Antimicrobial Agents and Chemotherapy, February 1998, p. 209-215, Vol. 42, No. 2
Service de Microbiologie,
Received 4 April 1997/Returned for modification 12 June
1997/Accepted 11 November 1997
Three clinical isolates, Enterobacter cloacae EC1562
and EC1563 and Citrobacter freundii CFr564, displayed an
aminoglycoside resistance profile evocative of low-level
6'-N acetyltransferase type II [AAC(6')-II] production,
which conferred reduced susceptibility to gentamicin but not to
amikacin or isepamicin. Aminoglycoside acetyltransferase assays
suggested the synthesis in the three strains of an AAC(6') which
acetylated amikacin practically as well as it acetylated gentamicin in
vitro. Both compounds, however, as well as isepamicin, retained good
bactericidal activity against the three strains. The aac
genes were borne by conjugative plasmids (pLMM562 and pLMM564 of ca.
100 kb and pLMM563 of ca. 20 kb). By PCR mapping and nucleotide
sequence analysis, an aac(6')-Ib gene was found in each
strain upstream of an ant(3")-I gene in a
sulI-type integron. The size of the AAC(6')-Ib variant
encoded by pLMM562 and pLMM564, AAC(6')-Ib7, was deduced to
be 184 (or 177) amino acids long, whereas in pLMM563 a 21-bp
duplication allowing the recruitment of a start codon resulted in the
translation of a variant, AAC(6')-Ib8, of 196 amino acids,
in agreement with size estimates obtained by Western blot analysis.
Both variants had at position 119 a serine instead of the leucine
typical for the AAC(6')-Ib variants conferring resistance to amikacin.
By using methods that predict the secondary structure, these two amino
acids appear to condition an
0066-4804/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Aminoglycoside 6'-N-Acetyltransferase
Variants of the Ib Type with Altered Substrate Profile in Clinical
Isolates of Enterobacter cloacae and Citrobacter
freundii

-helical structure within a putative
aminoglycoside binding domain of AAC(6')-Ib variants.
*
Corresponding author. Mailing address: L.R.M.A.,
Université Paris VI, 15, rue de l' Ecole de Médecine,
75270 Paris Cedex 07, France. Phone: 33-1-42.34.68.65. Fax:
33-1-43.25.68.12. E-mail: collatz{at}ccr.jussieu.fr.
Present address: Hoechst Marion Roussel, 93235 Romainville Cedex,
France.
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