Chromosomal β-lactamases in enterobacteria and in vivo evolution of β-lactam resistance
Sammanfattning: The ß-lactam antibiotics are the most important antibacterial agents in the treatment of infectious diseases. A severe problem in ß-lactam therapy is the emergence of ß-lactam resistant bacteria. Clinical ß-lactam resistance is most often due to the production of ß-lactamases. ß-lactamase genes reside either on plasmids or on the chromosome. The aim of this study was to acquire an understanding of organisation and regulation of chromosomal ß- lactamase genes in different Gram negative species and to elucidate the mechanisms for ampC hyperproduction in the in vivo situation.By DNA hybridization with an ampC probe from Escherichia coli K-12 it was shown that other Gram negative bacteria contained an artpC like chromosomal gene, suggesting a common evolutionary origin. Furthermore, the preceding frd operon that overlaps the ampC gene in E. coli K-12 was found to be much more conserved than the ampC gene in the bacterial species investigated.The ampC and frd opérons in Shigella sonnei and Citrobacter freundii were cloned and characterized by physical mapping. The respective maps were compared to the ampC and frd region in E. coli K-12. The physical map of Sh. sonnei was almost identical to the E. coli K-12 map, whereas in C. freundii only the frd region exhibited any considerable homology. Moreover, in C. freundii, the anpC and frd regions were separated by 1100 basepairs. It is suggested that this DNA is involved in the induction of ß-lactamase production in this organism. A hypothesis for the evolution of the anpC operon in enterobacteria is presented.By isolating and characterizing six ß-lactam resistant clinica], isolates of E. coli hyperproducing the dhrcmosomal ß-lactamase, genetic mechanisms for in vivo evolved resistance was aimed at. These isolates exhibited a 24-48 fold increase in ß-lactamase production. The ß-lactamase produced was found to be biochemically and immunologically identical to the ß-lactamase produced by E. coli K-12. The ampC control region of these six E. coli isolates was DNA-seqenced. The cause of ß-lactamase hyperproduction in five of the clinical E. coli isolates, identical in the DNA segment sequenced, was due to a strong novel ampC promoter displaced 5 bp upstream of the ampC promoter defined in E. coli K-12. The ß-lactamase hyperproduction in the sixth clinical isolate was shown to be caused by two mutations affecting both the promoter and the attenuator in the regulatory region defined by E. coli K- 12. The obtained changes were sufficient to explain the increase in ampC ß- 1 act ama se expression exhibited in these clinical E. coli isolates.Sequence analysis of the ampC control region in Sh. sonnei revealed that it was, with one exception, identical to the one found in the five clinical E. coli ß-lactamase hyperproducers. The only difference was in a position that creates the strong novel ampC promoter in the E. coli hyperproducers. By isolating spontaneous Sh. sonnei mutants with a 40-fold increase in ß-lactamase production carrying the same novel ampC promoter as the clinical E. coli isolates it was concluded that this DNA segment has been transferred in vivo frcm Shigella to E. coli across the species barrier.
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