4.1 The origin of CTX-M-type ESBLs

Unlike most acquired β-lactamases, for which the source remains unknown, the source of CTX-M-type ESBLs has been identified as members of the bacterial genusKluyvera.

Kluyvera spp. are soil bacteria which are associated with plant roots and are non-pathogenic to humans. Precursors of the CTX-M genes found in E. coli have been identified as chromosomal genes in Kluyvera spp. where they can confer resistance to third-generation cephalosporins (Humeniuk et al., 2002).

These chromosomal CTX-M precursor genes have been captured and incorporated into plasmids through mechanisms that you do not need to know about in this course.

In the next section, you will look at how these plasmid-encoded CTX-M genes have been rapidly spread by horizontal gene transfer to other bacterial types, including E. coli and Klebsiella pneumoniae.