Figure 3. Phylogenetic analysis of nikB in IncI complex plasmids from Escherichia coli. DNA sequences of nikB PCR amplicons and sequences obtained from public resources were aligned and analyzed by using MEGA 4.0 (29). A neighbor-joining tree was constructed by using complete deletion modeling and computed by using the maximum composite likelihood method (30). The phylogenetic tree was linearized assuming equal evolutionary rates in all lineages. Circles, nikB sequences from plasmids isolated from veterinary isolates from the United Kingdom; triangles, nikB sequences of plasmids from Escherichia coli isolated from humans; squares, nikB sequences of plasmids obtained from GenBank or the Sanger Institute; shaded shapes, plasmids identified as pCT-like by using PCR in this study; asterisks, plasmids encoding blaCTX-M-14. pCT, IncK plasmid. Scale bar indicates nucleotide substitutions per site.