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Journal of Clinical Microbiology, June 2007, p. 1978-1980, Vol. 45, No. 6
0095-1137/07/$08.00+0 doi:10.1128/JCM.00563-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
Identification of an Emerging Pathogen, Mycobacterium massiliense, by rpoB Sequencing of Clinical Isolates Collected in the United States
Keith E. Simmon,1
June I. Pounder,1
John N. Greene,2
Frank Walsh,3
Clint M. Anderson,4
Samuel Cohen,4 and
Cathy A. Petti1,4,5*
Associated Regional and University Pathologists (ARUP) Institute for Clinical and Experimental Pathology, Salt Lake City, Utah,1
H. Lee Moffitt Cancer and Research Institute, Tampa, Florida,2
Department of Medicine, University of South Florida College of Medicine, Tampa, Florida,3
ARUP Laboratories, Salt Lake City, Utah,4
Departments of Medicine and Pathology, University of Utah School of Medicine, Salt Lake City, Utah5
Received 13 March 2007/
Accepted 26 March 2007

ABSTRACT
Mycobacterium massiliense is a rapidly growing mycobacterium
that is indistinguishable from
Mycobacterium chelonae/
M. abscessus by partial 16S rRNA gene sequencing. We sequenced
rpoB,
sodA,
and
hsp65 genes from isolates previously identified as being
M. chelonae/M. abscessus and identified
M. massiliense from
isolates from two patients with invasive disease representing
the first reported cases in the United States.

TEXT
Rapidly growing mycobacterium infections are increasing in the
United States (
7) and are difficult to speciate by conventional
methods. Partial 16S rRNA gene sequencing is the most widely
used method for the identification of nontuberculous mycobacteria
(
6,
8,
12), but this gene target is often limited by the lack
of sequence divergence among closely related
Mycobacterium species
(
15).
Mycobacterium chelonae and
M. abscessus are two species
that share the same 16S rRNA gene sequence, and since distinguishing
these two species is clinically relevant, assays targeting base
pair differences within the 16S-23S rRNA internal transcribed
spacer (ITS) region have been developed (
5). The ITS assay has
proven to be valuable but cannot differentiate
M. abscessus from
M. massiliense and
M. bolletii, two new species of mycobacteria
that share the same 16S rRNA gene sequence with
M. chelonae/M. abscessus (
1,
4). Although
M. massiliense and
M. bolletii have
not been described in the United States, these two species may
have been misclassified by previous assays and may remain undetected
as emerging pathogens.
We sequenced portions of the rpoB, sodA, and hsp65 genes to gain a better understanding of the frequency of detection of M. massiliense or M. bolletii among clinical isolates identified as being M. chelonae/M. abscessus by 16S and ITS assays. From this analysis, we found four isolates from two patients with identifications consistent with the novel species M. massiliense and report their clinical case histories. To our knowledge, these are the first reported cases of invasive infections from M. massiliense in the United States.
Case reports. (i) Patient 1.
A 43-year-old female from Nevada with multiple sclerosis and pacemaker placement 11 months previously presented with pacemaker pocket infection. An erythematous "lump" developed at her pacemaker site that required local incision and drainage (no culture). She was treated empirically with vancomycin but developed fever and increasing pain at the site. Intraoperative cultures from surgical debridement grew colonies of acid-fast bacilli, which were identified as being M. abscessus. She remained on vancomycin. The fever persisted, and 2 weeks later, all components of the pacemaker were removed surgically, with intraoperative cultures again being positive for M. abscessus. The isolate was susceptible only to clarithromycin (MIC < 0.12 µg/ml) and amikacin (MIC < 16 µg/ml). She was discharged and received 6 months of clarithromycin treatment.
(ii) Patient 2.
A 29-year-old female from Florida with chronic myelogenous leukemia received an allogeneic hematopoietic stem cell transplant that was complicated by chronic graft-versus-host disease. Five months following the hematopoietic stem cell transplant, she developed fever and cough. Cultures from bronchoalveolar lavage and multiple blood cultures recovered rapidly growing mycobacteria identified as being M. abscessus. The isolate was susceptible to linezolid (MIC < 8 mg/ml), clarithromycin (MIC < 0.12 µg/ml), and amikacin (MIC < 16 µg/ml). She received 6 weeks of treatment with intravenous tigecycline, oral moxifloxacin, and oral azithromycin.
Over a 7-month period, 63 clinical isolates, representing 58 patients, that were previously identified as harboring M. chelonae (n = 8) and M. abscessus (n = 55) by 16S and ITS sequence analyses (5) were retrieved retrospectively. DNA extractions, PCR, and sequencing reactions were performed as previously described (13) with amplification and sequencing primers targeting the rpoB (2), sodA (3), and hsp65 (14) genes. Neighbor-joining trees were constructed by using MEGA v3.1 (9). Isolates were identified by using the rpoB sequence criteria described previously by Adekambi et al. (2). Susceptibility testing was performed by broth microdilution according to CLSI (formerly NCCLS) standard M24-A (11). Doxycycline susceptibility testing was performed by Etest (AB Biodisk, Solna, Sweden).
A comparison of rpoB, sodA, and hsp65 sequences is shown in Table 1. Four isolates identified as being M. abscessus by ITS assay were identified as being M. massiliense by rpoB sequencing (Fig. 1). The rpoB sequencing results agreed with results for the ITS assay for the remaining 59 isolates. With sodA sequencing, only M. chelonae was differentiated from M. massiliense and M. abscessus (Figure
). A total of 11 isolates had 100% identity with the previously published sodA sequence for M. massiliense. However, the rpoB sequence identified these 11 isolates as being M. massiliense (n = 4) or M. abscessus (n = 7). A comparison of the hsp65 sequences showed results that were similar to those of sodA comparisons. The 11 isolates that shared hsp65 identity with M. massiliense correlated with the 11 isolates that shared 100% sodA identity with M. massiliense. No isolates were identified as being M. bolletii. Susceptibility patterns for all 63 isolates are shown in Table 2. All M. massiliense isolates were resistant to doxycycline.
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TABLE 1. Comparison of gene sequences from clinical isolates and reference strains of M. abscessus, M. chelonae, and M. massiliense
|
With
rpoB sequencing, 51 isolates originally identified as
M. abscessus isolates by ITS assay were confirmed to be
M. abscessus isolates, with 4 isolates recharacterized as harboring
M. massiliense.
To our knowledge, this is the first report to describe the isolation
and identification of
M. massiliense isolates collected in the
United States associated with invasive infections, and it suggests
that
M. massiliense may be more commonly encountered but may
be potentially misclassified as
M. abscessus.
Unlike partial 16S rRNA gene sequencing, where interspecies similarity is high, with
0.4% sequence differences, the rpoB gene sequence is more variable, with 2 to 3% sequence differences correctly classifying most nontuberculous mycobacteria (9). In this study, we found partial rpoB gene sequencing to be a more discriminating gene target than sodA and hsp65 for M. massiliense, a unique observation that conflicts with data from previous reports (4, 10). Additionally, other investigators previously proposed that susceptibility to doxycycline may serve as a surrogate marker to differentiate M. abscessus from M. massiliense (4), but we could not confirm this finding.
Distinguishing M. chelonae from M. abscessus is clinically important because of their unique susceptibility patterns and disease manifestations. Our understanding of M. massiliense representing a distinct clinical and taxonomical entity from M. abscessus is still evolving. For our two patients, no histories of unusual exposure to animals or environmental sources were present, and their clinical presentations and susceptibility patterns were similar to those of infections caused by M. abscessus.
Overall, rpoB gene sequencing is emerging as the preferred tool to identify mycobacteria taxa (1-4) and enabled us to identify M. massiliense from two patients with invasive infection. Although the clinical significance of routinely identifying certain mycobacteria remains unclear, the 16S rRNA gene and ITS regions are often inadequate to completely capture the microbial diversity of mycobacteria, and alternative gene targets such as the rpoB gene should be considered to recognize emerging mycobacterial pathogens with the potential for invasive disease.

ACKNOWLEDGMENTS
This work was supported by the Associated Regional and University
Pathologists Institute for Clinical and Experimental Pathology,
an enterprise of the University of Utah and its Department of
Pathology.
We have no conflict of interest regarding the software, products, or concepts used in this study.

FOOTNOTES
* Corresponding author. Mailing address: ARUP Laboratories, 500 Chipeta Way, Salt Lake City, UT 84108. Phone: (801) 583-2787. Fax: (801) 584-5207. E-mail:
cathy.petti{at}aruplab.com 
Published ahead of print on 4 April 2007. 

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Journal of Clinical Microbiology, June 2007, p. 1978-1980, Vol. 45, No. 6
0095-1137/07/$08.00+0 doi:10.1128/JCM.00563-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
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