A metallo-β-lactamase is responsible for the degradation of ceftiofur by the bovine intestinal bacterium Bacillus cereus P41

- Ceftiofur degradation by the bovine isolate Bacillus cereus P41 is examined.
- The major stable metabolite is derived from the furoic acid side chain.
- The related cephalosporins ceftriaxone and cefpodoxime are similarly degraded.
- The metallo-β-lactamase responsible for this degradation was cloned and sequenced.
- Degradation by metallo-β-lactamases may explain ceftiofur's instability in the gut.

Ceftiofur is a highly effective veterinary cephalosporin, yet it is rapidly degraded by bacteria in the gut. The goal of this work was to directly determine the mechanism of ceftiofur degradation by the bovine intestinal isolate Bacillus cereus P41. B. cereus P41 was isolated from the feces of a cow that had not been treated with cephalosporins, and was found to rapidly degrade ceftiofur in culture. Analysis of spent culture media by HPLC/UV and HPLC/MS revealed one major metabolite of ceftiofur, with a negative ion m/z of 127. Comparison of ceftiofur, ceftriaxone, and cefpodoxime degradation suggested that the major stable ceftiofur metabolite was the thiofuroic acid group eliminated from the C-3 position of the drug after hydrolysis by β-lactamase. Genomic DNA from B. cereus P41 was cloned into Escherichia coli, and the transformants were screened for growth in the presence of ceftiofur. DNA sequencing of the plasmid pHSG299-BC-3 insert revealed the presence of a gene encoding a metallo-β-lactamase. Incubation of ceftiofur with either the E. coli transformant or a commercial B. cereus metallo-β-lactamase showed degradation of the drug and formation of the same major metabolite produced by B. cereus P41. These data demonstrate that a metallo-β-lactamase plays a major role in the degradation of ceftiofur by the bovine intestinal bacterium B. cereus P41.