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Journal of Bacteriology, October 2003, p. 6220-6223, Vol. 185, No. 20
0021-9193/03/$08.00+0 DOI: 10.1128/JB.185.20.6220-6223.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Metagenomic Analyses of an Uncultured Viral Community from Human Feces
Mya Breitbart,1 Ian Hewson,2 Ben Felts,3 Joseph M. Mahaffy,3 James Nulton,3 Peter Salamon,3 and Forest Rohwer1,4*
Department of Biology,1
Department of Mathematical Sciences,3
Center for Microbial Sciences, San Diego State University, San Diego, California 92182,4
Department of Biology, University of Southern California, Los Angeles, California 900892
Received 12 May 2003/
Accepted 31 July 2003

ABSTRACT
Here we present the first metagenomic analyses of an uncultured
viral community from human feces, using partial shotgun sequencing.
Most of the sequences were unrelated to anything previously
reported. The recognizable viruses were mostly siphophages,
and the community contained an estimated 1,200 viral genotypes.

TEXT
The human gut is colonized by an abundant, active, and diverse
microbiota. This microbiota has been studied extensively using
culture-based assays and, more recently, by a variety of molecular
methods including fluorescent in situ hybridization, terminal
restriction fragment length polymorphism, membrane assays, microarrays,
and direct sequencing of 16S libraries (
15,
18,
22,
25,
32,
35). These studies have shown that there are 400 to 500 human
intestinal microbial species, with 30 to 40 species accounting
for 99% of the total population (
9,
15,
23,
24,
35).
Bacteriophages likely exert a strong influence on the diversity and population structure of bacterial communities in the human gut. Phages that infect Escherichia coli, Salmonella spp., and Bacteroides fragilis have been isolated from human fecal samples at concentrations ranging from 0 to 105 phages per g of dry feces (4, 7, 11-14, 16, 17, 19, 26, 28, 33). The presence and concentration of these phages differed among individuals and did not correlate with age or sex (12). Studies have also demonstrated prophage induction from human fecal bacteria upon treatment with DNA-damaging agents (5, 20). However, direct counts of bacteriophages in human feces have not been performed, nor have molecular methods been applied to study the phage populations. Here we present the first metagenomic analyses of the composition and population structure of an uncultured viral community from human feces.
Approximately 500 g of freshly voided fecal matter was collected from a 33-year-old healthy male. The fecal matter was resuspended in 5 liters of phosphate-buffered saline and shaken vigorously to dislodge the viral particles from the feces. The supernatant was poured through a Nitex filter (
100-µm pore size) and then concentrated using a 100-kDa tangential flow filter. The concentrate was loaded onto a cesium chloride step gradient and ultracentrifuged, and the 1.35- to 1.5-g ml-1 fraction was collected. A portion of the viral concentrate was stained with 1x SYBR Gold (Molecular Probes) for 10 min and visualized by using epifluorescent microscopy (Fig. 1). When stained with SYBR Gold, prokaryotic cells are extremely bright and have visible morphologies. Prokaryotic cells are easily distinguishable from viruses, which are distinct pinpricks of light. As shown in Fig. 1, the purified viral concentrate was not contaminated with any microbial cells.
DNA was extracted from the viral concentrate by using formamide
and cetyltrimethylammonium bromide extractions (
31). A linker-amplified
shotgun library was then created from the human fecal viral
DNA as described previously (
3; M. Breitbart, B. Felts, S. Kelley,
J. M. Mahaffy, J. Nulton, P. Salamon, and F. Rohwer, submitted
for publication;
www.sci.sdsu.edu/PHAGE/LASL/index.htm). Briefly,
the total viral community DNA was randomly sheared (HydroShear;
GenMachine, San Carlos, Calif.) and end repaired and double-stranded
DNA linkers were ligated to the ends. The fragments were amplified
using Vent DNA polymerase, ligated into the pSMART vector (Lucigen,
Middleton, Wis.), and electroporated into MC12 cells. This method
overcomes limitations of modified nucleotides and bactericidal
genes in viral genomes.
With the use of the AmpL2 forward primer (Lucigen), 532 clones from the library were sequenced (accession no. CC820769 to CC821300). TBLASTX comparison of these sequences against those in GenBank revealed that the majority (59%) of the sequences were not significantly similar (E value of <0.001) to anything previously reported (1, 2) (Fig. 2A). Sequences with significant hits were classified as phages, viruses, mobile elements, repeat elements, Bacteria, Archaea, or Eucarya based on GenBank annotation. Bacterial, archaeal, and eukaryotic hits were examined manually to identify repeat elements and potential prophages.
The most common known matches were with bacteria. We do not
believe that the presence of these sequences was due to contamination.
Cesium chloride purification efficiently separates the viral
particles from the prokaryotic cells and free DNA. As shown
in Fig.
1, no contaminating microbial cells were present in
the viral concentrate. Additionally, the open reading frames
in purified cultured phage genomes are often more similar to
bacterial open reading frames than to those of other phages
(see, e.g., references
27 and
30). For several clones from a
previously sequenced marine viral library which were sequenced
from both ends, the match with one end of the clone was with
a phage while the other end of the same clone had a significant
match with a bacterial sequence (
3). These bacterial hits may
also represent uncharacterized prophages or sequences from transducing
phages.
Significant matches with phages were the second most abundant category in the uncultured fecal library (Fig. 2B). Among the phage matches, the majority (81%) were with siphophages and prophages within bacterial genomes. Since many of these phages have the ability to be temperate, this suggests that temperate lifestyles may be important in the human colon. The most common phage matches were with bacteriophage A118 of Listeria monocytogenes, bacteriophage E125 of Burkholderia thailandensis, and bacteriophage bIL285 of Lactococcus lactis. Fifty-three percent of the phage matches were with known proteins, with structural proteins and terminases being the most common (Table 1). Several matches with mobile elements (plasmids, transposons, and insertion sequences) were also observed in the library (Fig. 2C).
In order to determine the genome size distribution of uncultured
bacteriophages in the human fecal sample, a fraction of the
viral concentrate was examined using pulsed-field gel electrophoresis
as described previously (
10). Major bands for the fecal phage
population were observed at 15 and 90 kb, with minor bands present
at 30, 40, and 60 kb (Fig.
3). This genome size distribution
was significantly different than that observed in other environments
(e.g., seawater, sediment, and rumen) (
10,
21,
36). Especially
notable was the presence of a dominant band at a small genome
size of approximately 15 kb.
The structure of the uncultured fecal viral population was determined
by running Monte Carlo simulations to match the distribution
of overlapping sequences (i.e., the contig spectrum) observed
in the library (
3; Breitbart et al., submitted). Among the 532
sequences from the fecal viral library, there were 18 contiguous
sequences made up of two sequences (2-contigs), two 3-contigs,
and two 4-contigs, as well as 482 sequences that did not overlap
with any other sequences (1-contig). Based on the normalized
band intensities of the pulsed-field gel, an average genome
size of 30 kb was assumed for the population modeling. The population
structure of the uncultured fecal viral community was assumed
to follow a power law distribution (
ni = ai-b, where
ni is the
relative frequency of the
ith genotype,
a is relative abundance
of the most abundant genotype,
i is the rank index from 1 to
the total number of genotypes, and
b is the evenness parameter)
based on previous results (
3). Using these assumptions, and
an average fragment size of 699 bp, we calculated that the viral
community contained

1,200 genotypes. The most abundant virus
made up

4% of the total (Table
2). The nonparametric estimator
Chao1 predicted 162 genotypes for this population (
3,
6). Effects
of varying the average genome size on the model predictions
are shown in Table
2. Based on previous estimates of gut microbial
diversity, there are approximately two to five times as many
viral genotypes as the number of bacterial species in the human
intestinal microbiota. Diversity of the fecal viral community
was high, with a Shannon index (
Hnats) of 6.4 nats. This is
a higher Shannon index than that observed for most microbial
communities, but it is lower than the Shannon index found for
uncultured viral communities in seawater (
Hnats =

7) and sediment
(
Hnats > 9) samples (
3; Breitbart et al., submitted).
In all the uncultured viral communities examined so far (
3;
Breitbart et al., submitted), the majority of the sequences
have been novel. Similarly, culture-based studies of phage genomes
have found that much, if not most, of phage diversity is unsampled
(
27,
29). Significant matches with each of the major groups
of double-stranded DNA phagessiphophages, podophages,
and myophageshave been observed in all uncultured viral
communities studied. Siphophages and prophages were more abundant
in the marine sediment (Breitbart et al., submitted) and fecal
libraries than in the seawater libraries (
3). The fecal viral
community contained very few matches with the T7-like podophage
and

-like siphophage, which were the most abundant groups in
the marine viral communities (Breitbart et al., submitted).
In the fecal viral community, numerous similarities to phages
that are known to infect gram-positive bacteria were seen. This
can be explained by the fact that over 62% of the cells in human
feces that can be detected with a bacterium-specific probe belong
to gram-positive groups (
15).
Understanding the population structure and dynamics of the normal human intestinal microbiota has important implications for human health, nutrition, and the development of probiotics for the treatment and prevention of gastrointestinal disorders. Since phages found in human feces are used as indicators of sewage-derived contamination of the environment, it is also important to know the identity of these phages (12). Phages likely influence the composition of bacterial populations in the intestine through specific predation on microbial hosts. As a particular strain becomes dominant, phages can infect and lyse that host, allowing another bacterial strain, either of endogenous or exogenous origin, the opportunity to become abundant (34). Additionally, through lysogenic conversion of resident intestinal bacteria, phages may introduce new phenotypic traits, such as antibiotic resistance and the ability to produce exotoxins (8).
Phages are a diverse and largely unexplored component of the microbial community in human feces. Using culture-independent methods, we have described the population structure and genome size distribution of a human fecal phage community. In addition, we have elucidated the identity of some of these uncultured phages based on sequence similarities. Future studies need to focus on determining the ecological roles of phages in the human intestine and the degree of similarity of the phage communities populating different individuals.

ACKNOWLEDGMENTS
We thank Anca Segall, Beltran Rodriguez Brito, David Bangor,
and Linda Wegley for helpful comments on the mathematical modeling
and manuscript, as well as the San Diego State University Microchemical
Core Facility for sequencing.
This work was supported by NSF DEB 03-16518 and DEB-BE0221763.

FOOTNOTES
* Corresponding author. Mailing address: San Diego State University, 5500 Campanile Dr., San Diego, CA 92182-4614. Phone: (619) 594-7829. Fax: (619) 594-5676. E-mail:
forest{at}sunstroke.sdsu.edu.


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Journal of Bacteriology, October 2003, p. 6220-6223, Vol. 185, No. 20
0021-9193/03/$08.00+0 DOI: 10.1128/JB.185.20.6220-6223.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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