The biological cost of genetic elements.

Abstract

The emergence and increase of antimicrobial resistance among bacterial populations has raised interest in the factor that contributes to this situation. Mobile genetic elements and in particular integrons have been found to play a role in the spread of resistance genes due to their ability to capture and integrate one or more gene cassettes by site-specific recombination. The genes can be integrated within the same attachment sites resulting in the formation of composite clusters of antibiotic resistance genes that facilitate multiple resistance phenotypes. The aim of this study was to test the hypotheses that stated that the widespread dispersal of integrons in gram-negative bacteria is due to (1) low if any biological cost associated with harboring a class 1 integrons, (2) integron encoded cassettes are stably maintained in the absence of antimicrobial selection. The class 1 integron was obtained from the clinical strains A.baumannii Ab64 and Ab65 FFC and integrated into A.baylyi ADP1, an integron-free strain, by natural transformation. The fitness cost of three different strains was determined in pair wise competitions with the otherwise isogenic model organism ADP1. The strain Ab64.T1b that contained the newly acquired integron was found to have a fitness cost of 7% and stabile gene cassettes. The biological cost of the integrase was assessed with an integrase knock out strain Ab64.T1b int::cat and was observed to be 2% where as the fitness cost associated with harboring the gene cassettes nptII and sacB inserted in the selectively neutral locus was found to be zero. The stability of the integron encoded cassettes was tested in a 10 day experiment that corresponded to approximately 70 generation and it was found that the gene cassettes in the integron where 100% stable.