Molecular epidemiology of antimicrobial resistant escherichia coli isolated from wild and domestic rats trapped in and around Nairobi, Kenya
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Drug-resistant micro-organisms belonging to the species F-',coli remain a major health problem worldwide. They are of particular importance in developing countries where few therapeutic choices are available for treating infections. Few studies have addressed the epidemiological issues and the dynamics of antimicrobial resistance genes of E. coli causing disease in these countries. Antimicrobial resistance in rats as in other animals may have great impact on human health, particularly in an environment where humans and rats share the same ecosystem because of the zoonotic nature of E coli infections. Furthermore, the tendency of rats to contaminate water sources and human food may lead to concentration of antimicrobial resistant E. coli with subsequent transmission to humans and other animals. This study was designed to determine if antimicrobial resistance occurs in E coli isolated from rats living in the vicinity of human settlements. An attempt was made to determine if the resistance was encoded on plasmids and if it is transferable from one bacteria to another. Rats were trapped from inside and outside the houses. E. coli and other members of the family enterobacteriaceae which included Proteus spp., Enterobacter spp., Citrobacter spp., Klebsiella spp., Morganella spp. and Salmonella spp., were isolated from contents of their gastro-intestinal tracts. Antimicrobial susceptibility tests were perfomed on E. coli isolates and plasmids isolated from the E. coli showing resistance. Transferable antimicrobial resistance was determined by the ill vitro conjugation tests. Rats were radomly trapped from the densely populated (slums) area of Kibera and the less densely populated areas of Kawangware and Kabete in Nairobi, Kenya. The significance of the three areas is that the densely populated areas have poor sewer systems and dumping sites hence there is high likelihood of contact between rats and human as opposed to the less densely populated areas. The proportions of E. coli isolated from rats trapped in Kibera and Kabete areas did not differ significantly (P=0.5658). The proportions did not also differ significantly depending on whether rats were trapped from inside or outside the houses (P=0.8696). Kawangware area was not used in the comparisons as no E. coli isolates were obtained. Antimicrobial susceptibility to I I antimicrobials was done using disc diffusion method. The antimicrobials tested were ampicillin (10 J.1g), co-amoxyclav (20: 10 J.1g), sulphamethoxazole (10 J.1g), streptomycin (10 J.1g), tetracycline (10 J.1g), trimethoprim (5 ug), cefuroxime (30 J.1g), nalidixic acid (30 J.1g), gentamicin (IO J.1g), ceftazidime (30 J.1g) and ciprofloxacin (5 J.1g). Twelve (20%) of the 60 E. coli isolates tested were resistant to a single antimicrobial mainly ampicillin, streptomycin and sulphamethoxazole. Eight (13.3%) of the isolates showed multidrug resistance. Eight (13.3%) of the isolates were fully susceptible to all the I I antimicrobials. The minimum inhibitory concentrations (MICs) of the 60 I..,..•. coli isolates were also determined by the doubling agar dilution method. Fourteen (23.3%) of the isolates were resistant to ampicillin, 9 (15%) to streptomycin, 6 (6.6%) to cotrimoxazole, 2 (3.3%) to tetracycline and I (1.7%) to co-amoxyclav. The MIC50 and MIC<)()were exceptionally low except for ampicillin which were 16 and 64 ug, respectively. Exceptionally high MIC (128 ugunl) was observed for only 2 (3.3%) isolates, one to ampicillin and the other to streptomycin. There was no significant (P=0.2627) difference between the number of resistant L'. coli isolates using the MIC or. the disc susceptibility tests. A significant (P=0.043I ) difference occurred between the number of resistant E coli isolates from rats trapped in Kibera and Kabete areas but no significant (P=0.2884) difference occurred between resistant E. coli isolates from rats trapped inside and outside the houses. Plasmid profile analysis grouped the 22 antimicrobial resistant E coli into 5 plasmid groups depending on the number of plasmids each isolate carried. Sixteen (72.7%) of these isolates carried plasmids while 6 (27.3%) did not carry any plasmids. Plasmids of approximately 90-100 Mda, 55-65 Mda and 40-50 Mda were found at high frequency in resistant isolates and were thought to be responsible for the resistance. In conjugation tests, resistance was only transferred from E coli isolates resistant to ampicillin to E. coli KI2. Five (35.7%) out of the 14 isolates resistant to ampicillin showed transferable resistance. Three (60%) out of 5 transconjugants carried plasmids while for the other 2 no plasmids were recovered. In conclusion, rats may act as a pool of antimicrobial resistant E coli and their plasmids. These antimicrobial resistant E. coli may be transmitted to humans and form their normal gastro-intestinal tract microtlora with subsequent transfer of the resistance to other pathogenic micro-organisms. Further studies are recommended to determine if any relationships exist between plasmids carried \}y antimicrobial resistant E. coli isolates from rats, humans and other domestic animals.
CitationMaster of Science in Veterinary Epidemiology and Economics
University of NairobiDepartment of Public Health, Pharmacology and Toxicology, University of Nairobi, Kenya