Extended-spectrum cephalosporinase in Acinetobacter baumannii

Abstract

An AmpC-type beta-lactamase conferring high-level resistance to expanded-spectrum cephalosporins and monobactams was characterized from an Acinetobacter baumannii clinical isolate. This class C beta-lactamase (named ADC-33) possessed a Pro210Arg substitution together with a duplication of an Ala residue at position 215 (inside the Omega-loop) compared to a reference AmpC cephalosporinase from A. baumannii. ADC-33 hydrolyzed ceftazidime, cefepime, and aztreonam at high levels, which allows the classification of this enzyme as an extended-spectrum AmpC (ESAC). Site-directed mutagenesis confirmed the role of both substitutions in its ESAC property.

FIG. 1.

Amino acid sequence alignment including ADC-33 (-33) as an ESAC, ADC-50 (-50) and ADC-11 (-11) as regular AmpCs, and ADC-7 (-7) taken as the reference sequence as published (10). Conserved amino acid identities are indicated by dashes. The typical AmpC β-lactamase domains (SVSK, YSN, and KTG) are underlined. Helices H-2 and H-10 are boxed in gray. The Ω-loop is boxed in gray and double underlined. Differences observed inside the Ω-loop are in boldface. The vertical arrow indicates the position of the +1 amino acid (cleavage site for signal peptide). Numbering is according to the sequence of the mature protein.