J. M. Ling

Molecular studies of plasmids of multiply-resistant Shigella spp. in Hong Kong

One hundred and two Shigella spp. isolated in two hospitals in Hong Kong were analysed for antibiotic resistances, resistance plasmids and plasmid profiles. Three quarters of the isolates were S. flexneri. All isolates harboured plasmids, up to a maximum of ten within one strain. Plasmids of 220 kb encoding resistances to tetracycline, chloramphenicol and sulphonamide and probably also associated with invasiveness in the Sereny test were found in 80 strains and were transferable in 18% of cases. Resistance plasmids of 92 and 99 kb were found in 27 and 15 strains respectively and encoded resistances to ampicillin, tetracycline, chloramphenicol, kanamycin, sulphonamide, trimethoprim, cotrimoxazole and gentamicin; these plasmids were usually transferable. Four plasmids of 3.9, 2.8, 2.2 and 1.8 kb were commonly found in S. flexneri strains, but were rare in other species. In contrast, there was no predominant plasmid profile in S. sonnei. S. flexneri is endemic in Hong Kong and these plasmid studies suggest that the strains in circulation are derived from only a few clones.

Epidemiological Analysis of Salmonella enterica Serotype typhimurium from Hong Kong by Ribotyping and Pulsed-field Gel Electrophoresis

We investigated the antimicrobial susceptibilities and molecular epidemiology of 200 strains of Salmonella enterica serotype typhimurium isolated from 1989 to 1996 in Hong Kong. Only 22% of strains were susceptible to all 19 antibiotics tested but all were susceptible to second- and third-generation cephalosporins. Up to 9% of strains were resistant to 0.12 mg/l concentrations of ciprofloxacin or ofloxacin but none were resistant to 1 or 2 mg/l concentrations of these 2 drugs, respectively. The isolates were grouped into 15 types by ribotyping with restriction endonuclease EcoRI and into 53 types by pulsed-field gel electrophoresis of XbaI-restricted DNA fragments. When DNA fragments of the ribotypes and pulsotypes were pooled and analyzed 87 types resulted, 76 (87%) of which were of > 90% similarity and were grouped into 15 clusters. About 60% of the isolates belonged to 3 clusters, which probably represented 3 clones endemic in the community. The rest of the isolates were of a large variety of types or clusters. For epidemiological purposes analysis of pooled results from different molecular techniques would be more discriminative than results from individual techniques alone.We investigated the antimicrobial susceptibilities and molecular epidemiology of 200 strains of Salmonella enterica serotype typhimurium isolated from 1989 to 1996 in Hong Kong. Only 22% of strains were susceptible to all 19 antibiotics tested but all were susceptible to second- and third-generation cephalosporins. Up to 9% of strains were resistant to 0.12 mg/l concentrations of ciprofloxacin or ofloxacin but none were resistant to 1 or 2 mg/l concentrations of these 2 drugs, respectively. The isolates were grouped into 15 types by ribotyping with restriction endonuclease EcoRI and into 53 types by pulsed-field gel electrophoresis of XbaI-restricted DNA fragments. When DNA fragments of the ribotypes and pulsotypes were pooled and analyzed 87 types resulted, 76 (87%) of which were of > 90% similarity and were grouped into 15 clusters. About 60% of the isolates belonged to 3 clusters, which probably represented 3 clones endemic in the community. The rest of the isolates were of a large variety of types or clusters. For epidemiological purposes analysis of pooled results from different molecular techniques would be more discriminative than results from individual techniques alone.

DEVELOPMENT OF TRIMETHOPRIM-RESISTANCE IN SALMONELLA TYPHI DURING THERAPY

Summary A typhoid patient was infected with a fully‐sensitive, plasmidless strain of Salmonella typhi which acquired resistance to kanamycin, sulfamethoxazole and trimethoprim during cotrimox‐azole therapy. The resistant post‐treatment strain harboured 4 plasmids of 62, 4.1, 3.8 and 3.0 Md in size. Kanamycin‐, sulfamethoxazole‐ and trimethoprim‐resistance were borne on a transferable 62 Md plasmid which was compatible with groups FI, Fime, FII, FIV, H1, H2, B, l1, l2, J, K, M, N, T, X, W and P. Sulfamethoxazole‐resistance was also borne on the 4.1 Md plasmid. The 3.8 Md plasmid was not transferable; the 3.0 Md plasmid was transferable but did not confer antibiotic resistance. Excluding the strain described here, only 5 out of 35 other S typhi isolates contained plasmids. This is the first report of trimethoprim‐resistant S typhi in Hong Kong.