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The diluted essential oil of Mentha longifolia (L.) var. chlorodictya Rech F. foliage enhanced the bactericidal activity of nitrofurantoin decreasing the minimum inhibitory concentration (MIC) of nitrofurantoin for nitrofurantoin-resistant strains of Enterobacteriaceae. Thin-layer chromatography (TLC) analysis of the essential oil detected a fraction (R(f) = 0.35, UV lambda(max) of 232.5), which was the most effective in enhancement of nitrofurantoin activity. Using gas liquid chromatography and known standards, the active fraction was identified as piperitone. 1 microl of the piperitone fraction decreased the MIC of nitrofurantoin 3-20 fold for the different strains of Enterobacteriaceae tested.

Two hundred and four Escherichia coli strains were isolated from external and visceral cavity surfaces of 102 slaughtered broiler carcasses. The isolates were screened to determine the phylogenetic background and presence of Shiga toxins (stx1, stx2), intimin (eae) and beta-lactamase (blaTEM, blaSHV) genes. Phylotyping results revealed that the E. coli isolates segregated in four phylogenetic groups A (56.86%), B1 (19.12%), B2 (4.90%) and D (19.12%). PCR assays revealed that 13 isolates (6.37%) from 12 carcasses were positive for eae (12 isolates) and/or stx2 (2) genes. The eae positive isolates belonged to phylogenetic groups A (A0, A1), B1, B2 (B22) and D (D2). Two stx2 positive and seven eae positive isolates were recovered from visceral cavity surface, whereas only 5 eae positive isolates were from the external surface of the carcasses. On the other hand, thirty one E. coli strains isolated from visceral cavity and external surface of 26 carcasses carried the blaTEM (27) and blaSHV (4) genes and belonged to different phylo-groups. This study suggests that broiler carcasses could be considered as an important source of EPEC and STEC pathotypes in southeast of Iran; as well as the examined antibiotic resistance genes, which were carried by some isolates and could be transferred to pathogens through the food chain.

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