysis of Native rRNAs Hybridized to Oligonucleotide Microarrays
Said El Fantroussi,1,
Hidetoshi Urakawa,1,
Anne E. Bernhard,1 John J. Kelly,2 Peter A. Noble,1 H. Smidt,1,
G. M. Yershov,3 and David A. Stahl1*
Department of Civil and Environmental Engineering, University of Washington, Seattle, Washington 98195,1
Department of Biology, Loyola University Chicago, Chicago, Illinois 60626,2
Biochip Technology Center, Argonne National Laboratory, Argonne, Illinois 604393
Received 29 July 2002/
Accepted 4 December 2002

ABSTRACT
Oligonucleotide microarrays were used to profile directly extracted
rRNA from environmental microbial populations without PCR amplification.
In our initial inspection of two distinct estuarine study sites,
the hybridization patterns were reproducible and varied between
estuarine sediments of differing salinities. The determination
of a thermal dissociation curve (i.e., melting profile) for
each probe-target duplex provided information on hybridization
specificity, which is essential for confirming adequate discrimination
between target and nontarget sequences.

INTRODUCTION
The vast and previously unrecognized diversity of the microbial
world discovered during the last decade reflects the rapid progress
made in the development of molecular techniques for direct characterization
of environmental systems. Among techniques now commonly employed,
recovery of 16S rRNA sequences by using standard methods of
PCR amplification, cloning, and sequencing has provided the
most encompassing perspectives of microbial diversity (
8). However,
such descriptions of diversity have been accompanied by modest
progress in characterizing the spatial and temporal patterns
of abundance and activity of microorganisms. Thus, future advances
will likely be technology driven and dependent upon new methods
for both extensive and intensive monitoring of population abundance,
distribution, and activity (
22). In this regard, we anticipate
that high-throughput DNA microarrays, which provide for parallel
hybridizations to hundreds or thousands of genes per single
experiment, will play a major role in the study of environmental
systems (
9,
11,
20,
27).
The major advantages of the microarray format are a low-sample-size requirement and the possibility for characterizing environmental nucleic acid pools without the biases associated with PCR and other amplification techniques. The level of sensitivity is particularly relevant to the development of this technology, since many molecular detection methods currently rely upon amplification to detect target sequences (12, 16). Amplification, however, often introduces biases that make quantification of natural populations difficult (5, 21, 24, 26). The ability to detect target rRNA sequences without amplification (i.e., direct profiling of rRNA sequences) would greatly improve our capacity to determine the relative number of sequences representing natural microbial populations. In our present work, we demonstrate the direct profiling of environmental microbial populations by hybridization of native rRNA recovered from two different sediment samples to a microarray designed to identify major microbial groups (PhyloChip).
The level of probe and hybridization specificity is also important to the development of DNA microarray technology, since probes targeting certain groups differ by as little as a single base pair. For example, probe BET42a (L-Sc-bProt-1027-a-A-17), which targets the ß subclass of Proteobacteria, and probe GAM42a (L-Sc-gProt-1027-a-A-17), which targets the
subclass of Proteobacteria, differ by a single T or A nucleotide (Table 1). The position and type (i.e., nucleotide) of a single-base-pair mismatch in a probe-target duplex (in addition to hybridization and wash conditions) have significant effects on hybridization specificity (25). Hence, it is necessary to recognize when a high level of discrimination is not feasible and to acknowledge instances when the identification of a particular target sequence was not achieved. This includes recognizing detection limits and/or the extent to which nucleic acid cross-hybridization compromises our ability to adequately distinguish target from nontarget sequences.
In conventional hybridization assays, the level of specificity
is typically determined by adjusting the experimental conditions
(e.g., temperature, ionic strength, and concentration of formamide)
of association (i.e., hybridization) and the subsequent dissociation
(i.e., washing or melting) of the probe-target duplex (
28).
In DNA microarray assays, however, this approach is difficult
to apply since one set of conditions does not provide optimal
target discrimination for all probes. Ideally, hybridization
would be interrogated by using the association or dissociation
condition that provides appropriate specificity for each probe.
To this end, we have previously demonstrated that a thermal
dissociation analysis of all probe hybrids, conducted in parallel
on a single microarray, could be used to achieve higher discrimination
between matched and mismatched probe-target duplexes and to
provide a measure of the extent of cross-hybridization (
13,
25). In the present study, we incorporated a thermal dissociation
step in the characterization of native environmental rRNAs,
demonstrating a format suitable both for direct detection and
for confirmation that appropriate target sequence specificity
has been realized.

Rationale for probe design.
Twenty-one oligonucleotide probes were used in this study (Table
1). Each probe targeted a given group or subgroup, was not complementary
to any nontarget rRNA sequence in the ARB software (
http://www.mikro.biologie.tu-muenchen.de)
or GenBank database, and had a low number of self-complementarities
(<4 nucleotides) as determined by online analysis of the
Probe_Match program in the Ribosomal Database Project II (
14).
The positions of single-base-pair mismatches in nontarget sequences
were designed to be not less than 2 bases from the 5' or 3'
termini (
25). To facilitate the correct interpretation of hybridization
patterns on the microarray and to ensure probe fidelity, replicate
and redundant probes were incorporated. While replicate probes
served as internal hybridization controls, redundant and hierarchically
nested probes served to confirm identification (
13). Identification
of a specific target molecule can be confirmed from the hybridization
patterns of redundant probes that hybridize to multiple unique
sites in rRNA. The hierarchical nesting of the probes provides
phylogenetic information at several levels of taxonomic specificity
(e.g., from domain to species) as well as information on hybridization
specificity, since hybridization to multiple probes provides
additional coverage of a single molecule. Lastly, since the
secondary structures of probes and target sequences affect signal
intensity, some redundant probes were designed to hybridize
to different sites of the same rRNA molecule while maintaining
taxonomic specificity (e.g., GPOS1192 and GPOS1199).

Sampling and RNA extraction.
Sediment cores (6.5-cm diameter) were collected during low tide
from a high-salinity (20 ppt) site and a low-salinity (0 ppt)
site in Plum Island Sound, Mass., in April 2001. Sediment samples
at the high-salinity site consisted almost exclusively of sand,
while those collected at the low-salinity site consisted primarily
of silt and clay (
10). Cores were kept in the dark and transported
at the in situ water temperature to the laboratory where the
top 0.5 cm was collected, homogenized, and stored at -80°C
until processing.
Total RNA was extracted from approximately 0.5 g of sediment by bead beating (two times for 30 s each), phenol-chloroform extraction, and isopropanol precipitation as previously described (23). Replicate extracts from the high-salinity site (n = 4) and the low-salinity site (n = 3) were analyzed.

Fragmentation and labeling of rRNAs.
The RNA was fragmented and labeled simultaneously by using a
silica-based column as previously described, with slight modifications
(
4). The silica matrix material was suspended twice with 500
µl of the binding buffer containing 50 µg of total
RNA and centrifuged at 14,000
x g for 30 s. The column was washed
twice with 70% ethanol, followed by a second wash using 100%
ethanol, and air dried. The silica column containing bound RNA
was sealed and incubated at 95°C for 4 min. Freshly prepared
labeling cocktail (150 µl) containing 5 mM 1,10-phenanthroline,
0.5 mM CuSO
4, 1 mM lissamine-rhodamine B ethylenediamine (Molecular
Probes, Inc., Eugene, Oreg.), and 20 mM sodium phosphate (pH
7.0) was heated at 95°C for 30 s. Hydrogen peroxide (2 mM)
and freshly prepared 20 mM NaCNBH
3 were added to the cocktail,
and the mixture was added to the column. After incubation of
the mixture for 10 min at 95°C, the reaction was stopped
by adding 9 µl of 500 mM EDTA (pH 8.0). Fragmented nucleic
acids were immobilized on the column by adding 15 µl of
5 M ammonium acetate and 450 µl of 100% (vol/vol) ethanol
followed by a 5-min incubation at room temperature. Excess fluorescent
label was removed by washing the column twice with 500 µl
of 100% (vol/vol) ethanol. The column was then dried with forced
air, and the labeled product was eluted with 45 to 60 µl
of diethyl pyrocarbonate-treated water.

Microarray fabrication and hybridization.
The DNA microarray was fabricated as previously described (
25),
with each gel pad containing a total of 3 pmol of probe. Hybridization
buffer (40% formamide, 0.9 M NaCl, 20 mM Tris-HCl [pH 8.0])
containing approximately 2 µg of labeled RNA in 40 µl
of buffer was passed through a 0.22-µm-pore-size filter
to remove any particles that may damage the gel pads. The hybridization
solution was added to a hybridization chamber (Grace Biolabs,
Bend, Oreg.), and the hybridization chamber was affixed to a
microarray. The microarray was then allowed to hybridize overnight
at room temperature (20°C in the dark). After hybridization,
the chamber and hybridization solution were removed and the
microarray was washed twice for 10 s with 100 µl of washing
buffer (20 mM Tris-HCl [pH 8.0], 5 mM EDTA, 4 mM NaCl). Analyses
of images and thermal dissociation curves (i.e., melting profiles)
were carried out as previously described (
25).

Direct profiling of environmental rRNAs.
We achieved direct profiling of sediment microbial populations
by hybridizing rRNA extracted from estuarine sediment samples
to the microarray (Fig.
1). The profiles of sediment samples
were reproducible and distinctly different between the sites
(Fig.
2). The signal intensities of replicated bacterial domain
probe EUB927 were not significantly different from one another
(Student's
t test), indicating that identical probes located
at different positions on the microarray yielded identical results
(Fig.
1 and
2).
On the other hand, the signal intensities of some redundant
probes varied significantly for samples collected within the
same field site. For example, in the low-salinity samples (Fig.
2B), probe UNIV907 had significantly lower (
P < 0.02 for
both quadrants A and B [Student's
t test];
n = 3) signal intensities
than probe UNIV1390. However, this within-site variation was
not observed for the same probes hybridized to the high-salinity
samples (Fig.
2A). Within-site variation was also observed for
other redundant probes hybridized to rRNA extracted from high-salinity
samples (Fig.
2A). For example, results obtained for probe EUB336
(Fig.
2A) yielded significantly lower (
P < 0.03 for quadrant
A and
P < 0.01 for quadrant B [Student's
t test];
n = 4)
signal intensities than probes EUB927 and EUB338. Yet, there
were no statistical differences among signal intensities for
the same probes when using rRNA extracted from low-salinity
samples (Fig.
2B). Site-specific differences in signal intensities
originating from redundant probes might be due to subtle differences
in fragmentation and/or fluorescent labeling of the target sequences
(
4). However, we anticipate that more-significant differences
originate from minor sequence variation within the relatively
highly conserved regions of the rRNAs targeted by these probes.
Clearly, more-extensive experiments (beyond the scope of this
study) are needed to reveal the source of sample-specific variability
within redundant probe sets. Nonetheless, the results demonstrated
that oligonucleotide microarrays can be used to (i) detect rRNA
extracted from 0.5 g of sediment and (ii) provide information
on the composition of microbial communities at different levels
of taxonomic specificity.
Melting profiles were used to evaluate the specificity of hybridization of environmentally derived rRNA to specific probes. We accomplished this by comparing the melting profiles of probe-target duplexes using rRNA extracted from pure cultures with those obtained from environmental samples. Figure 3 shows the melting profiles of universal probe UNIV1390 to (i) a perfect-match target sequence (Sulfobacillus acidophilus ATCC700253) and (ii) rRNA extracted from sediment collected at the high-salinity site. The melting profiles were virtually identical, demonstrating that environmentally derived rRNA included perfect-match target sequences to this probe. Similar results were also obtained for probes EUB338 and EUB927 (data not shown).
In the case of probe ALF968 (S-Sc-aProt-0968-a-A-18), targeting
the

subclass of
Proteobacteria (Fig.
4), there were considerable
differences in melting profiles of rRNA extracted from sediment
samples and those of perfect-match rRNA extracted from a pure
culture of
Sulfobacillus acidophilus. Apparently, the melting
profile of the environmentally derived rRNA was more similar
to that of rRNA extracted from a pure culture,
Brevibacillus agri ATCC51663, which had a single-base-pair mismatch to probe
ALF968. These results indicate that a significant fraction of
the environmentally derived rRNA had at least one single-base-pair
mismatch to probe ALF968.
The advantage of comparing melting profiles of probe-target
duplexes rather than comparing signal intensities at a single
wash temperature of 20°C is that melting profiles provide
more information on hybridization specificity. Moreover, since
the relative abundances of target and closely related nontarget
molecules in the environment are not known, it is difficult
to determine the extent of cross-hybridization events from signal
intensity data alone. However, the incorporation of melting-profile
measurements in the oligonucleotide microarray analysis serves
to identify circumstances when cross-hybridization clouds our
ability to accurately discriminate target from nontarget sequences.
In summary, oligonucleotide microarrays were used to profile directly extracted rRNA from environmental microbial populations without PCR amplification. Recently, Small et al. (20) described a microarray system capable of detecting 0.5 µg of total rRNA by using a chaperone-detector probe strategy. This detection limit was in the level of PCR detection limits applied to environmental systems (109 to 1010 copies of 16S rRNA) and similar to the sensitivity we report here. The results achieved by Small et al. (20) were obtained by mixing purified reference nucleic acids with environmental extracts. Our study now demonstrates the feasibility of directly characterizing native rRNA. In our initial inspection of two distinct estuarine study sites, hybridization patterns were reproducible and varied between estuarine sediments of differing salinities. The incorporation of a melting-profile analysis for each probe duplex provided information on hybridization specificity, which is essential for confirming adequate discrimination between target and nontarget sequences.

ACKNOWLEDGMENTS
We thank B. Chernov, A. Gemmell, J. Golova, A. Kukhtin, and
S. Surzhikov for their efforts in manufacturing the oligonucleotide
microarrays and for synthesis of the oligonucleotide probes.
We are grateful to Y. Koizumi and S. Siripong for valuable discussions.
We are also grateful to J. C. Smoot for reading the manuscript.
This work was support by grants from DARPA and NASA to D.A.S. and National Science Foundation grants DEB-0088879 to P.A.N. and MCB-9977897 to D.A.S. and from DARPA to J.J.K.

FOOTNOTES
* Corresponding author. Mailing address: Civil and Environmental Engineering, University of Washington, 302 More Hall, Box 352700, Seattle, WA 98195. Phone: (206) 685-3464. Fax: (206) 685-9185. E-mail:
dastahl{at}u.washington.edu.

Present address: Unit of Bioengineering, University of Louvain, B-1348 Louvain-la-Neuve, Belgium. 
Present address: National Institute for Environmental Studies, Tsukuba-Shi, Ibaraki
305-8506, Japan. 
Present address: Laboratory of Microbiology, Wageningen University, 6703 CT Wageningen, The Netherlands. 

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Applied and Environmental Microbiology, April 2003, p. 2377-2382, Vol. 69, No. 4
0099-2240/03/$08.00+0 DOI: 10.1128/AEM.69.4.2377-2382.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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