Abstract.-This study was carried out to determine the antimicrobial susceptibility patterns of 584 Shigella flexneri isolated from diarrheal patients admitted in hospitals in Azad Kashmir Pakistan and to evaluate their changing trends against twenty antibiotics. The isolates showed highest resistance against penicillin followed by carbenicillin, ampicillin, tetracycline, erythromycin, ceftizoxime, kanamycin, co-trimoxazole, piperacillin, amoxicillin, amikacin, streptomycin, nalidixic acid, gentamicin, chloramphenicol, cephalothin and ceftriaxone. All S. flexneri isolates were sensitive to cefixime, ciprofloxacin and enoxacin. Multiple drug resistance (MDR) was observed against 3-10 drugs and was resistant against three or more antibiotics was at the level as high as 300µg/ml. The resistant isolates showed different patterns of antibiotics resistance. The most common pattern was PCaA. The plasmids were observed in 32.8% MDR strains of S. flexneri which were found resistant against three or more antibiotics. The number of plasmids varied from one to seven. Analysis of plasmid DNA of S. flexneri revealed that all the strains contained a heterogeneous population of plasmids ranging between >23.1 kb to <2.0 kb. Based on molecular weight, the pattern of different plasmids was also very diverse. Depending on the number of plasmids, individual strains were grouped into nine different plasmid patterns. Some of the antibiotic resistance determinants were cured by acridine orange, indicating that widespread antibiotic resistance is mediated through plasmid. Transformation experiments showed that the factors for resistance against ampicillin, chloramphenicol and sulfamethoxazole-trimethoprim resided in >23.1 Kb and 23.1 Kb plasmids
