Antibiotic resistance factors and other plasmids in bacterial isolates from hospitalized patients.

Among 186 arbitrarily selected gram-negative as well as gram-positive bacterial strains isolated from hospitalized patients, 63% were found to harbour plasmids, 42% of the strains with extrachromosomal DNA showed more than one plasmid DNA band in the agarose gel electrophoresis. The highest plasmid frequency was found in the enteric gram-negative microorganisms: 95% to 76% of the analyzed Enterobacter species, Klebsiella pneumoniae and E. coli isolates harboured plasmids. The plasmid frequency in the Staphylococci and group D Streptococci ranged between 36% and 75%. The examined Pneumococci strains were sensitive to all antibiotics and were found to be free of plasmids. In 45% of the enteric strains studied, the resistance markers were found to be transferable to a sensitive E. coli recipient by conjugation. Some of the resistance genes had not formed a stable complex with a plasmid and were lost during in vitro passage of their bacterial host. The loss of R-factors in vitro causes false diagnostic findings in regard to the in vivo antibiotic sensitivity of such strains. With purified plasmid DNA, individually isolated from E. coli strains, transformants were obtained carrying ampicillin resistance specifying plasmids of different molecular size. This result indicates that plasmids of different molecular size in a single bacterial cell can simultaneously carry ampicillin resistance genes. All findings indicated that the plasmids play an important role for the transfer and the maintenance of antibiotic resistance specifying genes in the bacterial isolates from clinical patients. However, most of the functions coded for by the numerous plasmids in the bacterial isolates remained unknown.