Molecular analysis of ampicillin-resistant sporadic Salmonella typhi and Salmonella paratyphi B clinical isolates

Objective: To study the mechanisms of antibiotic resistance in Salmonella typhi and Salmonella paratyphi B clinical isolates, and the clonality of resistant strains.
Method: Antibiotic susceptibility was tested by disk-agar diffusion. Conjugation experiments and plasmid analysis by agarose gel electrophoresis after Eco RI digestion were followed by hybridization to a digoxigenin-labeled TEM-type β-lactamase probe. DNA fingerprints were obtained by pulsed-field gel electrophoresis of Xba I-digested chromosomal DNA.
Results: Three S. typhi isolates (7% of the isolates studied), of which one was ampicillin resistant and the other two multiresistant (ampicillin, chloramphenicol, tetracycline, sulfamethoxazole/trimethoprim and streptomycin), and two ampicillin-resistant S. paratyphi B isolates (25% of the isolates studied) were further evaluated. A 34-MDa conjugative plasmid, previously isolated from Salmonella enteritidis , conferred ampicillin resistance. A 100-MDa conjugative plasmid encoded resistance to chloramphenicol, tetracycline and sulfamethoxazole/trimethoprim, as well as ampicillin. Chromosomal fingerprinting revealed two distinct resistant strains for each serovar which were different from a matched set of sensitive S. typhi strains.
Conclusions: Two conjugative, TEM-type β-lactamase-encoding plasmids conferred ampicillin resistance to S. typhi and S. paratyphi B. The 34-MDa plasmid was identical to that previously characterized from S. enteritidis , while the 100-MDa plasmid also encoded resistance to chloramphenicol, tetracycline and sulfamethoxazole/trimethoprim. Resistant isolates did not belong to a single clone but rather represented distinct strains.