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Antimicrobial Agents and Chemotherapy, December 1999, p. 3042-3043, Vol. 43, No. 12
0066-4804/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
LETTERS TO THE EDITOR
Increase in Resistance of Methicillin-Resistant
Staphylococcus aureus to 
-Lactams Caused by
Mutations Conferring Resistance to Benzalkonium Chloride, a
Disinfectant Widely Used in Hospitals
 |
LETTER |
Methicillin-resistant Staphylococcus aureus (MRSA) has
been found worldwide and is one of the major nosocomial agents. When MRSA strains were first discovered in hospitals, resistance of MRSA to
low concentrations of 
-lactams was evident, but increasing resistance to high concentrations of -lactams is now common. Most of
the key elements responsible for methicillin resistance of MRSA have
been well defined; however, mechanisms related to the acquisition of
those genetic phenotypes in hospitals remain uncertain (4).
Appearance of MRSA resistant to benzalkonium chloride, a disinfectant
widely used in hospitals, has been reported (1). To
determine if the disinfectant might be related to increases in
resistance to -lactams, we selected strains of MRSA which showed
resistance to low concentrations of oxacillin (MIC, 2 to 32 µg/ml),
isolated mutants resistant to benzalkonium, and then monitored the
resistance to oxacillin. Approximately 2 × 108 cells
of parent MRSA were seeded on a Luria-Bertani (LB) agar plate
containing 5 µg of benzalkonium chloride per ml and incubated at
37°C for 3 days. The colonies which appeared were isolated as
benzalkonium-resistant mutants. The MIC of benzalkonium for parent MRSA
was 5 µg/ml, and the value for the isolated mutants increased to 10 µg/ml. Surprisingly, for 15 to 55% of benzalkonium chloride-resistant mutants from these parental strains the MICs of
oxacillin were over eightfold higher than the MICs for the parent
strains. MRSA strains highly resistant to oxacillin were also obtained
with benzethonium chloride, another cationic detergent (data not shown).
Most benzalkonium chloride-resistant mutants with resistance to
high concentrations of oxacillin formed small colonies on LB agar.
Revertants which formed large colonies showed sensitivity to both
benzalkonium chloride and oxacillin, as did the parent MRSA (data not
shown). Thus, resistance to oxacillin and resistance to benzalkonium
chloride are closely related and may be caused by a single mutation.
Isolation of S. aureus mutants resistant to benzalkonium
chloride has been reported (1, 5-7), but a related increase
in resistance to -lactam antibiotics has not been documented. We introduced into a wild type of S. aureus plasmid pTZ20
(6) containing the qacC gene, which is shown to
cause resistance to disinfectants. The qacC gene alone
increased resistance of S. aureus to benzalkonium chloride
but not to -lactams (data not shown).
MRSA mutants resistant to benzalkonium chloride showed a higher
resistance than parent strains to various -lactam antibiotics including cloxacillin, moxalactam, flomoxef, and cefmetazole (Table 1), as seen in the case of oxacillin.
These mutants showed no remarkable increase in resistance to
cefazolin and cephalothin (narrow-spectrum cephems),
chloramphenicol (an inhibitor of protein synthesis) or ampicillin but
did show resistance to ofloxacin, the lethal mechanisms of which differ
from those of -lactams. It is possible that the mutation may affect
the efficiency of uptake or may activate an efflux pump of drugs, in a
nonspecific manner. Another possibility is that mutation of genes
encoding elements regulating the expression of methicillin resistance, such as the femA gene (2), is responsible for the
phenotype. Mutations responsible for the concomitant increase in
resistance to benzalkonium chloride and -lactams will need to be
identified.
| |
ACKNOWLEDGMENTS |
We are grateful to Teruyo Ito, Keiichi Hiramatsu, Masahisa Noguchi,
Hiroaki Nakayama, Matsuhisa Inoue, and Yasuyuki Ogata for helpful
discussions and for materials.
| |
FOOTNOTES |
*
Phone: 81-3-5841-4820
Fax: 81-3-5684-2973
E-mail:
sekimizu{at}mol.f.u-tokyo.ac.jp
| |
REFERENCES |
| 1.
|
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|
| | | | |
Nobuyoshi Akimitsu
Hiroshi Hamamoto
Ryu-ichi Inoue
Department of Molecular Microbiology
Graduate School of Pharmaceutical Sciences
Kyushu University
Fukuoka, Japan
|
| | | | |
Mikio Shoji
Akifumi Akamine
Department of Operative Dentistry and Endodontology
Faculty of Dentistry
Kyushu University
Fukuoka, Japan
|
| | | | |
Koh-ichi Takemori
Naotaka Hamasaki
Central Clinical Laboratory
Kyushu University Hospital
Fukuoka, Japan
|
| | | | |
Kazuhisa Sekimizu*
Laboratory of Developmental Biochemistry
Graduate School of Pharmaceutical Sciences
University of Tokyo
7-3-1, Hongo, Bunkyo-ku
Tokyo, Japan
|
Antimicrobial Agents and Chemotherapy, December 1999, p. 3042-3043, Vol. 43, No. 12
0066-4804/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
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