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Preliminary characterization of a food-borne multiple-antibiotic-resistant Salmonella typhimurium strain
J D Schuman1, E A Zottola, S K Harlander
1Department of Food Science and Nutrition, University of Minnesota, St. Paul 55108.
Abstract:
Plasmid characterization studies were conducted on a Salmonella typhimurium strain isolated from pasteurized milk and from a symptomatic patient during the 1985 Illinois salmonellosis outbreak. This strain (Hf) was reported to possess an unusual plasmid profile which distinguished it from all Salmonella strains isolated in the United States prior to 1984. Antibiotic susceptibility testing revealed that the strain was resistant to tetracycline, erythromycin, clindamycin, sulfisoxazole, sulfadiazene, triple sulfa, cefoperazone, streptomycin, mezlocillin, piperacillin, carbenicillin, penicillin, ampicillin, and kanamycin. Plasmid analysis revealed that the strain possessed four plasmids with sizes of approximately 158, 98, 10.2, and 6.0 kilobase pairs (kb). Successive transfer at 43 degrees C led to increased antibiotic sensitivity in 75.5% of the isolates screened. Electroporation and calcium chloride treatment were each used to transform plasmid-free Escherichia coli strains with the plasmid pool from S. typhimurium Hf. Plasmids introduced by transformation ranged in size from 4.4 to 23.2 kb and correlated with resistance to penicillin G, ampicillin, carbenicillin, cephalothin, cefoperazone, cefamandole, mezlocillin, piperacillin, and in some cases, tetracycline and kanamycin. DNA-DNA hybridization experiments localized these resistance genes to a highly duplicated 6.3-kb fragment of the total EcoRI restriction digest of the S. typhimurium Hf plasmid pool.
Insights
This study investigated a unique Salmonella typhimurium strain from a 1985 outbreak, identifying four plasmids. These plasmids conferred multiple antibiotic resistances, which were transferable and localized to a specific DNA fragment.
Area of Science:
- Microbiology
- Molecular Biology
- Public Health
Background:
- A Salmonella typhimurium strain (Hf) isolated during the 1985 Illinois salmonellosis outbreak exhibited an unusual plasmid profile.
- This strain was resistant to a broad spectrum of antibiotics, including tetracycline, erythromycin, and ampicillin.
Purpose of the Study:
- To characterize the plasmids of the S. typhimurium Hf strain.
- To determine the antibiotic resistance profiles associated with these plasmids.
- To investigate the transferability and genetic localization of antibiotic resistance genes.
Main Methods:
- Antibiotic susceptibility testing was performed on the S. typhimurium Hf strain.
- Plasmid analysis was conducted using size determination and transformation experiments (electroporation, calcium chloride treatment) into Escherichia coli.
- DNA-DNA hybridization was used to localize resistance genes within plasmid fragments.
Main Results:
- The S. typhimurium Hf strain possessed four plasmids (approx. 158, 98, 10.2, and 6.0 kb).
- Heat treatment (43°C) resulted in increased antibiotic sensitivity in 75.5% of isolates, suggesting plasmid loss.
- Transformed E. coli strains acquired resistance to multiple antibiotics, with plasmids ranging from 4.4 to 23.2 kb, and resistance genes were mapped to a 6.3-kb fragment.
Conclusions:
- The characterized plasmids in S. typhimurium Hf are responsible for its extensive antibiotic resistance.
- The resistance genes are located on a transferable, highly duplicated DNA fragment.
- This finding provides insight into the genetic basis of antibiotic resistance in Salmonella during the 1985 outbreak.