Emergence of a cefepime- and cefpirome-resistant Citrobacter freundii clinical isolate harbouring a novel chromosomally encoded AmpC β-lactamase, CMY-37

Citrobacter freundii strain 4306 was isolated from a urine specimen of a patient in March 2006 in Palestine. This strain showed a unique multidrug resistance phenotype, as it was resistant both to 7-α-methoxy- and oxyimino-cephalosporins, including cefepime, cefpirome and monobactams, in addition to quinolones, streptomycin and trimethoprim/sulfamethoxazole. Clavulanic acid did not act synergistically with cephalosporins by the double-disk synergy test. Molecular characterisation showed that the resistance to 7-α-methoxy- and oxyimino-cephalosporins was due to a novel AmpC β-lactamase, designated CMY-37, with an isoelectric point of ∼9.0. CMY-37 is a variant of C. freundii chromosomal AmpC enzymes with at least seven amino acid substitutions. One of these substitutions, L316I, is located within the R2 loop that is considered the hotspot region responsible for the extended substrate spectrum in class C β-lactamases. The blaCMY-37 gene was cloned and expressed in Escherichia coli TG1. CMY-37 is chromosomally encoded and is not associated with ISEcp1-like element. Phylogenetic analysis suggested that CMY-37 is the origin of many plasmid-mediated AmpC β-lactamases. This study highlights the emergence of cefepime and cefpirome resistance in C. freundii owing to a new type of AmpC β-lactamase.