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The most common mechanism of trimethoprim (TMP)-resistance is the acquisition of dihydrofolate reductase enzyme resistant to this drug. Previous molecular characterization of TMP-genes resistance in Chilean isolates of  Shigella sonnei  searching for  dfrA1  and  dfrA8 , showed solely the presence of  dfrA8  (formerly  dhfrIIIc ). However, these genetic markers were absent in  S. sonnei  strains further isolated during an outbreak in 2009. To identify the TMP-resistance gene in these strains, a genomic DNA library from a TMP-resistant (TMP R )  S. sonnei  representative strain for the outbreak was used to clone, select and identify a TMP-resistance marker. The TMP R  clone was sequenced by primer walking, identifying the presence of the  dfrA14  gene in the  sul2-strA'-dfrA14-‘strA-strB  gene arrangement, harbored in a native 6779-bp plasmid. The same plasmid was isolated by transforming with a ~4.2 MDa plasmid extracted from several TMP R   S. sonnei  strains into  Escherichia coli . This plasmid, named pABC-3, was present only in  dfrA14 -positive strains and was homologous to a previously described pCERC-1, but different due to the absence of an 11-bp repetitive unit. The distribution of  dfrA1, dfrA8 , and  dfrA14  TMP-resistance genes was determined in 126 TMP R   S. sonnei  isolates. Most of the strains (96%) carried only one of the three TMP-resistance genes assessed. Thus, all strains obtained during the 2009-outbreak harbored only  dfrA14 , whereas,  dfrA8  was the most abundant gene marker before outbreak and, after the outbreak  dfrA1  seems have appeared in circulating strains. According to PFGE,  dfrA14-positive  strains were clustered in a genetically related group including some  dfrA1 - and  dfrA8 -positive strains; meanwhile other genetic group included most of the  dfrA8 -positive strains. This distribution also correlated with the isolation period, showing a dynamics of trimethoprim genetic markers prevalent in Chilean  S. sonnei  strains. To our knowledge,  dfrA14  gene associated to a small non-conjugative plasmid was detected for the first time in  Shigella . Apparently, the strain causing the outbreak must have been introduced, changing drastically the genetic distribution of trimethoprim resistance in Chilean  S. sonnei  strains.

Emergence of Plasmid-Borne dfrA14 Trimethoprim Resistance Gene in Shigella sonnei