Stereo Dissecting Microscope

The CTX-M-25 family of β-lactamases is a closely related family of enzymes found rarely in the world. We aimed to describe the occurrence and to understand the dissemination of this extended-spectrum β-lactamase family among Enterobacteriaceae strains in our hospital.

Fifty-four CTX-M-producing Enterobacteriaceae strains collected from 2000 to 2005 were screened for bla_CTX-M-25 genes by PCR and sequencing. Genetic relatedness was analysed by PFGE. Antibiotic susceptibilities were determined by VITEK-2. Plasmids encoding bla_CTX-M-25-type genes were isolated, transformed and analysed by Southern blot using a bla_CTX-M-25 probe. Chromosomal location of bla_CTX-M-25-type was studied by I-CeuI restriction analysis. The bla_CTX-M-25 genetic environment was characterized by PCR mapping and partial sequencing.

Ten out of 54 CTX-M-producing isolates (18.5%) carried bla_CTX-M-25 genes, including Klebsiella pneumoniae (n = 4), Escherichia coli (n = 3), Enterobacter cloacae (n = 1) and Proteus mirabilis (n = 2). Isolates were genetically unrelated. Four β-lactamases were found: CTX-M-25, CTX-M-26, CTX-M-39 and CTX-M-41, a new member of the family (accession no. DQ023162) that differed from CTX-M-25 in three amino acids, Ala80Val, Val106Ile and Ile126Ser. bla_CTX-M-25-type genes were plasmid-mediated in all genera but P. mirabilis, organized in a class I integron and located downstream of an ISEcp1 element. The genes were encoded on different plasmids with varying degree of similarities. Several antibiotic-resistant determinants conferring resistance to trimethoprim and aminoglycosides existed on the same integron.

bla_CTX-M-25 exists in Israel in different enteric species. Spread of these enzymes within and between species is due to transfer of plasmids with common regions and by dissemination of determinants encoding these genes. CTX-M-41, a novel member of this family, was identified in the chromosome of P. mirabilis.