A Novel Type of AmpC β-Lactamase, ACC-1, Produced by a Klebsiella pneumoniae Strain Causing Nosocomial Pneumonia

A Klebsiella pneumoniae strain resistant to oxyimino cephalosporins was cultured from respiratory secretions of a patient suffering from nosocomial pneumonia in Kiel, Germany, in 1997. The isolate harbors a bla resistance gene located on a transmissible plasmid. An Escherichia coli transconjugant produces a β-lactamase with an isoelectric point of 7.7 and a resistance phenotype characteristic of an AmpC (class 1) β-lactamase except for low MICs of cephamycins. The bla gene was cloned and sequenced. It encodes a protein of 386 amino acids with the active site serine of the S-X-X-K motif at position 64, as is characteristic for class C β-lactamases. Multiple alignment of the deduced amino acid sequence with 21 other AmpC β-lactamases demonstrates only very distant homology, reaching at maximum 52.3% identity for the chromosomal AmpC β-lactamase of Serratia marcescens SR50. The β-lactamase of K. pneumoniae KUS represents a new type of AmpC-class enzyme, for which we propose the designation ACC-1 (Ambler class C-1).