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Journal of Clinical Microbiology, December 2004, p. 5565-5570, Vol. 42, No. 12
0095-1137/04/$08.00+0 DOI: 10.1128/JCM.42.12.5565-5570.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
"Bacteroides nordii" sp. nov. and "Bacteroides salyersae" sp. nov. Isolated from Clinical Specimens of Human Intestinal Origin
Y. L. Song,1*
C. X. Liu,1
M. McTeague,2 and
S. M. Finegold3,4,5
Research Service,1
Clinical Microbiology Laboratory,2
Infectious Diseases Section, VA Medical Center West Los Angeles,3
Department of Medicine,4
Department of Microbiology, Immunology, and Molecular Genetics, UCLA School of Medicine, Los Angeles, California5
Received 21 May 2004/
Returned for modification 12 July 2004/
Accepted 13 August 2004

ABSTRACT
Two groups of unknown bacteria, which phenotypically resemble
members of the
Bacteroides fragilis group but phylogenetically
display >5% 16S rRNA gene sequence divergence from their
nearest validly described species,
Bacteroides thetaiotaomicron,
were characterized by phenotypic and molecular taxonomic methods.
Phylogenetically and phenotypically, the unidentified bacteria
displayed a relatively close association with each other. However,
a 16S rRNA gene sequence divergence of approximately 4% between
the two unknown bacteria, as well as distinguishable biochemical
characteristics, demonstrates that these organisms are genotypically
and phenotypically distinct, and each group may represent a
previously unknown subline within the
Bacteroides phylogenetic
cluster. Subsequent DNA-DNA hybridization studies confirmed
that the two novel organisms were indeed distinct from each
other. The previously described species closest to both of them
is
B. thetaiotaomicron (approximately 94% sequence similarity),
but they can be differentiated easily from
B. thetaiotaomicron by virtue of not utilizing trehalose. DNA-DNA pairing studies
also documented the separateness of the unknown species and
B. thetaiotaomicron. Based on the phenotypic and phylogenetic
findings, two new species, "
Bacteroides nordii" sp. nov. and
"
Bacteroides salyersae" sp. nov, are proposed. The G+C content
of the DNA is 41.4 mol% for
Bacteroides nordii and 42.0 mol%
for
Bacteroides salyersae. The type strains of
Bacteroides nordii and
Bacteroides salyersae are WAL 11050 (ATCC BAA-998 or CCUG
48943) and WAL 10018 (ATCC BAA-997 or CCUG 48945), respectively.

INTRODUCTION
The
Bacteroides fragilis group is part of the commensal flora
in humans and is commonly associated with a variety of human
infections, such as intra-abdominal abscesses, wound infections,
and bacteremia (
5).
B. fragilis group bacteremia contributes
significantly to morbidity and mortality (
12). The choice of
antibiotics for therapy is limited because the species of the
B. fragilis group are among the most resistant of all anaerobes
to antimicrobial agents, and this resistance has increased recently
(
4,
14). Although
B. fragilis group species have been found
to be clinically very important, studies of the significance
of isolates of the
B. fragilis group might have been hindered
by an inadequate taxonomy. The taxonomy of
Bacteroides has undergone
significant changes in the past few years (
8). It has recently
been proposed that the genus
Bacteroides be restricted to highly
fermentative species that phenotypically resemble
B. fragilis,
the type species of the genus (
13), and related taxa. The taxonomic
positions of some other species still included in the genus
remain uncertain; all of these species will ultimately be transferred
to other genera. Furthermore, several clinically important species
still await formal description.
In this paper, we report on the characterization of two groups of clinical isolates which were isolated from clinical specimens of human intestinal origin. Phenotypically, the two novel species are very much like Bacteroides stercoris and Bacteroides uniformis, respectively; therefore, they were misidentified as B. stercoris and B. uniformis previously. However, 16S rRNA sequencing reveals approximately 7% sequence divergence between the novel species and B. stercoris or B. uniformis. Phylogenetically, the previously described species closest to both of them is Bacteroides thetaiotaomicron (approximate 94% similarity). Although these two novel species have 96% sequence similarity, they can be distinguished easily by phenotypic characteristics. DNA-DNA hybridization studies also confirmed that these two groups of unknown organisms were indeed distinct from each other and from their nearest valid species B. thetaiotaomicron. Based on the phenotypic and phylogenetic findings presented here, two new species, "Bacteroides nordii" sp. nov. and "Bacteroides salyersae" sp. nov, are proposed. In addition, we also describe the phenotypic tests useful in distinguishing between the organisms mentioned above.

MATERIALS AND METHODS
Bacterial strains and growth conditions.
The study included 12
B. stercoris-like strains (group I); 9
B. uniformis-like strains (group II); 8 strains each of
B. fragilis,
B. stercoris,
B. thetaiotaomicron,
B. uniformis; and 3 strains
each of
Prevotella nigrescens,
Prevotella disiens,
Prevotella corporis,
Prevotella intermedia,
Porphyromonas asaccharolytica,
Porphyromonas endodontalis, and
Porphyromonas gingivalis (Table
1). The novel isolates were recovered from clinical sources
such as peritoneal fluid, appendix tissue, and intra-abdominal
abscess; therefore they are likely of intestinal origin. All
the clinical isolates of each species were identified by 16S
DNA sequencing in our laboratory. All the strains were cultivated
on brucella agar (Difco, Detroit, Mich.) supplemented with 5%
sheep blood and incubated anaerobically at 37°C under N
2 (86%), H
2 (7%)
, and CO
2 (7%) gas phase.
Biochemical characterization.
The strains were characterized biochemically by using a combination
of conventional tests described previously in the Wadsworth
and VPI anaerobe manuals (
7,
9), plus the API ZYM and Rapid
ID 32A systems (API bioMérieux, Marcy l'Etoile, France)
and the RapID ANA II system (Remel, Inc., Lenexa, Kans.), according
to the respective manufacturers' instructions. All biochemical
tests were performed in duplicate. Fermentation tests were performed
using prereduced, anaerobically sterilized peptone-yeast-sugar
broth tubes (Anaerobe Systems, Morgan Hill, Calif.). The strains
were grown in peptone-yeast broth and peptone-yeast-glucose
broth (Anaerobe Systems) for metabolic end product (short-chain
volatile and nonvolatile fatty acids) analysis by gas-liquid
chromatography (
9). Antimicrobial susceptibility studies were
done using various antimicrobial agents, which were selected
either as representative of a class of compound or as drugs
for which MICs for quality control strains were published, by
means of the NCCLS-approved Wadsworth plate dilution method
(
11).
Cellular fatty acid composition.
Long-chain cellular fatty acids were analyzed as previously described (16).
DNA base composition.
The mol percent G+C content of DNA was determined by high-performance liquid chromatography as described by Mesbah et al. (10) except that the methanol content of the chromatographic buffer was decreased to 8% and the temperature was increased to 37°C.
16S rRNA sequencing and phylogenetic analysis.
The 16S rRNA genes were amplified by PCR with universal primers 8UA (positions 8 to 28, Escherichia coli numbering) and 1485B (positions 1485 to 1507) as described previously (2). The amplified product was purified by using the QIAamp PCR purification kit (QIAGEN, Inc., Chatsworth, Calif.) and directly sequenced with a Biotech Diagnostic (Laguna Niguel, Calif.) Big Dye sequencing kit on an ABI 377 sequencer (Applied Biosystems, Foster City, Calif.). The closest known relatives of the new isolates were determined by performing database searches using the BLAST software (1). Almost the full lengths of the 16S rRNA gene sequences (>1,400 nucleotides) of the unidentified bacteria and of closely related bacteria were aligned with CLUSTAL-W (http://genome.kribb.re.kr). A phylogenetic tree was reconstructed with DNA analysis software PAUP*, version 4.0 (Sinauer Associates, Inc., Sunderland, Mass.). The stability of the groupings was estimated by bootstrap analysis (1,000 replications) using the same program.
DNA-DNA reassociation.
DNA-DNA reassociation experiments were carried out according to the spectrophotometric method of De Ley et al. (3), using a Gilford System model 2600 spectrophotometer equipped with a Gilford model 2527-R thermal programmer.
Nucleotide sequence accession numbers.
The 16S rRNA sequences of strains WAL 11050T and WAL 10018T have been deposited in GenBank under accession numbers AY608697 and AY608696, respectively.

RESULTS
The unusual bacteria recovered from clinical infections were
always isolated together with other anaerobes (mostly other
species of the
B. fragilis group) and/or aerobes. The majority
of isolates were recovered in heavy growth on primary isolation.
They were found to be gram-negative, rod-shaped organisms. Typical
cells of both groups were 0.8 to 1.5 µm by 0.5 to 5.0
µm. Colonies on brucella blood agar plates at 48 h were
grey, circular, convex, entire, and opaque and attained a diameter
of 1 to 2 mm. All of the isolates grew well (resistant to 20%
bile) and blackened the
Bacteroides bile esculin agar by hydrolyzing
esculin. They all grew well anaerobically, but no growth occurred
following subculture in air or in atmospheres of 2 or 6% O
2.
All of the strains were resistant to the kanamycin (1,000 µg),
vancomycin (5 µg), and colistin sulfate (10 µg)
special-potency disks. They were indole positive and lipase,
catalase, urease, and nitrate negative. They were capable of
hydrolyzing esculin and gelatin. They all produced acid from
cellobiose, glucose, rhamnose, sucrose, and xylose but did not
produce acid from salicin, trehalose, and xylan. The strains
of group I (
B. stercoris-like) did not utilize arabinose, in
contrast to strains of group II (
B. uniformis-like). Tests with
the API ZYM, Rapid ID 32A, and RapID ANA II systems showed that
all isolates of the same group produced the same profile. Positive
reactions were obtained for ß-galactosidase,

-glucosidase,
ß-glucosidase, alkaline phosphatase, ß-
N-acetyl-glucosaminidase,
leucyl glycine arylamidase, alanine arylamidase, glutamyl glutamic
acid arylamidase, acid phosphatase, naphthol-AS-Bl-phosphohydrolase,
and
p-nitrophenylphosphatase. Mannose and raffinose were fermented
in tests with the Rapid ID 32A system. In tests with the RapID
ANA II system,
p-nitrophenyl-ß,
D-disaccharidase reactions
were strongly positive for isolates of group II but were either
negative or weakly positive for group I. All the other tests
were negative.

-Fucosidase testing by all three systems was
negative. In peptone-yeast broth and peptone-yeast-glucose broth,
a major amount of acetic acid and minor amounts of isovaleric
acid, propionic acid, and formic acid were produced by all isolates.
The principal long-chain cellular fatty acid of the isolates
was C
15:0 ANTEISO FAME (20 to 23% of total). Significant amounts
of C
15:0 ISO FAME, C
16:0 FAME,C
16:0 3OH FAME, and C
18:1 CIS 9 FAME (9 to 18% each of the total) were also present. Agar
dilution tests showed that most of the strains were susceptible
to metronidazole (MIC

4 µg/ml), imipenem (MIC

2 µg/ml),
clindamycin (MIC

4 µg/ml), amoxicillin-clavulanate (MIC

8 µg/ml), and ertapenem (MIC

1 µg/ml). Resistance
to penicillin G (MIC

32 µg/ml), ceftizoxime (MIC

32
µg/ml), cefotetan (MIC

64 µg/ml), and vancomycin
(MIC

128 µg/ml) was shown by all strains. All strains
were ß-lactamase positive.
To assess the genealogical affinity between the unknown bacteria and their relationship with other taxa, their 16S rRNA gene sequences were determined. Pairwise analysis showed all of the isolates of the same group were phylogenetically closely related to each other (>99.5% sequence similarity). Sequence searches of GenBank and Ribosomal Database Project libraries for validly described species revealed that the unknown organisms were members of the Cytophaga-Flavobacter-Bacteroides (CFB) phylum, with a loose affinity with Prevotella and Porphyromonas phylogenetic clusters (>15% sequence divergence from species of the genera Prevotella and Porphyromonas sensu stricto) and a relatively close association with the Bacteroides cluster (>93% sequence similarity). A tree, constructed by the maximum-parsimony method, depicting the phylogenetic affinity of the two novel bacteria as exemplified by strains ATCC BAA-998T and ATCC BAA-997T is shown in Fig. 1; it confirmed the placement of the unknown bacteria in the Bacteroides phylogenetic cluster. It is evident from the branching pattern in the tree that the two novel species possess a significantly close relationship with each other (90% bootstrap resampling value). Chlorobium vibrioforme was used as the outgroup, since this species represents the phylum that branches closest to the CFB phylum (6). Pairwise comparison revealed approximately 4% sequence divergence between the two novel bacteria and approximately 5% sequence divergences between the unknown bacteria and the type strain of their closest valid species, B. thetaiotaomicron, based on almost the full lengths of the 16S rRNA gene sequences (>1,400 nucleotides). Although there is no precise correlation between percentages of 16S rRNA sequence divergence and species delineation, it is now generally accepted that organisms displaying divergence close to 3% or more do not belong to the same species (15). Furthermore, DNA-DNA reassociation values of 30.9, 13.5, and 16.8% for isolates WAL 11050T and WAL 10018T, WAL 11050T and ATCC 29741T (B. thetaiotaomicro type strain), and WAL 10018T and ATCC 29741T, respectively, were observed, thereby confirming that the unidentified bacteria represent two different species. The data clearly show that each group represents a previously unknown subline within the Bacteroides phylogenetic cluster.
Support for the separation of the unknown bacteria from their
related bacterial species also comes from the phenotypic characterizations.
The two groups of unknown organisms can be readily distinguished
from species of the genera
Prevotella and
Porphyromonas by their
resistance to 20% bile. In addition, the unknown bacteria are
highly fermentative, in contrast to asaccharolytic species in
the genus
Porphyromonas and moderately saccharolytic species
in
Prevotella. Within the
Bacteroides phylogenetic cluster,
although two groups of unknown bacteria are very much like each
other in terms of biochemical characteristics, they can be differentiated
from each other by virtue of arabinose fermentation: group II
produces acid from arabinose, in contrast to group I. In addition,
RapID ANA II testing indicated that group II showed a strongly
positive reaction for
p-nitrophenyl-ß,
D-disaccharidase,
whereas group I was either negative or only weakly positive.
The unknown organisms of groups I and II are also similar biochemically
to
B. stercoris and
B. uniformis, respectively; therefore they
were misidentified as
B. stercoris and
B. uniformis previously.
However, organisms of group I can be differentiated from
B. stercoris by several features, such as utilization of cellobiose
but not xylan, production of glutamic acid decarboxylase but
not arginine dihydrolase, and reaction to
p-nitrophenyl-ß,
D-disaccharidase.
Organisms of group II can also be differentiated from
B. uniformis by not producing

-fucosidase,

-arabinosidase, or

-chymotrypsinase,
whereas
B. uniformis does. Although 16S rRNA gene sequence analysis
showed the closest phylogenetic relative to both of the unknown
bacteria is
B. thetaiotaomicron, they can be distinguished easily
from it by several biochemical characteristics. In particular,
B. thetaiotaomicron produces acid from trehalose but the unknown
organisms do not. The characteristics that distinguish the unknown
bacteria from their phenotypically or phylogenetically closely
related species are summarized in Table
2. Figure
2 presents
a flow chart of key characteristics for identification and differentiation
of the unknown organisms from the other species in the
B. fragilis group.
View this table:
[in this window]
[in a new window]
|
TABLE 2. Some properties by which B. nordii sp. nov. and B. salyersae sp. nov. can be differentiated from related Bacteroides spp.a
|

DISCUSSION
In this study, we report on the characterization of two groups
of unknown bacteria that are commonly isolated from clinical
infections and are likely of intestinal origin. They appear
to be of relatively low virulence since they were always found
in mixed culture and were not recovered in blood cultures or
in very serious infection. These two groups of clinical isolates
were misidentified as
B. stercoris and
B. uniformis by routine
biochemical tests. However, 16S rRNA sequencing revealed approximately
7% sequence divergence between the novel species and
B. stercoris or
B. uniformis, and the previously described species closest
to both of them phylogenetically is
B. thetaiotaomicron (approximate
94% sequence similarity). Although the two unknown bacteria
display a number of similarities, 16S rRNA gene sequence divergence
of approximately 4% between them, as well as distinguishable
biochemical characteristics, demonstrated that these organisms
are distinct. Subsequent DNA-DNA hybridization studies confirmed
that the two novel organisms were indeed distinct from each
other. A flow chart summarizes the key characteristics for identification
and differentiation of the two organisms that we describe here
from the other species in
B. fragilis group. Based on both phenotypic
and genotypic evidence, it is clear that the two groups of unknown
isolates recovered from infections of intestinal origin in humans
represent two novel species. We propose that the unknown isolates
of group I and group II be classified as two new
Bacteroides species,
B. nordii and
B. salyersae, respectively.
Description of B. nordii sp. nov.
B. nordii (to honor Carl Erik Nord, who has contributed so much to our knowledge of anaerobic bacteriology in general and intestinal bacteriology) cells are rod shaped and stain gram negative. Typical cells are 0.8 to 1.5 µm by 0.5 to 5.0 µm. Colonies on brucella blood agar plates at 24 h are grey, circular, convex, entire, and opaque and attain a diameter of 1 to 2 mm. The cells are obligately anaerobic, indole positive, and lipase, catalase, urease, and nitrate negative. Esculin and gelatin are hydrolyzed. Cells are resistant to 20% bile. Acid is produced from cellobiose, glucose, mannose, raffinose, rhamnose, sucrose, and xylose but not from arabinose, salicin, trehalose, and xylan. In peptone-yeast broth and peptone-yeast-glucose broth, a major amount of acetic acid and minor amounts of isovaleric acid, propionic acid, and formic acid are produced by all isolates. Positive reactions were obtained for ß-galactosidase,
-glucosidase, ß-glucosidase, alkaline phosphatase, ß-N-acetyl-glucosaminidase, leucyl glycine arylamidase, alanine arylamidase, glutamyl glutamic acid arylamidase, acid phosphatase, naphthol-AS-Bl-phosphohydrolase, and p-nitrophenylphosphatase with the API ZYM, Rapid ID 32A, and RapID ANA II systems. Negative reactions for
-fucosidase were obtained with all three systems. The major long-chain cellular fatty acid is C15:0 ANTEISO FAME (20 to 23% of total). Significant amounts of C15:0 ISO FAME, C16:0 FAME, C16:0 3OH FAME, and C18:1 CIS 9 FAME (9 to 18% of the total each) are also present. Cells showed resistance to kanamycin (1,000 µg), vancomycin (5 µg), and colistin sulfate (10 µg) identification disks; agar dilution tests showed that most of the strains are susceptible or had intermediate susceptibility to metronidazole (MIC
4 µg/ml), imipenem (MIC
2 µg/ml), clindamycin (MIC
4 µg/ml), amoxicillin-clavulanate (MIC
8 µg/ml), and ertapenem (MIC
1 µg/ml). Resistance to penicillin G (MIC
32 µg/ml), ceftizoxime (MIC
32 µg/ml), cefotetan (MIC
64 µg/ml), and vancomycin (MIC
128 µg/ml) is shown by all strains. All strains were ß-lactamase positive (Table 3).
The cells were isolated from human clinical specimens of intestinal
origin. The habitat is probably the human gut. The type strain
is WAL 11050 (ATCC BAA-998 or CCUG 48943). The G+C content of
the type strain is 41.4 mol%.
Description of B. salyersae sp. nov.
B. salyersae (to honor Abigail Salyers, who has contributed so much to our knowledge of intestinal bacteriology and anaerobic bacteriology in general) cells are rod shaped and stain gram negative. Typical cells are 0.8 to 1.5 µm by 0.5 to 5.0 µm. Colonies on brucella blood agar plates at 24 h are grey, circular, convex, entire, and opaque and attain a diameter of 1 to 2 mm. Cells are obligately anaerobic, indole positive, and lipase, catalase, urease, and nitrate negative. Esculin and gelatin are hydrolyzed. Cells are resistant to 20% bile. Acid is produced from arabinose, cellobiose, glucose, mannose, raffinose, rhamnose, sucrose, and xylose but not from salicin, trehalose, and xylan. In peptone-yeast broth and peptone-yeast-glucose broth, a major amount of acetic acid and minor amounts of isovaleric acid, propionic acid, and formic acid are produced by all isolates. Positive reactions are obtained for
-galactosidase, ß-galactosidase,
-glucosidase, ß-glucosidase, alkaline phosphatase, ß-N-acetyl-glucosaminidase, leucyl glycine arylamidase, alanine arylamidase, glutamyl glutamic acid arylamidase, acid phosphatase, naphthol-AS-Bl-phosphohydrolase, p-nitrophenylphosphatase and p-nitrophenyl-ß,D-disaccharidase with the API ZYM, Rapid ID 32A, and RapID ANA II systems. Negative reactions for
-fucosidase were obtained with all three systems. The long-chain cellular fatty acid produced by most of the isolates is C15:0 ANTEISO FAME (20 to 23% of total). Significant amounts of C15:0 ISO FAME, C16:0 FAME, C16:0 3OH FAME,and C18:1 CIS 9 FAME (9 to 18% each of total) are also present. Cells showed resistance to kanamycin (1,000 µg), vancomycin (5 µg), and colistin sulfate (10 µg) identification disks; agar dilution tests showed that most of the strains are susceptible or had intermediate susceptibility to metronidazole (MIC
4 µg/ml), imipenem (MIC
2 µg/ml), clindamycin (MIC
4 µg/ml), amoxicillin-clavulanate (MIC
8 µg/ml), and ertapenem (MIC
1 µg/ml). Resistance to penicillin G (MIC
32 µg/ml), ceftizoxime (MIC
32 µg/ml), cefotetan (MIC
64 µg/ml), and vancomycin (MIC
128 µg/ml) is shown by all strains. All strains were ß-lactamase positive (Table 3).
Cells were isolated from human clinical specimens of intestinal origin. The habitat is probably the human gut. The type strain is WAL 10018 (ATCC BAA-997 or CCUG 48945). The G+C content of the type strain is 42.0 mol%.

ACKNOWLEDGMENTS
This work has been carried out, in part, with financial support
from Veterans Administration Merit Review funds.

FOOTNOTES
* Corresponding author. Mailing address: VA Medical Center WLA., Bldg. 304, Rm. E3-227, 11301 Wilshire Blvd., Los Angeles, CA 90073. Phone: (310) 478-3711, ext. 49151. Fax: (310) 268-4458. E-mail:
yulis1{at}yahoo.com.


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Journal of Clinical Microbiology, December 2004, p. 5565-5570, Vol. 42, No. 12
0095-1137/04/$08.00+0 DOI: 10.1128/JCM.42.12.5565-5570.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
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