R Plasmids and Antibiotic Resistances

Le résumé fourni par la source

It has been estimated that at least 356 extra-chromosomal elements can introduce genetic information into Escherichia coli, an amount of exogenously contributed DNA, in the form of plasmids, equal to or greater than the DNA content of the E. coli chromosome. The mole fraction G+C values would provide a clue to their origin(s) since the E. coli chromosome has a G+C ratio of 0.50, in comparison with a G+C ratio of 0.38 for the Haemophilus influenza chromosome. The author and his coworkers found mole fraction G+C values of 0.39 and 0.40 for plasmids RSF007 and RSF0885, respectively, values that are closer to those for H. influenza than E. coli. To visualize in more detail the exact nature of the TnA sequence in both Haemophilus plasmids, they heteroduplexed each plasmid with R648, a replicon containing the entire ampicillin transposon with the added feature of one EcoRI cleavage site that served to orient TnA in heteroduplex analyses. The incidence of R-plasmid-mediated drug resistance among clinical isolates is most evident in highly selective environments such as intensive care units and burn wards, where the daily administration of high-potency antibiotics is often a necessity. The identities of two conjugative R plasmids isolated from bacterial strains from two gram-negative genera originating in a hospital burn unit were proved, by a variety of molecular techniques. Drug-resistant Pneumococcus strains cause otitis media meningitis among children in day-care centers, and in hospitals, clinicians grapple with multidrug-resistant Staphylococcus and Enterococcus species.

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