title: Molecular analysis of ampicillin-resistant sporadic Salmonella typhi and Salmonella paratyphi B clinical isolates.
abstract: OBJECTIVE: To study the mechanisms of antibiotic resistance in Salmonella typhi and Salmonella paratyphi B clinical isolates, and the clonality of resistant strains. METHODS: Antibiotic susceptibility was tested by disk-agar diffusion. Conjugation experiments and plasmid analysis by agarose gel electrophoresis after EcoRI digestion were followed by hybridization to a digoxigenin-labeled TEM-type beta-lactamase probe. DNA fingerprints were obtained by pulsed-field gel electrophoresis of Xbal-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 beta-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.
authors.affiliation: Department of Microbiology and.
authors.forename: Panayotis T.
authors.initials: PT
authors.lastname: Tassios
authors.forename: Alkiviadis C.
authors.initials: AC
authors.lastname: Vatopoulos
authors.forename: Efstratios
authors.initials: E
authors.lastname: Mainas
authors.forename: Dimitra
authors.initials: D
authors.lastname: Gennimata
authors.forename: John
authors.initials: J
authors.lastname: Papadakis
authors.forename: Asterios
authors.initials: A
authors.lastname: Tsiftsoglou
authors.forename: Victoria
authors.initials: V
authors.lastname: Kalapothaki
authors.forename: Nicholas J.
authors.initials: NJ
authors.lastname: Legakis