Distribution of the blaTEM gene and blaTEM-containing transposons in commensal Escherichia coli

Objectives: The context of antibiotic resistance genes can provide valuable information about the epidemiology of mobile genetic elements. This study examined the distribution of the closely related blaTEM transposons Tn1, Tn2 and Tn3, or blaTEM-containing fragments of them, in ampicillin-resistant human commensal Escherichia coli isolates. Methods: A PCR mapping protocol was used to detect different segments of the transposons or to link partial copies to the insertion sequence IS26. Restriction digestion of one amplicon was used to assign transposons to Tn1, Tn2 or Tn3 groups and sequencing validated this approach. Restriction digestion and sequencing were used to determine how much of the transposon remained when blaTEM was linked to IS26. Sequences were compared with those in GenBank. Results: Of 25 ampicillin-resistant E. coli strains recovered from the faecal flora of healthy humans that carried the blaTEM gene, 15 carried a complete copy of Tn2 or a Tn2 variant; one was interrupted by IS4. A further isolate carried Tn3. Tn2 was also most abundant in sequences available in GenBank. Two isolates carried Tn2 and an IS26-blaTEM fragment. The remaining 10 isolates carried only the blaTEM end of the transposon and 9 of these partial copies were flanked by IS26 at varying distances upstream of blaTEM. One configuration corresponded to that in Tn6029B and the complete transposon was shown to be present. Conclusions: Tn1, Tn2 and Tn3 can be simply and rapidly identified. Tn2 appears to be the most widely distributed. However, the blaTEM-containing end associated with an IS26 is also widely distributed.