Escherichia coli present the ampC naturally, and the observation of phenotypical resistance to cefoxitin is related to this gene deregulation. Mutations in the regulatory region in ampC cause exaggerated expression. The most frequent alterations in the E. coli AmpC promoter/attenuator leading to this overexpression is described at the positions: -88, -82, -42, -18, -1 and +58. Mastitis studies were carried in Rio de Janeiro and Mato Grosso, Brazil. Two cefoxitin and amoxicillin-clavulanic acid-resistant E. coli from farms animals were unusually detected once these characteristics are not observed together in this species. The objective of this work was to determine if these isolates had a chromosomal gene mutation, determining AmpC hyperproduction. After DNA sequencing, mutations were observed at -88, -82, -73, -18, -1 and +58 positions, confirming the initially suspected AmpC hyperexpression. In Brazil, this is the first work to report E. coli hyperproducing this enzyme.
Key words: Ampc attenuator, AmpC hyperproduction, ampC promoter, bovine feces, mastitic milk.
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