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Journal of Clinical Microbiology, November 1998, p. 3399-3407, Vol. 36, No. 11
Division of Infectious Diseases and
Department of Internal Medicine,1
Division of Clinical Microbiology,2
and
Division of Biochemistry and Molecular
Biology,3 Mayo Clinic and Foundation,
Rochester, Minnesota 55902
Received 6 May 1998/Returned for modification 22 June 1998/Accepted 18 August 1998
The 16S rRNA sequences of enterococcal species E. faecium, E. faecalis, E. gallinarum, E. casseliflavus/flavescens, E. dispar, E. pseudoavium, E. sulfureus,
E. malodoratus, E. raffinosus,
E. cecorum, E. hirae, E. saccharolyticus, E. seriolicida, E. mundtii, E. avium, E. durans,
E. columbae, and E. solitarius are
presented herein. These data were utilized to confirm the species
identification of two nonmotile E. gallinarum isolates
which had been previously phenotypically identified as
E. faecium. The implications of this finding are
discussed.
We previously reported detection of the
vanC-1 vancomycin resistance-associated gene in two
enterococcal isolates (isolates 41 and 88), which by phenotypic
analysis appeared to be Enterococcus faecium
(19). Because the vanC-1 gene is believed to be
an intrinsic characteristic of Enterococcus gallinarum, our
findings raised the question as to whether the species identification
of these two isolates, which was performed by conventional phenotypic
methodologies, was erroneous (1, 6, 15, 18). To answer this
question, we performed small-subunit rRNA (16S rRNA) sequencing of
these isolates as well as a collection of enterococci.
0095-1137/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Determination of 16S rRNA Sequences of Enterococci and
Application to Species Identification of Nonmotile
Enterococcus gallinarum Isolates
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TABLE 1.
Homology values derived from 16S
rRNA sequencesa
To date, molecular analyses of the 16S rRNA sequences of enterococci have only been partially performed (32). Objective, clean 16S rRNA sequencing data for enterococci are important to determine the relationship of clinically relevant enterococcal species, especially in situations such as that delineated above. Herein we present the 16S rRNA sequences of multiple isolates of the enterococcal species E. faecium, E. faecalis, E. gallinarum, and E. casseliflavus/flavescens and single isolates of E. dispar, E. mundtii, E. pseudoavium, E. sulfureus, E. malodoratus, E. raffinosus, E. cecorum, E. hirae, E. saccharolyticus, E. seriolicida, E. avium, E. durans, E. columbae, and E. solitarius and use these findings to demonstrate that isolates 41 and 88 are nonmotile E. gallinarum isolates.
Thirty-four clinical isolates of vancomycin-resistant enterococci (VRE) or vanC gene-carrying enterococci (E. faecium, E. faecalis, E. gallinarum, E. casseliflavus/flavescens, and isolates 41 and 88) as well as one isolate of E. avium and one isolate of E. raffinosus identified by the Mayo Clinic microbiology laboratory were identified as previously reported (19). We previously reported detection of the vanC-1 gene in isolates 41 and 88, which initially had appeared to be E. faecalis and E. faecium, respectively (19). Subsequent analysis suggests that isolate 41, referred to in our previous publication as E. faecalis (19), is a mixture of E. faecium and E. faecalis and that the E. faecium component (nonmotile; arginine, mannitol, arabinose, raffinose, sucrose, and bile esculin positive; and sorbitol, sorbose, tellurite, and pyruvate negative) has the vanC-1 gene (19). Therefore, both isolates 41 and 88 appear phenotypically to be E. faecium.
In addition to the 36 aforementioned isolates, the following American Type Culture Collection strains were studied: E. mundtii ATCC 43186, E. dispar ATCC 51266, E. pseudoavium ATCC 49372, E. sulfureus ATCC 49903, E. malodoratus ATCC 43197, E. cecorum ATCC 43198, E. hirae ATCC 8043, E. saccharolyticus ATCC 43076, E. seriolicida ATCC 49156, E. durans ATCC 59607, E. columbae ATCC 51263, and E. solitarius ATCC 49428.
16S rRNA PCR amplification and sequencing were performed with previously described cycling conditions and primers (10) and previously described PCR mixtures (19, 20). The sequence data were analyzed with Sequencher 3.0 (Gene Codes Corporation, Ann Arbor, Mich.).
We sequenced the 16S rRNA gene from isolates 41 and 88 and compared the sequences to those from 14 E. faecium isolates, 3 E. faecalis isolates, 10 E. gallinarum isolates, 4 E. casseliflavus isolates, and 1 isolate each of E. flavescens, E. mundtii, E. dispar, E. pseudoavium, E. sulfureus, E. malodoratus, E. raffinosus, E. cecorum, E. hirae, E. saccharolyticus, E. seriolicida, E. avium, E. durans, E. columbae, and E. solitarius (Fig. 1). A distance matrix tree constructed from the 16S rRNA gene sequences is presented in Fig. 2, and homology values are presented in Table 1. All three E. faecalis isolates had identical sequences, as did all 14 E. faecium isolates. All 15 of the E. gallinarum, E. casseliflavus, and E. flavescens isolates were identical to each other except as follows. At position 287 (as shown in Fig. 1), the E. casseliflavus and E. flavescens isolates had an adenine and the E. gallinarum isolates had a cytosine. At positions 476 and 479 (as shown in Fig. 1), all of the E. gallinarum isolates except one and one of the E. casseliflavus isolates had a guanosine and a cytosine, one of the E. gallinarum isolates had an adenine and a cytosine, and all of the E. casseliflavus isolates except one and the one E. flavescens isolate had an adenine and a thymidine. Although E. flavescens differs from E. casseliflavus in only one biochemical reaction (21), both possess the vanC-2 gene (5). In three of four isolates of E. casseliflavus, the 16S rRNA sequences were identical to each other and to that of E. flavescens. The 16S rRNA of the fourth E. casseliflavus isolate differed from the rest by 2 bp. This suggests that E. casseliflavus and E. flavescens comprise a single species.
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Isolates 41 and 88 had 16S rRNA sequences identical to those of E. gallinarum (19). These isolates were previously misidentified as E. faecium because they were not motile, despite being raffinose positive. The vancomycin MICs for these two isolates were 4 and 8 µg/ml, respectively. This finding has important implications for the clinical microbiology laboratory. E. gallinarum is pathogenic for humans (21, 22); in a recent study, enterococci with vanC-associated vancomycin resistance were isolated from nonstool specimens of 9 of 538 patients (1.7%), including two patients with bacteremia who subsequently died (27). Furthermore, vancomycin treatment failure has been associated with intrinsic low-level vancomycin resistance both in humans and in animal models of experimental endocarditis (8, 12, 15). There therefore exists a need to accurately and quickly differentiate enterococci with vanC-associated vancomycin resistance from vancomycin-susceptible enterococci. On the other hand, it has been suggested that vanC-associated vancomycin resistance is not a concern for infection control, because no nosocomial transmission of the involved organisms has been reported (27-29). Costly and cumbersome infection control precautions, such as those recommended for enterococci with vanA- and vanB-associated vancomycin resistance, may therefore not be necessary in the management of patients with vanC enterococci (9, 11, 13, 16). There also exists a need to accurately and quickly differentiate enterococci with vanC-associated vancomycin resistance from VanA or VanB VRE. Stool screening for VRE commonly involves the use of media supplemented with vancomycin at concentrations as low as 6 µg/ml and may also not accurately differentiate E. gallinarum from VanA or VanB VRE or from vancomycin-susceptible enterococci (14, 24, 28).
Unfortunately, both conventional species identification, including
commercial test systems, and vancomycin susceptibility tests, including
the classical disk diffusion method, are unreliable at detecting
E. gallinarum (and E. casseliflavus/flavescens) (25, 26, 31). E. gallinarum can be difficult to differentiate from other
enterococci, particularly E. faecium, with commercial
biochemical test systems, which may not even include this
organism in their databases (1). The motility test is
not totally reliable for E. gallinarum (or
E. casseliflavus) (4, 18, 27, 30). The
MIC breakpoint of vancomycin for one of our alleged vanC-1 E. faecium isolates was low (4 µg/ml), which would have
been categorized as susceptible according to current guidelines
(17, 18). Recently, the use of a
methyl-
-D-glucopyranoside reagent has shown promising results and may prove valuable in the species identification of E. gallinarum; our isolates 41 and 88 produced acid
from methyl-
-D-glucopyranoside (2). As an
alternative, 16S rRNA sequencing may be helpful in selected
situations.
Finally, our findings regarding the 16S rRNA sequences of E. seriolicida (11.4 to 11.8% different from the other Enterococcus spp.) and E. solitarius (5.2 to 7.0% different from the other Enterococcus spp.) are consistent with the findings of previous investigators, who have suggested that these proposed species should not be included in the genus Enterococcus (3, 7, 32). E. seriolicida has been renamed as Lactococcus garvieae (6a).
In summary, we have reported the 16S rRNA sequences of E. faecium, E. faecalis, E. gallinarum, E. casseliflavus/flavescens, E. dispar, E. mundtii, E. pseudoavium, E. sulfureus, E. malodoratus, E. raffinosus, E. cecorum, E. hirae, E. saccharolyticus, E. seriolicida, E. avium, E. durans, E. columbae, and E. solitarius and used these sequences to confirm the species identification of two nonmotile E. gallinarum isolates.
Nucleotide sequence accession number. Representative nucleotide sequences have been submitted to GenBank and have accession no. AF039898 to AF039903 and AF061000 to AF061013.
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FOOTNOTES |
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* Corresponding author. Mailing address: Division of Infectious Diseases, Department of Internal Medicine, Mayo Clinic and Foundation, Rochester, MN 55902. Phone: (507) 255-7762. Fax: (507) 255-7767. E-mail: patel.robin{at}mayo.edu.
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