Characteristics of the antibiotic resistance plasmid in Salmonella typhi isolated in Tunis in 1990.

A multiresistant Salmonella typhi (S typhi, strain 302) was isolated from a blood culture of a patient in the Infectious Diseases department of Rabta Hospital in Tunis. The following tests were carried out: antibiotic susceptibility testing by the agar diffusion method; determination of the minimum inhibitory concentration against four beta-lactam antibiotics (amoxicillin, ceftriaxone, ceftazidime, imipenem), chloramphenicol, gentamicin, and amikacin by the agar dilution method; conjugation with E coli K12 for study of transferability of resistance markers; and electrophoresis of plasmid DNA extracts on agarose gel. S typhi 302 was resistant to amoxicillin, streptomycin, tetracycline, chloramphenicol, and sulfamide-trimethoprim, and this resistance was transferable in toto with a frequency of 10(-4). The MICs of amoxicillin and chloramphenicol were, respectively, 1024 (due to the production of TEM-1 beta-lactamase) and 256 mg/l. These resistance markers were carried by a plasmid of about 40 kb, similar to the Salmonella wien plasmid. The easy acquisition of a multiresistance plasmid by S typhi suggests that epidemiological monitoring of this serovar should be carried out.