Identification of a New Allelic Variant of the Acinetobacter baumannii Cephalosporinase, ADC-7 β-Lactamase: Defining a Unique Family of Class C Enzymes

Abstract

Acinetobacter spp. are emerging as opportunistic hospital pathogens that demonstrate resistance to many classes of antibiotics. In a metropolitan hospital in Cleveland, a clinical isolate of Acinetobacter baumannii that tested resistant to cefepime and ceftazidime (MIC ‫؍‬ 32 g/ml) was identified. Herein, we sought to determine the molecular basis for the extended-spectrum-cephalosporin resistance. Using analytical isoelectric focusing, a ␤-lactamase with a pI of >9.2 was detected. PCR amplification with specific A. baumannii cephalosporinase primers yielded a 1,152-bp product which, when sequenced, identified a novel 383-amino-acid class C enzyme. Expressed in Escherichia coli DH10B, this ␤-lactamase demonstrated greater resistance against ceftazidime and cefotaxime than cefepime (4.0 g/ml versus 0.06 g/ml). The kinetic characteristics of this ␤-lactamase were similar to other cephalosporinases found in Acinetobacter spp. In addition, this cephalosporinase was inhibited by meropenem, imipenem, ertapenem, and sulopenem (K i < 40 M). The amino acid compositions of this novel enzyme and other class C ␤-lactamases thus far described for A. baumannii, Acinetobacter genomic species 3, and Oligella urethralis in Europe and South Africa suggest that this cephalosporinase defines a unique family of class C enzymes. We propose a uniform designation for this family of cephalosporinases (Acinetobacter-derived cephalosporinases [ADC]) found in Acinetobacter spp. and identify this enzyme as ADC-7 ␤-lactamase. The coalescence of Acinetobacter ampC ␤-lactamases into a single common ancestor and the substantial phylogenetic distance separating them from other ampC genes support the logical value of developing a system of nomenclature for these Acinetobacter cephalosporinase genes.Acinetobacter spp. are commonly associated with serious nosocomial infections (10,14,23,37,40,41). Most recently, American troops wounded in Iraq (Operation Iraqi Freedom) and Afghanistan (Operation Enduring Freedom) have suffered severe infections from antibiotic-resistant Acinetobacter baumannii, making this organism an important pathogen in military health

Acinetobacter baumannii : Emergence of a Successful Pathogen

“…However, in keeping with the complex nature of this organism, multiple mechanisms often work in concert to produce the same phenotype (47,165,446). Inherent to all A. baumannii strains are chromosomally encoded AmpC cephalosporinases (49,249,250,427,468), also known as Acinetobacter-derived cephalosporinases (ADCs) (249). Unlike that of AmpC enzymes found in other gramnegative organisms, inducible AmpC expression does not occur in A. baumannii (49,233).…”

Molecular characterization of carbapenem-resistant Acinetobacter species in an Irish university hospital: predominance of Acinetobacter genomic species 3

“…PCR was also performed to screen for other resistance genes that might be present in carbapenem-resistant isolates, including those encoding Ambler class A b-lactamase-encoding genes (bla TEM , bla SHV , bla CTX-M , bla VEB and bla PER ) (Hopkins et al, 2006;Naas et al, 2006) and Ambler class C Acinetobacter-derived cephalosporinase-encoding genes (bla ADC ) (Hujer et al, 2005). The presence of integrons was screened using primers specific for class 1 and class 2 integrases (Koeleman et al, 2001).…”

Crystal structure of the carbapenemase OXA-24 reveals insights into the mechanism of carbapenem hydrolysis

“…The ␤-lactamase was then purified to homogeneity following the manufacturer's instructions for the GST Gene Fusion System (Amersham Pharmacia Biotech, Europe GmbH, Barcelona, Spain). Kinetic experiments were performed as described (33).…”