High Rate of Mobilization for bla(CTX-M)s

We constructed a phylogenetic analysis of class A β-lactamases and found that the bla(CTX-M)s have been mobilized to plasmids ≈10 times more frequently than other class A β-lactamases. We also found that the bla(CTX-M)s are descended from a common ancestor that was incorporated in ancient times into the chromosome of the ancestor of Kluyvera species through horizontal transfer. Considerable sequence divergence has occurred among the descendents of that ancestral gene sequence since that gene was inserted. That divergence has mainly occurred in the presence of purifying selection, which indicates a slow rate of evolution for bla(CTX-M)s in the pre–antimicrobial drug era.