Regional dissemination of a trimethoprim-resistance gene cassette via a successful transposable element.
BACKGROUND
Antimicrobial resistance is a growing international problem. We observed a 50% increase in the prevalence of trimethoprim resistance among fecal Escherichia coli from healthy Nigerian students between 1998 and 2005, a trend to increase that continued in 2009.


RESULTS
A PCR-based screen revealed that 131 (43.1%) of isolates obtained in Nigeria in 2005 and 2009 carried integron-borne dfrA cassettes. In the case of 67 (51.1%) of these isolates, the cassette was a class 1-integron-borne dfrA7 gene, which has been reported at high prevalence from E. coli isolates from other parts of Africa. Complete sequencing of a 27 Kb dfrA7-bearing plasmid from one isolate located the dfrA7 gene within a Tn21-type transposon. The transposon also contained an IS26-derived bla/sul/str element, encoding resistance to β-lactams, sulphonamides and streptomycin, and mercury resistance genes. Although the plasmid backbone was only found in 12 (5.8%) of trimethoprim-resistant isolates, dfrA7 and other transposon-borne genes were detected in 14 (16.3%) and 32 (26.3%) of trimethoprim resistant isolates collected in Nigeria in 2005 and 2009, respectively. Additionally, 37 (19.3%) of trimethoprim-resistant E. coli isolates collected between 2006 and 2008 from Ghana were positive for the dfrA7 and a transposon marker, but only 4 (2.1%) harbored the plasmid backbone.


CONCLUSIONS
Our data point to transposition as a principal mechanism for disseminating dfrA7 among E. coli from Nigeria and Ghana. On-going intensive use of the affordable broad-spectrum antibacterials is likely to promote selective success of a highly prevalent transposable element in West Africa.
Department of Biology, Haverford College, Haverford, Pennsylvania, United States of America.
Amy S
AS
Labar
Jennifer S
JS
Millman
Ellen
E
Ruebush
Japheth A
JA
Opintan
Rima A
RA
Bishar
A Oladipo
AO
Aboderin
Mercy J
MJ
Newman
Adebayo
A
Lamikanra
Iruka N
IN
Okeke
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial / Q000235:genetics*; D018432:Drug Resistance, Multiple / Q000235:genetics; D004926:Escherichia coli / Q000187:drug effects / Q000235:genetics; D005798:Genes, Bacterial / Q000235:genetics*; D005822:Genetic Vectors / Q000235:genetics*; D006801:Humans; D032023:Integrons / Q000235:genetics*; D009549:Nigeria; D010957:Plasmids / Q000235:genetics; D014295:Trimethoprim / Q000494:pharmacology*
D000900:Anti-Bacterial Agents; D014295:Trimethoprim