The origin of bacteria resistance to antibiotics can either be chromosomal or extra-chromosomal (plasmid mediated) and one way of determining the origin of bacterial drug resistance is by plasmid elimination. In this study, the antibiotic susceptibility of seven Escherichia coli isolates (numbered 1-7) from urine and stool samples, were assessed using the disk diffusion method. The 10 antibiotics used were: nitrofurantoin (100 µg), ciprofloxacin (5 µg), tetracycline (50 µg), norfloxacin (10 µg), amoxycillin (20 µg), ofloxacin (5 µg), chloramphenicol (10 µg), cefuroxime (30 µg), ampicillin (10 µg) and gentamicin (10 µg). All isolates (100%) were observed to have shown resistance to ampicillin. Isolate No 6 was resistant to 70% of the antibiotics while isolate No 7 was observed to be resistant to 30% of the antibiotics. In order to determine if the resistance is plasmid mediated or chromosomal-borne, two of the isolates (29%) that showed resistance to more than one antibiotic were subjected to acridine orange mediated plasmid elimination. Isolate No 6 lost its resistance to 5 out of the 7 antibiotics (71%) while isolate number 7 lost its resistance to 2 out of the 3 antibiotics (67%) after the curing. Loss of resistance after the plasmid curing was an indication that the resistance was plasmid-mediated while the resistance mechanism for those that retained their resistance after plasmid curing was chromosomal-borne. It was suggested that further studies be done for the characterization of resistance plasmids on E. coli and policies be set that will minimize the indiscriminate use of antibiotics.
Key words: Antibiotics, chromosomal, Escherichia coli, plasmid, resistance, sensitivity, susceptibility.
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