Genetic Exchange in the Respiratory Tract

Abstract

This chapter focuses on three important pathogens and the role that genetic exchange has played in their continuing evolution. There are numerous examples where genetic exchange has been responsible for the evolution of antibiotic resistance and virulence determinants and wholesale acquisition of pathogenicity islands, and there is increasing evidence that recombination is important in populations of both naturally transformable and nontransformable organisms. The prevalence of genetic exchange varies among different bacteria, as does the major mechanism of exchange. If these mechanisms enable the transmission of extrachromosomal elements, plasmids transposons, or lysogenic bacteriophage, bacteria can acquire completely novel resistance or virulence determinants. For chromosomal DNA, genetic exchange is mediated primarily by homologous recombination. Genetic exchange involving the acquisition of plasmids or conjugative transposons carrying novel resistance determinants is undoubtedly the most widespread route by which bacteria have acquired resistance to antibiotics. In many countries, lack of susceptibility to tetracyclines is the most frequently observed resistance phenotype in pneumococci. It would appear that different bacterial species are genetically variable for most of these factors. For example, restriction systems which differ widely between organisms function primarily against incoming double-stranded DNA but may also play a role in recombination. Although the future for the pathogenic commensal organisms of the upper respiratory tract is uncertain, it is clear that genetic exchange will continue to play an important role in helping to shape that future.