JAC Advance Access published online on March 13, 2003
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkg148
© 2003 by The British Society for Antimicrobial Chemotherapy
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Original article
1 Department of Microbiology and Immunology, Queen's
University, Kingston, Ontario, Canada K7L 3N6
* Corresponding author. E-mail: poolek{at}post.queensu.ca.
Received 17 October 2002
; revised 6 December 2002
; accepted 11 January 2003
Stenotrophomonas maltophilia is an
emerging nosocomial pathogen that displays high-level intrinsic
resistance to multiple antibiotics including aminoglycosides. A
gene [aac(6')-Iz] encoding an aminoglycoside-modifying
enzyme, AAC(6')-Iz acetyltransferase,
was recently cloned and sequenced in S. maltophilia,
but its importance with respect to aminoglycoside resistance in this
organism was not determined. Using a homologous gene replacement
approach, mutants carrying unmarked chromosomal deletions of the aac(6')-Iz gene
were constructed in wild-type and in vitro-selected
aminoglycoside-resistant S. maltophilia. AAC(6')-Iz-deficient mutants derived
from both wild-type and aminoglycoside-resistant strains displayed
an increase in susceptibility to amikacin, netilmicin, sisomicin
and tobramycin (4- to 32-fold decrease in MICs), known substrates
for AAC(6')-I enzymes. The cloned aac(6')-Iz gene
restored the aminoglycoside resistance of the aac(6')-Iz mutants, and
could also confer aminoglycoside resistance upon Escherichia
coli. To assess the significance of the aac(6')-Iz gene with respect
to the aminoglycoside resistance of clinical strains, its distribution
was assessed in 65 clinical isolates from two hospitals. Using PCR,
Southern hybridization, RT-PCR and/or nucleotide sequencing, the aac(6')-Iz gene
was identified in 57% of the isolates. Susceptibility tests
indicated a good correlation between the presence of the aac(6')-Iz gene and the
resistance to tobramycin, netilmicin and sisomicin in these strains.
These results indicate that the aac(6')-Iz gene is an important contributor to aminoglycoside
resistance in clinical strains of S. maltophilia,
particularly to tobramycin.
Keywords: aminoglycoside, resistance, Stenotrophomonas
maltophilia
Role of the acetyltransferase AAC(6')-Iz modifying enzyme in aminoglycoside resistance
in Stenotrophomonas maltophilia
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