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Applied and Environmental Microbiology, July 2008, p. 4231-4235, Vol. 74, No. 13
0099-2240/08/$08.00+0 doi:10.1128/AEM.02545-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.
Use of Laser Microdissection for Phylogenetic Characterization of Polyphosphate-Accumulating Bacteria
Stefanie Gloess,1
Hans-Peter Grossart,1
Martin Allgaier,1,
Stefan Ratering,2* and
Michael Hupfer1*
Leibniz Institute of Freshwater Ecology and Inland Fisheries, Alte Fischerhütte 2, D-16775 Stechlin-Neuglobsow, Germany,1
Institute of Applied Microbiology, Justus-Liebig University Giessen, Heinrich-Buff-Ring 26-32, D-35392 Giessen, Germany2
Received 12 November 2007/
Accepted 22 April 2008

ABSTRACT
Our novel approach for taxonomic identification of uncultured
bacteria harboring specific physiological features in complex
environmental samples combines cell collection by laser microdissection
and subsequent DNA analysis. The newly developed approach was
successfully tested for collection and phylogenetic characterization
of polyphosphate-accumulating bacteria in activated sludge and
lake sediment.

INTRODUCTION
In order to examine the ecological role of bacteria in a (eco)system,
it is often necessary to detect and identify these microorganisms
in situ, particularly if their specific metabolic capabilities
need to be studied. For this purpose, different methods are
available; however, currently all of them have distinct disadvantages
and limitations. For example, PCR-based techniques using cultivation-independent
approaches are very useful for exploring the diversity of microorganisms
in environmental samples, but often their metabolic functions
remain unknown. More recently, techniques based on fluorescence
in situ hybridization (FISH), such as Raman-FISH (
17), FISH-secondary-ion
mass spectrometry (
31), and FISH-microautoradiography (
26,
32),
have been developed to overcome such problems. These techniques
work with substrates labeled with either stable or radioactive
isotopes to help identify microbiota and to study their ecophysiology.
For the design and application of specific oligonucleotide probes
used in FISH, however, knowledge of the target (e.g., 16S rRNA)
sequence is essential. The identification of microorganisms
involved in specific metabolic processes in a complex environmental
sample can also be achieved by additional methods, e.g., separation
via stable isotope probing (
33) based on the selective recovery
of "heavy"
13C-labeled biomarkers (e.g., DNA or RNA). However,
this probing technique is restricted by the somewhat limited
availability of labeled substrates. In addition to the restriction
of limited availability of highly enriched commercial compounds,
the actual addition of external substrates and incubation time
may affect the activity and composition of the natural bacterial
community.
Hence, we aimed to develop an alternative approach for direct phylogenetic characterization of bacteria with specific metabolic functions from various environmental samples, in our case activated sludge and lake sediment. Our novel approach is based on the precise collection of single microorganisms and overcomes all of the abovementioned methodological limitations. In this study, we have separated polyphosphate (poly-P)-accumulating bacteria (PAO) from poly-P-containing sludge and lake sediment. PAO are key players in the phosphorus (P) cycle in sewage treatment plants with enhanced biological P removal. Hence, for optimization of sewage treatment processes, a fundamental understanding of the bacterial sludge community, especially of PAO, is necessary. To date, only a few bacteria of this group have been isolated and characterized; however, they are often of minor relevance for P cycling in full-scale plants (1, 8, 24). Thus, their occurrence in aquatic sediments as well as their role in biotic P transformation is poorly understood and highly controversial (19). In an attempt to specifically characterize PAO in sludge and lake sediment, we used laser microdissection and subsequent phylogenetic characterization to precisely describe members of this bacterial group. Laser microdissection is a powerful technique which has been used previously to analyze eukaryotic cells for a variety of scientific applications (6), e.g., forensics (4), botany (3), pathology (reviewed in reference 7), neurosciences (10), and medical biology (13). It has also been used to analyze pathogenic bacteria in histological tissues, using fluorescently labeled oligonucleotide probes or specific primers (16, 23). To our knowledge, this is the first study using laser microdissection for phylogenetic characterization of specific bacteria in aquatic samples.

Methodological approach.
Activated sludge was obtained from the Wassmannsdorf wastewater
treatment plant (near Berlin, Germany) in January 2006. Sediment
cores were collected (June 2004) from the deepest point of the
oligotrophic Lake Stechlin with a modified Kajak sampler (Uwitech,
Mondsee, Austria) equipped with Plexiglas tubes (diameter, 6
cm). The uppermost sediment (ca. 5 mm) was immediately removed
using a 50-ml plastic syringe equipped with a flexible tube.
To obtain a representative composite sample, six replicate cores
were pooled. Poly-P was detected and quantified in both samples
by
31P nuclear magnetic resonance analyses according to a standard
protocol (
18). Poly-P accounted for 0.81 and 2.97 mg P g
–1 dry weight in Lake Stechlin and activated sludge, respectively.
This high poly-P content provided a good basis for further detection,
separation, and identification of potential PAO in these samples.

Detection of PAO.
Bacterial cells were extracted from activated sludge and sediment
with sodium pyrophosphate (0.1% [wt/vol]) by gentle sonication
and shaking in an overhead shaker (level 3) (Reax 2; Heidolph
GmbH & Co. KG, Schwabach, Germany) for 30 min. After 30
min of sedimentation, the supernatant was removed and sonicated,
and the extracts were fixed in 99% ethanol (1:1 [vol/vol]).
PAO were identified by examination of their intracellular poly-P
granules, using epifluorescence microscopy (magnification,
x400),
after extracts were filtered onto a 0.2-µm polycarbonate
filter (Nuclepore; Whatman, Germany) and stained with 5 µg
ml
–1 DAPI (4',6'-diaminido-2-phenylindole dihydrochloride)
for 30 min. At this high DAPI concentration, poly-P granules
show an intensive yellow fluorescence (
36). This characteristic
attribute was our criterion for cell separation via laser microdissection,
which was performed within 48 h after DAPI staining.

Collection of PAO by laser microdissection.
Laser microdissection and pressure catapulting were performed
with a P.A.L.M. MicroBeam system in combination with a P.A.L.M.
RoboMover controlled by P.A.L.M. RoboSoftware v2.0 (P.A.L.M.
Microlaser Technologies GmbH, Bernried, Germany). The MicroBeam
system included an Axiovert 200 M microscope (Carl Zeiss Microimaging
GmbH, Germany) equipped with a 100-W mercury lamp. The polycarbonate
filters were inspected with an FS18 fluorescence filter (P.A.L.M.
Microlaser Technologies GmbH, Bernried, Germany) at a magnification
of
x400. Single bacterial cells with blue fluorescence and yellow
poly-P granules were excised and catapulted into the lid of
a 200-µl adhesive-cap tube (P.A.L.M. Microlaser Technologies
GmbH, Bernried, Germany) under sterile conditions. For Lake
Stechlin sediment, PAO were spread across the whole filter,
from which ca. 100 individual cells were selected for phylogenetic
analyses. In contrast, sewage sludge PAO often occurred in microcolonies
and aggregates, and hence, more cells could be separated (>400
cells). Although single cells were excised, all cells of each
sample were pooled in one tube for further phylogenetic analysis.
As negative controls, pieces of a sterile polycarbonate filter
were used.

DNA extraction and PCR.
All molecular work was performed under sterile conditions in
a laminar flow biosafety cabinet. Before and after work, the
cabinet and all instruments were irradiated for several hours
with UV light and cleaned with DNA decontamination reagent (DNA-ExitusPlus;
AppliChem, Darmstadt, Germany). To ensure that no contaminating
cells or DNA was present in the Palm adhesive caps or PCR tubes
(Biopur Safe-Lock; Eppendorf, Hamburg, Germany), empty tubes
were tested by PCR with universal bacterial 16S rRNA gene primers
as described below. Since we obtained positive amplification
from negative PCR controls for DNA extracted with a Qiagen Micro
kit (Qiagen, Hilden, Germany) especially designed for laser
microdissection, we extracted DNA from each sample directly
in the capture cups. For this purpose, a sterile water-buffer
solution was pipetted into the lids of adhesive-cap tubes containing
either microdissected cells or negative controls.
With very small amounts of extracted DNA, contaminating DNA can have a large effect on PCR-based analyses. Therefore, in order to avoid this potential problem, DNA extraction and PCR buffers were incubated with DNase I (0.5 to 0.75 U) at room temperature for 12 min, followed by DNase inactivation by incubation at 95°C for 15 min.
In order to directly extract DNA, a DNase I-digested water-buffer solution was pipetted into the lids of adhesive-cap tubes with either microdissected cells or negative controls. The water-buffer solution contained 10 µl water, 1 µl 10x PCR buffer (Qiagen, Hilden, Germany), and 0.75 U DNase I (AppliChem, Darmstadt, Germany) per sample. After pipetting of the DNase I-digested water-buffer solution onto the sample, the adhesive-cap tubes were vortexed with the lid upside down. Further cell-cracking steps were as follows: (i) centrifugation for 2 min at 16,000 x g, (ii) sonication for 2 min, (iii) centrifugation for 5 min at 16,000 x g, and (iv) freeze-thaw. Steps i to iii were performed at room temperature. For the freeze-thaw step, the tubes were incubated three times each for 3 min at 98°C and then in liquid nitrogen.
PCR amplification of 16S rRNA genes was performed using the bacterial primer pair 341f and 907r (30) (MWG Biotech AG, Ebersberg, Germany), as this primer set allows for a rough but rapid phylogenetic analysis. The reaction mixture contained 250 nM of each primer, 0.5 µg of bovine serum albumin (Roth, Karlsruhe, Germany), a 200 µM concentration of each deoxynucleoside triphosphate (Bioline GmbH, Luckenwalde, Germany), 2 mM MgCl2, 1 µl of 10x PCR buffer, and 0.2 U HotStarTaq DNA polymerase (Qiagen, Hilden, Germany). Ultrapure water (Bioline GmbH, Luckenwalde, Germany) was used to bring the reaction volume up to 11 µl. Note that to avoid contamination by traces of Escherichia coli DNA, the PCR master mix was incubated with DNase I (final concentration, 0.51 U) at room temperature for 12 min, followed by a DNase inactivation and polymerase activation step at 95°C for 15 min.
Four microliters of the cell-cracked sample and 11 µl of the DNase-digested PCR master mix were transferred into a new sterile Biopur Eppendorf cap. Both the DNase-digested water-buffer solution and PCR master mix were checked for PCR amplification without the addition of template. Solutions showing DNA amplification were discarded. PCR amplification was performed in a Gradient Cycler PT200 instrument (MJ Research), using the following thermal profile: (i) initial denaturation at 95°C for 2 min, (ii) denaturation at 95°C for 1 min, (iii) annealing at 55°C for 40 s, (iv) extension at 72°C for 1 min 30 s (48 cycles of steps 2 to 4 were performed), and (v) final extension at 72°C for 10 min. Cooling down at 4°C completed the reaction.
The greater number and increased cell size of separated sewage sludge bacteria led to more extracted and amplified DNA than that obtained from Lake Stechlin sediments.
We also tested primer pair 8f and 907r to try to maximize PCR product length; however, this primer pair did not amplify Lake Stechlin sediment-derived DNA. For sewage sludge DNA, both primer pairs revealed PCR products but gave different results for phylogenetic community composition. Whereas members of the Alphaproteobacteria, Actinobacteria, and Firmicutes were detected with both primer sets, the primer pair 8f-907r additionally revealed the occurrence of members of the Betaproteobacteria and Bacteroidetes. This bias might be caused by differences in primer selectivity of the forward primer 8f (14), although detection of similar clones in Lake Stechlin sediment suggests that they may have a role in poly-P accumulation.
Other sources of error may have been (i) the undefined efficiency of the above-described DNA extraction procedure for environmental samples and (ii) the small amounts of PCR products obtained, especially for sediment samples, despite optimizing the PCR cycle number. Nevertheless, the protocol described above proved to be sufficient for cloning and subsequent phylogenetic analysis of microdissected cells, whether they were derived from sludge or sediment.

Clone libraries.
For construction of clone libraries, the 16S rRNA gene PCR products
obtained using the primer pair 341f and 907r were purified using
SureClean (Bioline GmbH, Luckenwalde, Germany) and then cloned
into competent
E. coli cells, using pGEM-T Easy vector system
II (Promega GmbH, Mannheim, Germany) according to the manufacturer's
protocol. For each clone library, over 100 clones were picked
and the plasmids purified with the Wizard Plus Minipreps DNA
purification system (Promega GmbH, Mannheim, Germany). Clones
with an incomplete insert and sequences of poor quality were
excluded from further analyses. Between 72 and 84 clones of
each clone library were processed for sequencing.

Sequencing and phylogenetic classification of PAO.
For sequencing, a Big Dye Terminator v3.1 cycle sequencing kit
and an ABI Prism 3100-Avant genetic analyzer (Applied Biosystems,
Darmstadt, Germany) were used. The cloned 16S rRNA gene fragments
were partially sequenced (about 400 to 550 bp for most and 250
to 300 bp for some) with primer M13f (
29) or 341f. Phylogenetic
classifications were determined by BLAST searches (
http://www.ncbi.nlm.nih.gov/BLAST)
and the sequence classification tool of the Ribosomal Database
Project RDP II (
http://rdp.cme.msu.edu/). Clone libraries constructed
for PAO found in both sewage sludge and Lake Stechlin sediment
were dominated by members of the
Alphaproteobacteria and
Actinobacteria (Fig.
1; Table
1). In Lake Stechlin sediment, a more diverse
population of PAO was found, which is in accordance with the
fact that sediments consist of different microniches and zones
(
37). In contrast, sewage sludge is more adept at harboring
specific microbial communities due to its more uniform composition
and conditions. In both samples, relatively large numbers of
Propionibacterium-related sequences were found. Poly-P storage
has been described previously for some members of the genus
Propionibacterium and for the genus
Paracoccus, which dominated
the sludge clone library. Other known poly-P bacteria detected
belonged to the genera
Micrococcus,
Rhodococcus,
Corynebacterium,
and
Staphylococcus (present in the sewage sludge), as well as
Pseudomonas (present in Lake Stechlin sediment). Since the full
genome of "
Candidatus Accumulibacter phosphatis" was recently
studied by metagenomic analysis of an enrichment culture from
enhanced biological phosphorus removal sludge (
27), it was slightly
surprising that no trace of this bacterial group was found.
View this table:
[in this window]
[in a new window]
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TABLE 1. Phylogenetic classification of clones (obtained with primer pair 341f-907r) from sewage sludge and Lake Stechlin sedimenta
|
In summary, our study shows that PAO could be identified in
both samples by use of laser microdissection and subsequent
phylogenetic analyses of the excised cells. In addition to already
known PAO, several new bacterial taxa for which poly-P storage
has not yet been described were identified in the lake sediment
clone library (e.g., the genera
Hyphomicrobium and
Microbacterium).
However, the phylogenetic characterization of PAO is an indication
of the potential ability to store poly-P and cannot be taken
as evidence of a real physiological role. In this study, intracellular
poly-P was visualized by DAPI staining only. Therefore, additional
studies, preferably with cultures, are necessary to verify the
intracellular presence and formation of poly-P in these bacterial
groups. Furthermore, the development and use of highly specific
oligonucleotide probes (e.g., for FISH) would reveal more detailed
information about the relative abundances of PAO in natural
bacterial communities and their contribution to the actual poly-P
content measured. Nevertheless, our clone libraries suggest
that poly-P accumulation by sewage sludge and sediment bacteria
is a common feature to both bacterial communities.
The Palm MicroBeam technology coupled with molecular analyses is an excellent approach for the phylogenetic classification of hitherto unknown putative PAO from environmental samples.
Our results show that this approach is a powerful tool for direct identification of uncultured sediment and other aquatic bacteria with microscopically detectable specific features. This approach has the potential to select different bacterial groups from a variety of environments by using markers other than intracellular poly-P granules, e.g., intracellular polyhydroxybutyrate, FISH probes (medical microbiology [23]), or specific radiolabeled substrates (microautoradiography).

Nucleotide sequence accession numbers.
The partial sequences of 16S rRNA genes obtained in this study
were deposited in GenBank under accession numbers EU152875 to
EU152914, EU196056 to EU196127, and EU515684 to EU515781.

ACKNOWLEDGMENTS
We thank Peter Schmieder for his assistance with the nuclear
magnetic resonance investigations carried out at the Research
Institute of Molecular Pharmacology, Berlin, Germany. We also
thank Christiane Herzog for her valuable help with chemical-analytical
work and Monika Degebrodt for running the sequencer. Sylvia
Schnell and Udo Jäckel (Institute of Applied Microbiology,
University of Giessen) contributed to this study by their inspiring
discussions. Gabriele Friedemann (Palm Microlaser Technologies
GmbH, Bernried, Germany) is thanked for her introduction to
P.A.L.M. MicroBeam technology and for usage of P.A.L.M. instruments.

FOOTNOTES
* Corresponding author. Mailing address for Stefan Ratering (microdissection): Institute of Applied Microbiology, Justus-Liebig University Giessen, Heinrich-Buff-Ring 26-32, D-35392 Giessen, Germany. Phone: 49 641 9937354. Fax: 49 641 9937359. E-mail:
Stefan.Ratering{at}agrar.uni-giessen.de. Mailing address for Michael Hupfer (poly-P bacteria): Leibniz Institute of Freshwater Ecology and Inland Fisheries, Müggelseedamm 310, D-12587 Berlin, Germany. Phone: 49 30 64181 605. Fax: 49 30 64181682. E-mail:
hupfer{at}igb-berlin.de 
Published ahead of print on 2 May 2008. 
Present address: DOE Joint Genome Institute, 2800 Mitchell Dr., Walnut Creek, CA 94598. 

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Applied and Environmental Microbiology, July 2008, p. 4231-4235, Vol. 74, No. 13
0099-2240/08/$08.00+0 doi:10.1128/AEM.02545-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.