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Antibiotic resistance factors and other plasmids in bacterial isolates from hospitalized patients
Abstract:
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.
Insights
Bacterial plasmids, or extrachromosomal DNA, are common in hospital strains and play a key role in antibiotic resistance transfer. Understanding these plasmids is crucial for accurate antibiotic sensitivity testing.
Area of Science:
- Microbiology
- Molecular Biology
- Genetics
Background:
- Plasmids are extrachromosomal DNA elements frequently found in bacteria.
- Antibiotic resistance is a growing global health concern, often mediated by genetic elements like plasmids.
- Hospitalized patients are a significant source of bacterial isolates with potential resistance mechanisms.
Purpose of the Study:
- To investigate the prevalence and characteristics of plasmids in bacterial strains from hospitalized patients.
- To determine the role of plasmids in the transfer and maintenance of antibiotic resistance genes.
- To assess the impact of plasmid loss on antibiotic sensitivity diagnostics.
Main Methods:
- Agarose gel electrophoresis to detect plasmid DNA.
- Conjugation experiments to assess transferability of resistance markers.
- Transformation experiments using purified plasmid DNA from E. coli.
- Analysis of bacterial isolates from hospitalized patients.
Main Results:
- 63% of 186 bacterial strains harbored plasmids, with higher frequencies in Gram-negative enteric bacteria (e.g., E. coli, Klebsiella pneumoniae).
- 45% of enteric strains showed transferable resistance markers via conjugation; some resistance genes were unstable and lost in vitro.
- Pneumococci strains were plasmid-free and sensitive to all tested antibiotics.
- Transformants with ampicillin resistance were obtained using plasmids of varying molecular sizes, indicating co-carriage of resistance genes.
Conclusions:
- Plasmids are crucial for the acquisition, transfer, and maintenance of antibiotic resistance genes in clinical bacterial isolates.
- In vitro loss of plasmids can lead to inaccurate in vivo antibiotic sensitivity diagnoses.
- The functions encoded by many plasmids in clinical isolates remain largely uncharacterized.