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Journal of Clinical Microbiology, January 2000, p. 415-418, Vol. 38, No. 1
Laboratoire Mixte Pasteur-Necker de Recherche
sur les Streptocoques et Streptococcies and Unité INSERM 411, Faculté de Médecine Necker-Enfants Malades, 75730 Paris
Cedex 15, France
Received 11 August 1999/Returned for modification 22 September
1999/Accepted 25 October 1999
Simple PCR and sequencing assays that utilize a single pair of
degenerate primers were used to characterize a 438-bp-long DNA fragment
internal (sodAint) to the sodA gene
encoding the manganese-dependent superoxide dismutase in 19 enterococcal type strains (Enterococcus avium,
Enterococcus casseliflavus, Enterococcus cecorum, Enterococcus columbae, Enterococcus
dispar, Enterococcus durans, Enterococcus
faecalis, Enterococcus faecium, Enterococcus flavescens, Enterococcus gallinarum,
Enterococcus hirae, Enterococcus malodoratus,
Enterococcus mundtii, Enterococcus pseudoavium,
Enterococcus raffinosus, Enterococcus
saccharolyticus, Enterococcus seriolicida, Enterococcus solitarius, and Enterococcus
sulfureus). Sequence analysis of the
sodAint fragments enabled reliable
identification of 18 enterococcal species, including E. casseliflavus-E. flavescens and E. gallinarum. The
sodAint fragments of E. casseliflavus and E. flavescens were almost identical
(99.5% sequence identity), which suggests that they should be
associated in a single species. Our results confirm that the
sodA gene constitutes a more discriminative target sequence
than 16S rRNA gene in differentiating closely related bacterial species.
Enterococci, although not highly
virulent microorganisms, have emerged worldwide in the last decade as
one of the leading causes of nosocomial bacteremia, surgical wound
infections, and urinary tract infections (9, 10, 13, 24).
This evolution is mainly due to the appearance of multiresistant
strains of enterococci that are resistant to most antibiotics used in
treatment (e.g., ampicillin, aminoglycosides, and glycopeptides). Most
human enterococcal infections ( The main characteristics of the bacterial strains used in this study,
including the type strains, are listed in Table
1 and 2.
Rapid extraction of bacterial genomic DNA was carried out by using the
InstaGene Matrix (Bio-Rad, Hercules, Calif.) on cells collected from 2 ml of an overnight culture. The sodA degenerate primers
d1 (5'-CCITAYICITAYGAYGCIYTIGARCC-3') and
d2 (5'-ARRTARTAIGCRTGYTCCCAIACRTC-3') were
used to amplify an internal fragment, designated
sodAint, representing approximately 85% of
their sodA genes. PCRs were performed on a Gene Amp System
9600 instrument (Perkin-Elmer Cetus, Roissy, France) in a final volume
of 50 µl containing 250 ng of DNA as template, 0.5 µM each primer,
200 µM each deoxynucleoside triphosphate, and 1 U of AmpliTaq Gold
DNA polymerase (Perkin-Elmer) in a 1× amplification buffer (10 mM
Tris-HCl [pH 8.3], 50 mM KCl, 1.5 mM MgCl2). The PCR
mixtures were denatured (3 min at 95°C) and then subjected to 30 cycles of amplification (60 s of annealing at 37°C, 60 s of
elongation at 72°C, and 30 s of denaturation at 95°C) and 7 min at 72°C for the last elongation cycle. A single DNA fragment
corresponding to the expected 480-bp amplification product,
sodAint, was observed in all cases following
agarose gel electrophoresis and ethidium bromide staining (data not
shown). PCR products were purified on an S-400 Sephadex column
(Pharmacia, Uppsala, Sweden) and directly sequenced on both strands
with the oligonucleotides d1 and d2 by using the
ABI-PRISM Big Dye terminator sequencing kit on a Genetic ABI-PRISM 310 Sequencer Analyzer (Perkin-Elmer). The cycle sequencing protocol was
optimized as follows: the sequencing mixtures were subjected to 40 cycles of amplification consisting of 10 s of denaturation at
96°C, 5 s of annealing at 40°C, and 4 min of elongation at
60°C.
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Sequencing the Gene Encoding Manganese-Dependent
Superoxide Dismutase for Rapid Species Identification of
Enterococci
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90%) are caused by Enterococcus
faecalis and Enterococcus faecium; however, the
incidence of other species, such as Enterococcus casseliflavus and Enterococcus gallinarum, could be
underestimated because of bacterial misidentification. In clinical
laboratories, accurate identification of enterococcal species is
required to carry out a proper epidemiologic surveillance and may help
in the management of infected patients in case of relapse. This is usually done by testing tolerance to bile esculine and tellurite, growth in 6.5% NaCl broth, and specific carbohydrate utilization (2, 6); by characterizing bacterial motility and pigment production (1); and by using commercial biochemical test
systems, such as the API 20STREP or Rapid ID 32 Strep systems. However, these phenotypic methods are often not reliable and the automated systems, such as the Vitek and MicroScan systems, do not properly identify enterococci other than E. faecalis and E. faecium in the absence of additional tests (11).
Consequently, several genotypic methods based on the analysis of PCR
products derived from selected target DNA have been developed for the
species identification of enterococci (3, 14, 22). This
includes the determination of the 16S ribosomal DNA (rDNA) sequence
(18), a strategy which is now greatly facilitated by the use
of universal 16S PCR primers associated with the development of
simplified, partially automated, and cost-effective sequencing
technologies. However, the interpretation of these data may be
complicated by the fact that divergent 16S rDNA sequences may exist
within a single organism (23) or, alternatively, that
closely related species may have identical 16S rDNA sequences (8), as recently shown in the genera Enterococcus
for E. casseliflavus and E. gallinarum
(18). To solve this problem, it is possible to use
alternative monocopy target sequences which exhibit a higher divergence
than that of the 16S rDNA. The sodA gene of the
gram-positive cocci which encodes the manganese-dependent superoxide
dismutase fulfills these criteria and we recently reported that
sequencing of the sodA PCR product with the use of a single
pair of degenerate primers constitutes a valuable approach to the
genotypic identification of the 29 streptococcal species
(20). In this work, the same universal primers
(19) were used to construct a sodA database of 19 enterococcal species, including E. casseliflavus and
E. gallinarum. We show the usefulness of this library for a
rapid sequence-based identification method of enterococcal isolates.
TABLE 1.
Enterococcal type strains used in this study
TABLE 2.
Identification of various enterococcal strains by
sequencing the sodAint fragment
The nucleotide sequences of the sodAint fragments from the type strains of Enterococcus avium, E. casseliflavus, Enterococcus cecorum, Enterococcus columbae, Enterococcus dispar, Enterococcus durans, E. faecalis, E. faecium, Enterococcus flavescens, E. gallinarum, Enterococcus hirae, Enterococcus malodoratus, Enterococcus mundtii, Enterococcus pseudoavium, Enterococcus raffinosus, Enterococcus saccharolyticus, Enterococcus seriolicida, Enterococcus solitarius, Enterococcus sulfureus, and Lactococcus garvieae were determined (Table 1). We assumed that the PCR products sequenced were actual sodAint fragments, since the corresponding deduced polypeptides all contained the amino acids characteristic of the manganese-dependent superoxide dismutase (16, 17) at the expected positions (data not shown). Multiple alignment of these sodAint DNA sequences plus those of L. garvieae (Table 1), Lactococcus lactis (19), Streptococcus bovis (20), and Streptococcus pyogenes (20) was carried out with the Clustal X program (12), and an unrooted phylogenetic tree was constructed by the neighbor-joining (NJ) method (21). The sequences of the degenerate oligonucleotides d1 and d2 and alignment gaps were not taken into consideration for calculations. The reliability of the tree nodes was evaluated by calculating the percentage of 1,000 bootstrap resamplings that support each topological element. Only the nodes having a bootstrap value greater than 95% are shown in Fig. 1, since this critical value could be used to define the monophyly of a clade of related organisms (7). This analysis revealed that, as expected, the members of the genus Enterococcus, with the exception of E. seriolicida, were clustered within a clade supported by 99.5% of the bootstrap replicates. The sodAint sequences of E. seriolicida and of L. garvieae were almost identical (99.5% sequence identity) and were clustered with that of L. lactis within a clade supported by 96.3% of the bootstrap confidence limit (Table 3 and Fig. 1). These results are consistent with the redesignation of E. seriolicida as L. garvieae (4). The phylogenetic tree representing the enterococcal sodAint sequences (Fig. 1) has the same topology as the NJ tree constructed from the analysis of their 16S rDNA sequences (18). It is worth noting that the sodAint sequences of the E. casseliflavus and E. gallinarum type strains displayed 16.9% sequence divergence, a value similar to the 19.7% sequence divergence observed between the ddl genes encoding the D-Ala-D-Ala ligases in these species (5). These results do not support the suggestion that E. casseliflavus and E. gallinarum comprise a single species (18). By contrast, the fact that the 16S rDNA sequence (18) and the ddl (15), vanC (3), and sodAint (Table 3) genes of the E. casseliflavus and E. flavescens type strains were almost identical (99.9, 99.5, 96, and 98% sequence identity, respectively) suggests that they should be associated in a single species.
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The phylogenetic tree shown in Fig. 1 revealed the presence of two major clusters within the enterococcal species which we have designated the faecium group (E. faecium, E. durans, E. hirae, and E. mundtii) and the avium group (E. avium, E. malodoratus, E. pseudoavium, and E. raffinosus). Within each group, the 16S rDNA sequences exhibited more than 99% sequence identity (18), whereas the highest percentage of similarity found between two sodAint sequences was 87.9% (Table 3). These results confirm that the gene sodA constitutes a more discriminative target sequence than 16S RNA in differentiating closely related bacterial species.
Fifteen enterococcal isolates were identified by using conventional microbiological tests, Rapid ID 32 Strep, and the sodAint systems (Table 2). In all cases, the sodAint sequences of the isolates displayed less than 1.5% divergence from the corresponding type strain. For 10 strains (NEM1616, NEM1617, NEM1621, NEM1623, NEM1624, NEM1625, NEM1626, NEM1627, NEM1628, and NEM1630), the two methods gave the same results. Four isolates (NEM1618, NEM1620, NEM1622, and NEM1629) were identified at the species level with the sodAint system but not with conventional microbiological tests or the Rapid ID 32 Strep system. The remaining isolate, NEM1619, was identified with the Rapid ID 32 Strep system as E. hirae but was identified with the sodAint system as E. durans (Table 2). The reliability of the molecular identification of NEM1164 was based on the fact that its sodAint fragment displays 99.5 and 85% sequence identity with the type strains of E. durans and E. hirae, respectively.
In conclusion, we have determined the sodAint sequences of the type strains of E. avium, E. casseliflavus, E. flavescens, E. cecorum, E. columbae, E. dispar, E. durans, E. faecalis, E. faecium, E. gallinarum, E. hirae, E. malodoratus, E. mundtii, E. pseudoavium, E. raffinosus, E. saccharolyticus, E. seriolicida, E. solitarius, and E. sulfureus and demonstrated the usefulness of this database for the species identification of enterococcal isolates. The identification method presented in this study is not accessible to routine clinical microbiology laboratories, but it may become the "gold standard" technique in reference and large research hospital laboratories for epidemiologic purposes and/or identifying problematic strains.
Nucleotide sequence accession number. Representative nucleotide sequences have been submitted to the EMBL database and have been given accession no. AJ387906 to AJ387925 and AJ387927 to AJ387941.
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ACKNOWLEDGMENTS |
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We thank C. Bizet for the gift of enterococcal type strains (CIP), O. Gaillot for the gift of clinical isolates, O. Gaillot and S. Nair for a critical reading of the manuscript, and P. Berche for his interest in this work.
This work was supported by the Institut Pasteur and by the University of Paris V.
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FOOTNOTES |
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* Corresponding author. Mailing address: Laboratoire Mixte Pasteur-Necker de Recherche sur les Streptocoques et Streptococcies, Faculté de Médecine Necker-Enfants Malades, 75730 Paris Cedex 15, France. Phone: (33) (1) 40 61 56 79. Fax: (33) (1) 40 61 55 92. E-mail: ptrieu{at}pasteur.fr.
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