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Journal of Clinical Microbiology, April 2007, p. 1350-1352, Vol. 45, No. 4
0095-1137/07/$08.00+0 doi:10.1128/JCM.02274-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
High Frequencies of Clindamycin and Tetracycline Resistance in Methicillin-Resistant Staphylococcus aureus Pulsed-Field Type USA300 Isolates Collected at a Boston Ambulatory Health Center
Linda L. Han,1*
Linda K. McDougal,2
Rachel J. Gorwitz,2
Kenneth H. Mayer,3,4
Jean B. Patel,2
Janet M. Sennott,1 and
John L. Fontana1
Massachusetts Department of Public Health, State Laboratory Institute, Jamaica Plain, Massachusetts 02130,1
Division of Healthcare Quality Promotion, Centers for Disease Control and Prevention, Atlanta, Georgia 30333,2
Fenway Community Health Center, Boston, Massachusetts 02115,3
Brown University/Miriam Hospital, Providence, Rhode Island 029064
Received 8 November 2006/
Returned for modification 5 January 2007/
Accepted 29 January 2007

ABSTRACT
Individual or multiple resistance to clindamycin, tetracycline,
erythromycin, levofloxacin, or mupirocin was detected in a large
proportion of methicillin-resistant
Staphylococcus aureus pulsed-field
type USA300 isolates collected at an ambulatory health center
in Boston. The clindamycin, tetracycline, and mupirocin resistance
genes identified in these isolates are commonly associated with
plasmids.

TEXT
Methicillin-resistant
Staphylococcus aureus (MRSA) has long
been recognized as an important cause of nosocomial morbidity
and mortality and, more recently, as a cause of disease arising
in the community among previously healthy persons without traditional
risk factors for infection (referred to as community-associated
MRSA). One particular strain of MRSA, designated USA300-0114
on the basis of pulsed-field gel electrophoresis (PFGE) typing,
has emerged as a predominant and widely disseminated strain
linked to transmission in community settings nationwide (
12).
Most USA300 isolates are resistant only to ß-lactam
and macrolide antimicrobial agents; however, isolates resistant
to tetracycline, clindamycin, fluoroquinolones, and mupirocin
have been reported (
3,
4,
12,
13). This report describes (i)
a high prevalence of clindamycin and tetracycline resistance
among USA300 isolates collected at a single urban ambulatory
health center in Boston and (ii) the identification of multiple
resistance to erythromycin, clindamycin, levofloxacin, tetracycline,
and mupirocin in an MRSA strain closely related to USA300-0114
(USA300-0247) and in one isolate of USA300-0114.
All of the MRSA isolates collected at the health center during the 19-month period between May 2004 and November 2005 were forwarded to the Massachusetts Department of Public Health State Laboratory Institute for PFGE typing. More than 50% of the patients attending this health center are men who report that they have sex with men, and many health center patients are human immunodeficiency virus infected (7, 9). PFGE was performed with SmaI enzyme digestion as previously described (8). Gel analyses were performed with BioNumerics software, version 4.0 (Applied Maths, Kortrijk, Belgium). Antimicrobial susceptibility testing (AST) was performed with the Dade MicroScan WalkAway instrument (Dade MicroScan, Inc., West Sacramento, CA). Isolates resistant to erythromycin and susceptible to clindamycin were subjected to disk diffusion testing for detection of inducible clindamycin resistance (D-zone test) (2). AST was also performed on a second set of USA300 isolates comprising all of the isolates collected during a similar time period from outpatients tested at a Boston area community health network serving adult and pediatric patients at three hospitals and 20 primary-care practices. A subset of isolates resistant to erythromycin, clindamycin, levofloxacin, and tetracycline was forwarded to the Centers for Disease Control and Prevention, Atlanta, GA, for broth microdilution susceptibility and PCR testing. Susceptibility to minocycline, doxycycline, and mupirocin was determined by the reference broth microdilution method described by the Clinical and Laboratory Standards Institute, with cation-adjusted Mueller-Hinton broth (Becton Dickinson Microbiology Systems, Cockeysville, MD) (2). Quality control strains included S. aureus ATCC 29213, Enterococcus faecalis ATCC 25922, and S. aureus ATCC 43300. PCR testing was performed to identify genes conferring tetracycline resistance (tetK, tetM), inducible or constitutive clindamycin resistance (ermA, ermC), and mupirocin resistance (mupA) (10, 11, 12).
Between May 2004 and November 2005, culture specimens yielding MRSA were obtained from 123 health center patients. Only the first MRSA isolate collected from each patient was included in this analysis. Among 115 isolates with a known source, 103 (90%) were collected from skin and soft-tissue sites, 11 (10%) were from the nares or the nasopharynx, and 1 (1%) was from urine. Among 123 total isolates, 102 (83%) had PFGE patterns corresponding to either MRSA strain type USA300-0114 (73 isolates, 59% of the total) or USA300-0247 (29 isolates, 24% of the total) (Fig. 1). AST data are summarized in Table 1. All 12 multiresistant isolates (11 USA300-0247 and 1 USA300-0114) tested at the Centers for Disease Control and Prevention contained the tetK and ermC genes; none contained the tetM or ermA gene. All were susceptible to minocycline and doxycycline, all contained the mupA resistance gene, and the mupirocin MICs were
128 µg/ml. Among 26 USA300-0114 and 4 USA300-0247 isolates collected from the nearby health network, only 2 were clindamycin resistant and none were tetracycline resistant.
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TABLE 1. Prevalence of resistance to antimicrobial agents among predominant USA300 MRSA strain types at a Boston ambulatory health center in 2004 and 2005
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In a recent report of MRSA skin infections among adult emergency
department patients in 11 U.S. cities, 97% of the MRSA isolates
were USA300, 5% were resistant to clindamycin, 8% were resistant
to tetracycline (
9a), and 1% were resistant to both clindamycin
and tetracycline (Gregory E. Fosheim, unpublished data). The
prevalence of resistance to these agents among USA300 isolates
in our investigation, particularly USA300-0247 isolates, was
considerably higher and was also higher than the prevalence
of resistance among isolates collected at the nearby health
network. Furthermore, we identified multiple resistance to erythromycin,
clindamycin, levofloxacin, and tetracycline in 55% of the USA300-0247
isolates collected, and elevated mupirocin MICs (

128 µg/ml)
were detected in all 12 isolates for which mupirocin MICs were
determined. USA300 isolates with multiple resistance to the
same agents have previously been reported among patients of
a San Francisco community health network (
4). Multiple resistance
has also been identified in non-USA300 MRSA strains isolated
from children in community settings in Taiwan (
1). In a study
involving human immunodeficiency virus-infected men with skin
infections in Los Angeles who report that they have sex with
men, 3.2% and 35.5% of the MRSA isolates were resistant to clindamycin
and tetracycline, respectively, and multiple resistance was
not reported (
6). Examination of clinical and demographic features
of our case patients, including antibiotic exposure and underlying
disease conditions, may prove useful in identifying risk factors
and elucidating mechanisms associated with resistance acquisition
in this community. On a molecular level, an important mechanism
of resistance acquisition in USA300 MRSA appears to be the transfer
of plasmids from bacterial reservoirs; in the isolates described
in this report, resistance to clindamycin, tetracycline, and
mupirocin was mediated by resistance genes that are typically
located on plasmids (
4,
11,
12).
Among USA300 isolates resistant to tetracycline, we consistently found in vitro susceptibility to minocycline, doxycycline, and trimethoprim-sulfamethoxazole. These findings suggest several inexpensive oral treatment options for skin infections with multidrug-resistant USA300 isolates. However, the clinical efficacy of these drugs in the treatment of MRSA infections has not been extensively documented and incision and drainage should still be considered the primary therapy when skin abscesses are present (5). The high prevalence of elevated mupirocin MICs described in this report raises concerns about the in vivo efficacy of mupirocin for eradication of MRSA nasal colonization in certain populations. The appearance of multidrug-resistant MRSA in the community setting emphasizes the importance of routine collection of specimens for culture and AST, not only for individual patient management but also for development of community-wide treatment and prevention strategies.

ACKNOWLEDGMENTS
We thank our colleagues Karen Anderson, Barbara Bolstorff, Dan
Cohen, Al DeMaria, Bela Matyas, and John Szumowski for their
contributions.
The findings and conclusions in this report are ours and do not necessarily represent the views of the Centers for Disease Control and Prevention.

FOOTNOTES
* Corresponding author. Mailing address: State Laboratory Institute, 305 South St., Jamaica Plain, MA 02130. Phone: (617) 983-4362. Fax: (617) 983-6618. E-mail:
linda.han{at}state.ma.us 
Published ahead of print on 7 February 2007. 

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Journal of Clinical Microbiology, April 2007, p. 1350-1352, Vol. 45, No. 4
0095-1137/07/$08.00+0 doi:10.1128/JCM.02274-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.
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