Characterization of <i>Escherichia coli</i> isolated from samples of different biological and environmental sources

Abstract

Escherichia coli from 10 different biological and environmental sources were isolated and characterized in the Department of Microbiology and Hygiene, Bangladesh Agricultural University, Mymensingh during the period from January to May 2007. A total of 100 samples, 10 from each of human feces and urine, rectal swab of cattle, sheep and goat, cloacal swab of chicken, duck and pigeon, drain sewage and soil were collected aseptically and subjected to primary isolation by propagating in nutrient broth followed by culture on different agar media. Gram's staining and hanging drop techniques were also performed. Biochemical properties of the isolates were studied and reaction in TSI agar slant was also observed. Pathogenicity of 10 representative E. coli isolates, one from each source were determined by lethality assay in 12 day-old embryonated eggs, in day-old chicks and in day-old suckling mice models. E. coli was isolated successfully from all the samples. All the E. coli isolates were found to produce bright pink colonies on MacConkey agar, yellowish green colonies surrounded by an intense yellow green zone on BG agar and characteristic metallic sheen colonies on the EMB agar. In case of E. coli isolated from cattle, slight variation in colony character on EMB agar was observed showing greenish red colonies with faint metallic sheen. In Gram's staining technique, all the isolates were pink coloured, small rod shaped Gram negative bacilli and in the hanging drop technique they were motile. Reactions in TSI agar slant revealed yellow slant and butt with gas but no hydrogen sulphide production. Almost all the E. coli isolates fermented dextrose, maltose, lactose, sucrose and mannitol with the production of both acid and gas except E. coli isolated from drain sewage which did not ferment maltose and isolates from pigeon showed less production of acid and gas during sucrose fermentation. The results of Catalase, MR and indole test of the E. coli isolates were positive but V-P test was negative. In the embryo lethality assay, E. coli isolates from chicken, pigeon, duck, human urine, cattle, sheep and goats were virulent causing 33.33-100% death of the embryo except isolates from human faeces and drain sewage which were moderately virulent and that from soil which was avirulent. E. coli isolate of chicken origin found to be more virulent which caused 100% death of the embryos. Most of the embryos died between day-1 and day-2 PI. Chick lethality assay indicated that all the E. coli isolates were virulent as the mortality rate was more than 50%. In mice lethality assay, all the E. coli isolates were in the killer group causing cent percent death of mice within 10 to 42 h following inoculation. Among these three lethality assay models, avian embryo lethality assay was found to be most suitable to discriminate between virulent and avirulent isolates compared to day-old chick lethality assay and day-old suckling mice lethality assay where inconsistent results were observed. In conclusion, our result showed that E. coli isolated from different biological and environmental sources were found to be varied in virulence and avian embryo lethality assay was assumed to be the best model for discriminating virulent and avirulent E. coli.

Key words: Escherichia coli, pathogenicity, human, cattle, poultry, soil

DOI = 10.3329/bjvm.v5i1.1305

Bangl. J. Vet. Med. (2007). 5 (1 & 2): 25-32

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