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Spacer sequences of 42 and 127 bp, respectively, between the ISEcp1 insertion sequence and bla CTX-M have been observed in Klebsiella pneumoniae. However, it is not known whether different spacers upstream of bla CTX-M and a promoter within the 127 bp spacer influence cephalosporin resistance. Three recombinant plasmids with different spacers and with or without ISEcp1 were constructed to compare bla CTX-M-3 expression and susceptibility to cephalosporins. Our experiment revealed enhanced bla CTX-M-3 expression and a relatively high level resistance to cefotaxime and cefepime in recombinant plasmid IS42CTX-M-3, which contained ISEcp1 and the 42 bp spacer. A minor difference in bla CTX-M-3 expression was observed in recombinants IS127CTX-M-3 and 127CTX-M-3, which contained a 127 bp spacer with or without ISEcp1, respectively. In conclusion, the promoter within ISEcp1 and a shorter spacer (42 bp but not 127 bp) between ISEcp1 and bla CTX-M are necessary for high-level bla CTX-M-3 expression.
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