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Journal of Clinical Microbiology, May 2001, p. 2037-2038, Vol. 39, No. 5
0095-1137/01/$04.00+0 DOI: 10.1128/JCM.39.5.2037-2038.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
LETTERS TO THE EDITOR
Discordant Carbapenem Susceptibility in
Methylobacterium Species and Its Application as a Method
for Phenotypic Identification
 |
LETTER |
We encountered a meropenem-resistant, imipenem-susceptible
clinical Methylobacterium isolate and sought to determine
if discordant carbapenem susceptibility (meropenem
resistance/imipenem susceptibility) was common among
different Methylobacterium species. We concurrently investigated whether this phenotype was expressed in the genus Roseomonas, the other recognized group of pink-pigmented
nonfermenting gram-negative bacilli.
The genus Methylobacterium is comprised of pink-pigmented
nonfermenting gram-negative bacilli. These organisms are widely distributed in nature and have been isolated from chlorinated potable
water supplies (6). Their presence and role as
opportunistic pathogens and presence in the nosocomial setting are well
documented (1, 4, 5, 7, 9, 10, 14).
Isolates can be cultivated on a variety of solid media, but small
colonies may only be detected after incubation for 4 to 5 days
(15). Optimal growth occurs at 25 to 30°C; most strains fail to grow at 37°C. Biochemical characterization allows definitive identification and differentiation from other pink-pigmented
nonfermenters (15). 16S rRNA-based molecular studies have
allowed phylogenetic analysis and rapid identification of
Methylobacterium species (6, 13).
Current interpretive standards for antibiotic susceptibility testing do
not apply to Methylobacterium species (12). Two large Methylobacterium studies assessing in vitro antibiotic
susceptibility have demonstrated broad resistance to beta-lactam
antibiotics (2, 6). To date, isolates tested have been
imipenem susceptible (1, 7, 9). No data with regard to
meropenem susceptibility has been published to our knowledge.
Four clinical isolates from our laboratory were identified as
Methylobacterium species, by morphologic and
biochemical characteristics, at a reference laboratory
(Laboratoire de Santé Publique du Québec). We also
acquired three Methylobacterium species and three
Roseomonas species from the American Type Culture Collection
(ATCC; Table 1).
Susceptibility testing was performed by E-test (imipenem and meropenem;
AB BIODISK) and disk diffusion (imipenem and meropenem, 10-µg
disks; OXOID) on Sabouraud dextrose agar (SDA) and nutrient agar
(NA). E-test and disk diffusion methods were carried out as
described in the manufacturer's instructions and published protocol
(11), respectively.
Incubation of plates at 25°C for 72 h allowed all isolates
to attain optimal growth for susceptibility determination.
All Methylobacterium species isolates were extremely
susceptible to imipenem but highly resistant to meropenem (Table 1).
All Roseomonas isolates were extremely susceptible to both carbapenems.
In vitro, meropenem is more active than imipenem
against members of the family Enterobacteriaceae
and most oxidase-positive and/or glucose-nonfermenting gram-negative
bacilli (3, 8). The marked discordance between meropenem
and imipenem susceptibility that we describe has not to our knowledge
been observed in other gram-negative bacilli.
We assessed three distinct Methylobacterium
reference strains. 16S rRNA-based phylogenetic analysis has
previously placed these species within two major subclusters
(subcluster I, M. extorquens and M. organophilum;
subcluster II, M. mesophilicum) (6). Given the
interspecies consistency of the susceptibility phenotype we describe,
it seems plausible that the mechanisms contributing to this phenotype
are conserved at the genus level. Further in vitro study is needed to
characterize the underlying mechanism for this discordant susceptibility.
Our findings could contribute to more rapid identification of clinical
Methylobacterium isolates and conceivably could be exploited
for screening potable water supplies and environmental samples.
Additionally, our data suggests that discordant carbapenem susceptibility may have the potential to differentiate between Methylobacterium and Roseomonas species. However,
our observations include a small number of isolates. Further studies
are necessary to confirm our findings and to elucidate the responsible mechanism.
| |
FOOTNOTES |
*
Phone: (514) 340-8294
Fax: (514) 340-7546
E-mail: mmiller{at}lab.jgh.mcgill.ca
| |
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|
| | | | |
G. J. Zaharatos
A. Dascal
M.
A. Miller*
Department of Microbiology, SMBD Jewish General
Hospital,
and McGill University
Montréal, Québec, Canada
|
Journal of Clinical Microbiology, May 2001, p. 2037-2038, Vol. 39, No. 5
0095-1137/01/$04.00+0 DOI: 10.1128/JCM.39.5.2037-2038.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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