Background: The use of conventional antibiotics to prevent this infection is a usual in the poultry industry, specially to reduce the mortality associated with colibacillosis. Essential oils have demonstrated broad spectrums in terms of antimicrobial activities against various pathogens, Escherichia coli among them. The aim of this study was to evaluate the in vitro susceptibility of avian E. coli strains to Eugenia caryophyllata (clove) and Origanum vulgare (oregano) essential oils, at three different concentrations (2, 5, and 8%), and to the conventional antimicrobials, gentamicin and nalidixic acid. Materials, Methods & Results: Seventeen strains of Escherichia coli isolated from broiler bedding were tested and a standard E. coli strain (ATCC 25922) was used as a susceptibility test control. The antimicrobial activity of plant extracts from Eugenia caryophyllata and Origanum vulgare, obtained commercially from Laszlo Aromatologia Ltda., was tested in three concentrations (2, 5 and 8%) using the method of disk diffusion on agar according to the M2 - A8 protocol (CLSI, 2003) adapted to plant extracts and to the conventional antimicrobials gentamicin (10 μg) and nalidixic acid (30 μg), commercially obtained from Oxoid. The results showed that the extract from Origanum vulgare had an inhibitory activity on E. coli regardless of its concentration whereas the extract from Eugenia caryophyllata showed antimicrobial activity in 29.4% (5/17) of the isolated strains only at the concentration of 8%. All avian isolates were sensitive to gentamicin (100% of the strains tested); however, only 23.5% (4/17) of the samples were sensitive to nalidixic acid Discussion: The impressive antimicrobial activity of the Origanum vulgare against E. coli avian origin was confirmed in previous studies against both Gram-positive and Gram-negative bacteria. The essential oil of Origanum vulgare was more antimicrobial activity that Eugenia caryophyllata. Thymol and carvacrol is main antimicrobial component of the essential oil of Origanum vulgare, being responsible for the distortion to the physical structure of cells, leading to destabilization of the cell membrane, changing its permeability and denaturing essential enzymes. As for, the major constituent of the essential oil of Eugenia caryophyllata is eugenol, and their pharmacological effects depend on the concentration of free eugenol in living tissue, which may account for the better antimicrobial activity in the treatment with 8% of this extract. All the strains tested in this study were sensitive to gentamicin, while most of them (80%) were resistant to nalidixic acid. The gentamicin has the highest spectrum of action among aminoglycoside, however, bacteria rapidly acquire plasmidmediated resistance to this ingredient in the presence of selective pressure. The beginning of antimicrobial resistance by zoonotic bacteria has important implications for public health. Data from several researchers suggest that improper selection and disorderly use of conventional antimicrobials can lead to the resistance of various pathogens and, consequently, their access to consumers through foodstuffs. Therefore, the use of Origanum vulgare oil extract was found to be effective on avian E. coli strains and may be an alternative for reducing the losses caused by these bacteria in poultry production whenever a limitation in the use of traditional antibiotics exist
