- Department of Food Science, Center for Food Safety, University of Arkansas, Fayetteville, AR, USA
Introduction
Plant materials including flowers, roots, bark,
leaves, seeds, peel, fruits, and wood can be used to extract aromatic
and volatile liquids known as essential oils (EOs) (1–3).
These EOs have a long history of use for medical purposes, in perfumes
and cosmetics, and as herbs and spices for foods. EOs are considered to
be secondary metabolites in plants; secondary metabolites are organic
compounds that are not directly involved in the normal growth,
development, or reproduction of the plant (4). These secondary metabolites are often involved in plant defense and thus may possess antimicrobial properties (4, 5).
The first experiment to determine the bactericidal properties of EOs is
said to have been carried out by de la Croix in 1881 (6). In more recent years, many EOs, or their components, have been shown to possess broad-range antibacterial properties (7).
Increased resistance to infectious diseases,
including parasitic infections such as coccidiosis, has been noted when
plant phytonutrients were fed to animals. For instance, Lee et al. (8)
found that feeding plum powder to laying hens increased their immune
response as well as conferring immunity to coccidiosis. Lillehoj et al. (9)
determined the effects of feeding capsicum oleoresin or cinnamaldehyde
on the global gene expression profiles of broilers. Capsicum oleoresin
induced gene changes in genes associated with metabolism and immunity,
whereas cinnamaldehyde affected genes related to antigen presentation,
humoral immune response, and inflammatory disease. Feeding of these
compounds also protected the birds against infection with live
coccidiosis parasites. Mathlouthi et al. (10) found that oregano or rosemary EOs had different antimicrobial effects in vitro
against pathogenic and non-pathogenic bacteria but had the same growth
promoting effects as avilamycin when added to broiler diets. Authors
speculated that the in vivo growth promotion effects were due to
ecological changes in the bacterial gut flora rather than antibacterial
effects against a single bacterial genus and species. Betancourt et al. (11)
confirmed a shift in gut flora in the foregut but not ceca and colon in
broilers fed oregano EOs during a 42-day grow out period.
Alali et al. (12)
tested a mixture of carvacrol, thymol, eucalyptol, and lemon for the
ability to prevent colonization and shedding in broilers intentionally
fed Salmonella Heidelberg. They determined that feeding 0.05%
(v/v) of the EO mixture significantly reduced the colonization of the
crops of challenged birds as well as lowering feed conversion and
improving weight gain in the birds. However, cecal colonization and
shedding were not significantly decreased. Cerisuelo et al. (13)
fed an EO mixture composed of cinnamaldehyde and thymol to broilers,
either with or without butyric acid. They determined that the EO blend
reduced cecal numbers of Salmonella, especially when combined with butyric acid, as compared to control feed. Ricke et al. (14) provide an overview of the anti-Salmonella effects of EOs in agriculture.
Benchaar et al. (15) investigated the effects of EOs in vitro
rumen microbial fermentation. They determined that only the phenolic
compounds, carvacrol, thymol, and eugenol affected ruminal fermentation,
relative to the control, increasing pH and butyrate and decreasing
propionate, indicating antibacterial activity which was not
nutritionally beneficial. Callaway et al. (16) studied the in vitro effects of orange peel and orange pulp, both sources of EOs, against Escherichia coli O157:H7 and Salmonella typhimurium in rumen fluid. Growth of both pathogens was reduced by addition of 0.002 g/ml of orange pulp or orange peel. Callaway et al. (17) were able to demonstrate that the orange peel products when fed to experimentally inoculated sheep reduced S. typhimurium populations in the gut, with a significant reduction reached in the ceca.
The antimicrobial properties of EOs are a recent focus
for agricultural applications because of a desire on the part of many
consumers to reduce the use of “hazardous or unnatural chemicals” in
their food (18–20).
Although there are many studies on the antimicrobial activities of EOs,
few take the next step and determine the mode of action of these
compounds. However, application of EOs as antibacterial substances for
food animals or as food preservatives requires detailed knowledge about
their properties, including the mode of action. The purpose of this
review is to provide an overview of current knowledge about the
antimicrobial mode of action of EOs and their constituents.