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Applied and Environmental Microbiology, April 2002, p. 2049-2053, Vol. 68, No. 4
0099-2240/02/$04.00+0 DOI: 10.1128/AEM.68.4.2049-2053.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Molecular Biological Detection and Characterization of Clostridium Populations in Municipal Landfill Sites
M. I. Van Dyke* and A. J. McCarthy
School of Biological Sciences, University of Liverpool, Liverpool L69 7ZB, United Kingdom
Received 27 July 2001/
Accepted 3 January 2002

ABSTRACT
Primer sets specific for 16S rRNA genes were designed for four
phylogenetic groups of clostridia known to contain mesophilic
cellulolytic species. Specific amplification of these groups
from landfill leachate DNA extracts demonstrated the widespread
occurrence of clostridia from the
Clostridium thermocellum and
C. leptum groups. In contrast, the
C. botulinum group was never
detected, and the
C. coccoides-
C. lentocellum group was only
occasionally detected. Amplification products were analyzed
by temporal thermal gel electrophoresis to generate profiles
of the clostridial groups and to identify dominant bands. Sequence
analysis of 17 landfill clones confirmed that the primers were
specific for the clostridial subgroups and that the cloned sequences
had a close relationship with known cellulose-degrading clostridia.
The primers have therefore been authenticated for use in the
rapid identification of clostridia in anaerobic environments.

INTRODUCTION
Anaerobic degradation in landfills involves several coordinated
groups of microorganisms and follows a process that is typical
of waste degradation in anaerobic environments, such as soils,
sediment, and sludge. As the primary stage of waste degradation,
polysaccharide breakdown is an important limiting factor in
anaerobic treatment of waste, which in municipal landfills primarily
involves the decomposition of complexed polymers, including
cellulose, hemicellulose, and lignin. Although cellulose is
an important substrate in landfills, anaerobic degradation is
poorly understood, and our knowledge is based on studies using
culture-based methods (
1). The enumeration of cellulolytic bacteria
in landfills has often resulted in low cell counts (
10), suggesting
that culture-based methods may be underestimating bacterial
numbers. The aim of the study reported here was to use information
on 16S rRNA gene sequences to develop tools for the specific
detection of cellulose-degrading bacteria in landfill sites.
It is likely that anaerobic cellulose degradation in landfills is due primarily to bacteria related to the genera Clostridium and Eubacterium. Although very few cellulolytic strains have been isolated from landfill sites, Westlake et al. (15) have identified isolates related to these two groups. The genus Clostridium and its relatives constitute an ancient group whose members exhibit a wide range of phenotypic characteristics. Phylogenetic analysis of 16S rRNA genes shows that the group is very diverse, with deeply branching clusters that include nonclostridial species. Comparison of 16S rRNA genes has allowed the division of the genus Clostridium into subgroups, and cellulose-degrading representatives from genera such as Clostridium, Eubacterium, and Ruminococcus can be found in a number of different clusters (3). However, mesophilic cellulose-degrading strains tend to be found in groups I, III, IV, and XIVab, with group III comprising only cellulose-degrading strains to date.
Consequently, we have exploited this clostridial 16S rRNA database to investigate the presence of clostridial subgroups I, III, IV, and XIVab in landfill sites. Although these subgroups do not comprise solely cellulose-degrading species, their detection could be used to indicate the distribution of saccharolytic and proteolytic degrading bacteria in landfill sites. Specific PCR amplification, temporal thermal gel electrophoresis (TTGE), and sequence analysis are used to detect and profile these key groups of clostridia. TTGE can separate DNA fragments of the same length but with different sequence compositions. When combined with specific or nonspecific gene amplification, this method can rapidly profile the genetic diversity of microbial populations.
Pooled leachate samples from sites designated R, C, H, W, B, P, and S were obtained from landfill sites in the northwest of England that contained primarily municipal solid waste and were provided by UK Waste Ltd., Terry Adams Ltd., and Cleanaway Ltd. Samples So and Br were obtained from test cell reactors containing municipal solid waste and were provided by J. Wayne, Centre for Applied Microbiology Research, Porton Down, United Kingdom, and the Energy Technology Support Unit. One-liter samples of leachate were concentrated by centrifugation at 27,000 x g for 40 min, and the solids were resuspended in 20 ml of 0.1 M K2HPO4 buffer. Aliquots derived from 75 ml of leachate were harvested by centrifugation at 16,000 x g for 5 min, and the pellets were stored at -70°C. A sample from sheep rumen, provided by D. Mercer, Rowett Research Institute, Aberdeen, United Kingdom, was used as a control known to contain a high concentration of cellulolytic bacteria. One-milliliter samples of rumen fluid were concentrated by centrifugation at 16,000 x g for 5 min, and the pellets were stored at -70°C. Reference strains and their sources are listed in Table 1.
DNA was extracted from pure cultures with a Hybaid Ribolyser.
A 2-ml tube containing 0.5 g of glass beads (0.17- to 0.18-mm
diameter), 0.5 ml of 0.12 M K
2HPO
4 (pH 8.0), 0.5 ml of saturated
phenol (pH 8.0), and 0.5 ml of cell suspension was processed
at 6 m/s for 30 s, placed on ice for 5 min, and centrifuged
at 16,000
x g for 5 min. Supernatant was extracted with phenol-chloroform-isoamyl
alcohol (25:24:1) and chloroform-isoamyl alcohol (49:1), and
the DNA was precipitated with ethanol and then resuspended in
water. Cell lysis and DNA purification from landfill leachate
and rumen samples were performed with the Bio 101 FastDNA SPIN
kit for soil (Anachem). Cell pellets were resuspended in 0.2
ml of 0.1 M K
2HPO
4 buffer and lysed with a Hybaid Ribolyser.
The DNA was recovered and purified according to the Bio 101
protocol and purified further using the Wizard DNA Clean-up
system (Promega).
16S rRNA gene sequences were obtained from the Ribosomal Database Project (RDP) (9) and GenBank (2) databases. The sequences were aligned using the Genetics Computer Group Wisconsin package version 8.1 and the Genetic Data Environment. Primer sequences were located visually, and the suite was completed using published oligonucleotides (Table 2). Primers were tested for specificity using the RDP CHECK_PROBE and FASTA (11) search programs. The appropriate annealing temperatures (Table 2) and primer specificities were determined using pure-culture control strains (Table 1).
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TABLE 2. Primer sequences and targets used to amplify 16S ribosomal DNA of Clostridium groups and for TTGE analysis
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PCR amplification of DNA with
Clostridium group primers was
performed as follows with (per 50-µl reaction volume)
50 ng of genomic DNA, 10 pmol of each primer, 2 U of SuperTaq
(HT Biotechnologies), 10 mM Tris-HCl (pH 9.0), 1.5 mM Mg
2+,
50 mM KCl, 0.01% (wt/vol) gelatin, 0.1% Triton X-100, and 200
µM deoxynucleoside triphosphate. Reaction mixtures were
covered with mineral oil, denatured for 5 min at 94°C before
addition of the polymerase, and cycled at 94°C for 1 min,
at the appropriate annealing temperature (Table
2) for 1 min,
and at 72°C for 1 min. This cycle was repeated 40 times
for the amplification of clostridial DNA from landfill leachate
and rumen and 30 times for the amplification of DNA from pure
cultures, followed by a final incubation at 72°C for 5 min.
Products were visualized by agarose gel electrophoresis and
ethidium bromide staining. Products from amplification of landfill
DNA with group-specific primers were purified using the Qiagen
PCR Preps Purification kit, cloned into the pGEM-T vector (Promega),
and transformed into
E. coli JM109 competent cells (Promega).
TTGE analysis was performed to profile amplified DNA from each Clostridium group and to compare banding patterns with clones derived from the amplification products. A nested PCR protocol was used, in which amplification products from Clostridium group-specific primers were reamplified using the TTGE primer sets (Table 2). The amplification reaction mixtures were as described above, with 2 ng of group-specific PCR product as a template and 25 amplification cycles at the annealing temperatures listed in Table 2.
Products from amplification with the TTGE-1 and TTGE-2 primers were separated using the Bio-Rad TTGE system. The gels comprised 6% acrylamide (37:1 acrylamide- bisacrylamide), 7 M urea, 20% formamide, 2% glycerol, and 1.25x TAE buffer (50 mM Tris, 25 mM acetic acid, 1.25 mM Na2EDTA [pH 8]) (TAE buffer). Approximately 20 to 30 ng of pure-culture or clone product and 200 ng of landfill leachate product were run at 75 V from 43 to 51°C (0.5°C h-1) for 16 h. The gels were stained using SYBR Green I nucleic acid gel stain (Flowgen) that was diluted 1:50,000 (vol/vol) in 1.25x TAE buffer (pH 8) and were visualized using a Storm 860 optical scanner and ImageQuant software (Molecular Dynamics). DNA preparations from pure cultures were used to optimize the conditions for band separation. TTGE profiles from each landfill site were repeated at least twice, and the banding patterns were shown to be reproducible. Representative TTGE profiles for each Clostridium group are shown in Fig. 1.
As exemplified in Fig.
2, clones that gave bands of the same
mobility as the most intense bands in landfill leachate (Fig.
1) were selected for sequencing. The entire cloned insert, obtained
using group-specific primers, was sequenced. Plasmids containing
cloned inserts were purified using the Qiagen plasmid miniprep
kit. Sequence analysis was performed using the ABI 373 (Perkin-Elmer)
and Li-Cor 4200 (MWG-Biotechnology) sequencing systems. Further
16S rRNA gene sequence data were obtained from the RDP. Sequence
alignments were manipulated using the Wisconsin package version
8.1 and the Genetic Data Environment. Phylogenetic analysis
was performed with TREECON for Windows version 1.3b (
14) using
the Jukes-Cantor (
6) and neighbor-joining (
13) distance calculations,
with bootstrap analysis performed on 100 replicates.
No amplification products were obtained from any landfill leachate
or rumen fluid samples with primers specific for the
Clostridium botulinum group (cluster I), indicating that this group of bacteria
is either not present or present in low numbers. Similarly,
Franks et al. (
4) could detect only low numbers of cells that
hybridized to the cluster I probe in human fecal samples.
Both forward and reverse primers for the C. thermocellum group (cluster III) were designed in this study. Cluster III primers amplified DNA from landfill leachate from each site tested and from rumen fluid, resulting in gene products of the expected size. The C. thermocellum group is of particular interest in the context of this work, as to date it contains sequences only from cellulose-degrading strains isolated from environmental sources (8). TTGE analysis of Clostridium cluster III 16S rRNA genes showed that there were one or two dominant species present in each landfill site (Fig. 1). Phylogenetic analysis showed that the dominant sequences amplified from landfill sites grouped within sequences from pure-culture representatives (Fig. 3). Only one clone from site S (g3S5) was closely related to the thermophile C. thermocellum. Cloned sequences from sites H (g3H3) and Br (g3Br5) were similar to C. aldrichii and its relatives, and the remaining five clones from sites P, C, B, W, and R (g3P17, g3C1, g3B5, g3W3, and g3R1) were most closely related to the group containing C. papyrosolvens.
The
C. leptum group (cluster IV) is closely related to cluster
III (
3) and contains representatives of a mixture of genera,
including
Clostridium,
Eubacterium, and
Ruminococcus. A number
of species are mesophilic and cellulolytic, including
Ruminococcus albus,
Ruminococcus flavefaciens, and
C. cellulosi. Noncellulolytic
strains are also present, although many of these will degrade
other polysaccharides (
12). Using primers specific for the
C. leptum group, amplification products were obtained from the
rumen fluid sample and from all landfill sites except R and
S. TTGE profiles showed that the landfill sites contained one
or two strong bands with a number of less intensely stained
bands (Fig.
1). Cloned sequences from landfill sites grouped
among sequences from pure-culture representatives of this phylum
(Fig.
4), confirming the specificity of the primer set. Clones
from sites C, H, P, and Br (g4C2, g4H4, g4P3, and g4Br3) were
all closely related to
Eubacterium plautii and
Sporobacter termitidis,
clone g4H6 was closely related to
Ruminococcus bromii, clone
g4W7 was closely related to
Fusobacterium prausnitzii, and g4B3
was not closely related to any of the
Clostridium cluster IV
pure-culture sequences. Clones from site So were not analyzed.
Clostridium cluster XIVab is a large and diverse group, with
isolates having both high and low G+C contents. Cluster XIVab
contains both cellulolytic and noncellulolytic members from
both human and animal gut, rumen, and environmental sources.
In a study of human feces, bacteria that hybridized to the cluster
XIVab probe Erec482 constituted 29% of the total (
4). We could
amplify cluster XIVab genes from sheep rumen fluid but could
detect this group in only one landfill site sample (site Br).
The clone from landfill site Br (g14Br5) was found to group
within XIVa and was most closely related to
C. aminovalericum (not shown).
The results of this study show that among the clostridial and eubacterial groups tested, the predominant and most ubiquitous groups in landfill are C. thermocellum (cluster III) and C. leptum (cluster IV). These two groups are closely related, suggesting that they may have derived from a precursor organism that was adapted to the conditions found in many anaerobic environments. For example, the low nitrogen availability in environments such as soils and landfills may select for bacteria with nitrogenase activity, which is present in the cluster III cellulose-degrading strains C. papyrosolvens and C. cellobioparum (8). The C. thermocellum group contains 16S rRNA sequences from only cellulolytic bacteria, which suggests that this phenotype is typical of the group. It is tempting to speculate that sequences obtained using cluster III-specific primers will also have a cellulolytic phenotype. However, to determine if the 16S rRNA sequences obtained using group-specific primers are saccharolytic, they will need to be compared with sequences from metabolically characterized pure-culture isolates.
Molecular detection of clostridial populations has been described for the human intestinal tract (4), but never in a landfill environment. Improved understanding of the microbial populations of landfill sites, and especially the groups involved in initial hydrolysis of waste material, can lead to an overall scheme to monitor bacterial populations involved in anaerobic degradation in situ. Changes in group-specific amplification and TTGE banding patterns can be used to monitor changing conditions at the landfill sites, especially if these techniques target RNA to give some measure of activity. The techniques will also have application to other areas of anaerobic ecology, such as soils, sediments, anaerobic digestors, and the gastrointestinal tract. The application of molecular biological techniques will therefore contribute to our understanding of the ecology of an important bacterial group that has been difficult to isolate and monitor.

Nucleotide sequence accession numbers.
The GenBank accession numbers for the nucleotide sequences are
AF401533 to
AF401549.

ACKNOWLEDGMENTS
This research was funded by the EPSRC/NERC (United Kingdom)
Waste and Pollution Management Programme.
We are grateful to Paul Loughnane for technical assistance.

FOOTNOTES
* Corresponding author. Present address: GAP EnviroMicrobial Services, 1020 Hargrieve Rd., Unit 14, London, Ontario, Canada N6E 1P5. Phone: (519) 681-0571. Fax: (519) 681-7150. E-mail:
mvandyke{at}gapenviromic.com.


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Applied and Environmental Microbiology, April 2002, p. 2049-2053, Vol. 68, No. 4
0099-2240/02/$04.00+0 DOI: 10.1128/AEM.68.4.2049-2053.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
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