Epidemiology and genetics of CTX-M extended-spectrum β-lactamases in Gram-negative bacteria

Abstract

CTX-M enzymes, the plasmid-mediated cefotaximases, constitute a rapidly growing family of extended-spectrum β-lactamases (ESBLs) with significant clinical impact. CTX-Ms are found in at least 26 bacterial species, particularly in Escherichia coli, Klebsiella pneumoniae and Proteus mirabilis. At least 109 members in CTX-M family are identified and can be divided into seven clusters based on their phylogeny. CTX-M-15 and CTX-M-14 are the most dominant variants. Chromosome-encoded intrinsic cefotaximases in Kluyvera spp. are proposed to be the progenitors of CTX-Ms, while ISEcp1, ISCR1 and plasmid are closely associated with their mobilization and dissemination.

Figure 1.

Phylogenetic tree of CTX-M family based on amino-acid sequences. DNASIS Pro v2.10 (Hitachi Software Engineering Co., Tokyo, Japan) was used to align the amino-acid sequences and construct the phylogenetic tree. The amino-acid sequences were downloaded from GenBank under the accession numbers cited in Table 1. The branch lengths are drawn to scale and are proportional to the number of different amino-acid residues. The scale bars of 0.05 and 0.005 represent 5% and 0.5% amino-acid difference, respectively.

Figure 2.

Comparison of amino-acid sequences of seven representative enzymes in the CTX-M family. Amino-acids are numbered according to the standard numbering scheme for the class A serine β-lactamases, giving the active site serine residue the Ambler number 70. Dots indicate identical amino-acids compared to CTX-M-2. Deletion mutations are expressed with short lines. The underlined amino-acids, ⁷⁰SXXK⁷³, ¹⁰⁷P, 130SDN¹³², ¹⁴³GG¹⁴⁴, ¹⁶⁶E and ²³⁴KXG²³⁶, represent the conserved residues in typical class A serine β-lactamases.

Figure 3.

Identification of intrinsic cefotaximase genes in Kluyvera spp. as the original sources of acquired CTX-Ms based on their amino-acid identities and the homologies of neighboring sequences of the associated genes. c-CTX-M, CTX-M identified on chromosome of Kluyvera spp.; p-KLUC-2, KLUC-2 identified on plasmid in a clinical isolate of Enterobacter cloacae.

Figure 4.

Typical genetic platforms of CTX-M enzymes. A & B: the bla _CTX-M gene cassettes bracketed upstream by ISEcp1/ISEcp1-like and downstream by IS903/IS903-like (A) or orf477/orf477-like (B); C: bla _CTX-M genes associated with class 1 integron-ISEcp1; D & E: bla _CTX-M genes associated with class 1 integron-ISCR1 complex. CS, conserved segment; intI, integrase gene; qacE▵1, quaternary ammonium resistance gene; sul1, sulphonamide resistance gene; 3′-CS2, the second copy of 3′-conserved segment.