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Introduction
In recent years, antibiotics and antimicrobial compounds produced by microorganisms have played an important role in animal diets to increase their performances and decrease mortality (Benchaar et al., 2008). However, public concern over routine use of antibiotics in animal nutrition has increased due to the potential development of antibiotic resistant in animals and humans (Benchaar et al., 2006). As a result, alternative feed additives such as aromatic plants and their extracts were researched instead of antibiotics and they have gained interest as growth and health promoters (Benchaar et al., 2006). Essential Oils (EOs) are volatile, lipophilic substances obtained from plant materials which are gained generally by steam distillation mechanical separation or solvent extraction (Wallace, 2002; Castillejos et al., 2007; Hart et al., 2008). Hart et al., (2008), described that the concentration of EOs may vary depending on the stage of plant growth, different part of plant, amount of moisture received, light, and temperature. Essential oils are complex mixtures of secondary plant metabolites that demonstrate a broad spectrum of antimicrobial properties, particularly against gram positive bacteria (Burt, 2004). They are naturally occurring plant components and are considered safe for human and animal consumption (FDA, 2003; Benchaar et al., 2006). These appear to be selective in their antifungal, antiviral, bactericidal, and bacteriostatic effects on microorganisms (Burt, 2004; Janssen et al., 1986). Oh et al., (1967) and then Nagy and Tengerdy (1967), were the first to investigate effects of EOs on in vitro ruminal microbial fermentation and observed that EOs inhibited gas production, however the degree of inhibition was dependent on the chemical structure of the EOs.
The research of Smith-Palmer et al., (1998), determined that out of their EOs test articles, the oils of cinnamon and thyme exhibited the greatest inhibitory effects with bacteriostatic activity at a concentration of 0.075% or less against Escherichia coli, Staphylococcus aureus, Listeria monocytogenes, Salmonella enteritidis, and Campylobacter jejuni. However, negative gram organisms in this study (and others) were slightly less susceptible to EOs than positive gram bacteria (Smith-Palmer et al., 1998; Burt, 2004). Certain phenolic compounds such as thymol (thyme) could inhibit negative gram bacteria as well as positive gram bacteria. The mechanism by which most EOs are thought to exert their antibacterial effects...