Antibiotic susceptibilities and plasmid profiles of Shigella flexneri isolates from children with diarrhoea in Islamabad, Pakistan

Sir, Shigellosis is a major problem in developing countries and is associated with high incidences of morbidity and mortality. In Pakistan, Shigella spp., particularly Shigella flexneri, are frequently isolated from patients with diarrhoeal illnesses. Effective antibiotic therapy of infections caused by these pathogens is often compromised by resistance to commonly used agents. However, there are no published data on the incidence of antimicrobial resistance amongst shigellae in Pakistan. The aim of this study, which we believe to be the first of its kind to be undertaken in this country, was to determine the susceptibilities to 11 antibiotics and the plasmid profiles of S. flexneri strains isolated from patients with diarrhoea in the Islamabad region. Fifteen non-replicate S. flexneri strains were isolated from the stools of patients with diarrhoea at the National Institute of Health, Islamabad, Pakistan between June and August 1995 and were identified according to standard laboratory procedures. The patients, all of whom were under 3 years of age, had not received antibiotics for at least 3 months before the samples were provided. The antibiotics studied, which were obtained from Sigma Chemical Co., included amikacin, ampicillin, chloramphenicol, gentamicin, kanamycin, novobiocin, penicillin, streptomycin, spectinomycin, tetracycline and trimethoprim. Susceptibilities were determined by an agar dilution method; the medium used was Mueller–Hinton (Oxoid) and the inoculum was 10–10 cfu. The MIC was taken as the lowest concentration of each antibiotic that completely inhibited growth after incubation for 18 h at 37°C. Plasmid DNA was isolated according to the method of Portnoy & White, separated by electrophoresis on 0.7% agarose gels in Tris–acetate buffer and visualized under UV light after staining with ethidium bromide. The resistance phenotype of each isolate, assigned according to a system described by Koneman et al., is shown in the Table. The incidences of resistance to the antimicrobials tested were high. With the exception of one strain which was resistant only to trimethoprim, all of the isolates were resistant to at least five drugs. Three strains, for which the patterns of resistance were indistinguishable (Ap Cm Gn Nb Pn Sm Sp Tc Tp; see Table for abbreviations), were resistant to nine antibiotics; the MICs of chloramphenicol, novobiocin, penicillin, spectinomycin and