Extension of resistance to cefepime and cefpirome associated to a six amino acid deletion in the H-10 helix of the cephalosporinase of an Enterobacter cloacae clinical isolate.

:Enterobacter cloacae CHE, a clinical strain with overproduced cephalosporinase was found to be highly resistant to the new cephalosporins, cefepime and cefpirome (MICs> or =128 microg ml(-1)). The strain was isolated from a child previously treated with cefepime. The catalytic efficiency of the purified enzyme with the third-generation cephalosporins, cefepime and cefpirome, was 10 times higher than that with the E. cloacae P99 enzyme. This was mostly due to a decrease in K(m) for these beta-lactams. The clinical isolate produced large amounts of the cephalosporinase because introduction of the ampD gene decreased ampC expression and partially restored the wild-type phenotype. Indeed, MICs of cefepime and cefpirome remained 10 times higher than those for a stable derepressed clinical isolate (OUDhyp) transformed with an ampD gene. Sequencing of the ampC gene showed that 18 nucleotides had been deleted, corresponding to the six amino acids SKVALA (residues 289--294). According to the crystal structure of P99 beta-lactamase, this deletion was located in the H-10 helix. The ampR-ampC genes from the clinical isolates CHE and OUDhyp were cloned and expressed in Escherichia coli JM101. The MICs of cefpirome and cefepime of E. coli harboring ampC and ampR genes from CHE were 100--200 times higher than those of E. coli harboring ampC and ampR genes from OUDhyp. This suggests that the deletion, confirmed by sequencing of the ampC gene, is involved in resistance to cefepime and cefpirome. However, the high level of resistance to cefepime and cefpirome observed in the E. cloacae clinical isolate was due to a combination of hyperproduction of the AmpC beta-lactamase and structural modification of the enzyme. This is the first example of an AmpC variant conferring resistance to cefepime and cefpirome, isolated as a clinical strain.

Barnaud G ，Labia R ，Raskine L ，Sanson-Le Pors MJ ，Philippon A ，Arlet G ... -  《FEMS MICROBIOLOGY LETTERS》

Genetic characterization of an extended-spectrum AmpC cephalosporinase with hydrolysing activity against fourth-generation cephalosporins in a clinical isolate of Enterobacter aerogenes selected in vivo.

Rodríguez-Martínez JM ，Fernández-Echauri P ，Fernández-Cuenca F ，Diaz de Alba P ，Briales A ，Pascual A ... -  《-》

Resistance to cefepime and cefpirome due to a 4-amino-acid deletion in the chromosome-encoded AmpC beta-lactamase of a Serratia marcescens clinical isolate.

Mammeri H ，Poirel L ，Bemer P ，Drugeon H ，Nordmann P ... -  《-》

Gene mutations responsible for overexpression of AmpC beta-lactamase in some clinical isolates of Enterobacter cloacae.

Kaneko K ，Okamoto R ，Nakano R ，Kawakami S ，Inoue M ... -  《-》

Diversity of chromosomal AmpC beta-lactamases from Enterobacter cloacae isolates in a Portuguese hospital.

Conceição T ，Faria N ，Lito L ，Melo Cristino J ，Salgado MJ ，Duarte A ... -  《-》