Barry G Hall — Page 2 | PubMed Health

PubMed2002

Origin and evolution of the AmpC beta-lactamases of Citrobacter freundii.

To determine whether the widespread clinical use of beta-lactams has been selective for Citrobacter freundii-derived alleles of plasmid ampC genes, we generated a Bayesian consensus phylogeny of the published ampC sequences and compared the MICs of 16 beta-lactam antibiotics for Escherichia coli strains containing cloned copies of the C. freundii ampC alleles. We found that for the majority of compounds investigated, there has been essentially no increase in beta-lactam resistance conferred by those alleles. We also found that ampC alleles from the chromosomes of two beta-lactam-sensitive C. freundii strains isolated in the 1920s, before the clinical use of antibiotics, were as effective at providing beta-lactam resistance in E. coli as were the plasmid-borne alleles from beta-lactam-resistant clinical isolates. These results suggest that selection for increased resistance to beta-lactam antibiotics has not been a significant force directing the evolution of the C. freundii ampC alleles found in beta-lactam-resistant clinical isolates.

Bacterial Proteins↗

PubMed2002

Predicting evolution by in vitro evolution requires determining evolutionary pathways.

In an early example of DNA shuffling, Stemmer (W. P. C. Stemmer, Nature 370:389-390, 1994) demonstrated a dramatic improvement in the activity of the TEM-1 beta-lactamase toward cefotaxime as the consequence of six amino acid substitutions. It has been pointed out (B. G. Hall, FEMS Microbiol. Lett. 178:1-6, 1999; M. C. Orencia, J. S. Yoon, J. E. Ness, W. P. Stemmer, and R. C. Stevens, Nat. Struct. Biol. 8:238-242, 2001) that the power of DNA shuffling might be applied to the problem of predicting evolution in nature from in vitro evolution in the laboratory. As a predictor of natural evolutionary processes, that power may be misleading because in nature mutations almost always arise one at a time, and each advantageous mutation must be fixed into the population by an evolutionary pathway that leads from the wild type to the fully evolved sequence. Site-directed mutagenesis was used to introduce each of Stemmer's six substitutions into TEM-1, the best single mutant was chosen, and each of the remaining five substitutions was introduced. Repeated rounds of site-directed mutagenesis and selection of the best mutant were used in an attempt to construct a pathway between the wild-type TEM-1 and Stemmer's mutant with six mutations. In the present study it is shown (i) that no such pathway exists between the wild-type TEM-1 and the supereffective cefotaxime-hydrolyzing mutant that was generated by six amino acid substitutions via DNA shuffling (Stemmer, Nature 370:389-390, 1994) but that a pathway to a fourfold more efficient enzyme resulting from four of the same substitutions does exist, and (ii) that the more efficient enzyme is likely to arise in nature as the result of a single mutation in the naturally occurring TEM-52 allele.

Alleles↗