Even as an immunologically sensitive embryo, poultry already interact with pathogenic microbes originating from any stage prior to hatching [7]. This interaction may be a consequence of horizontal transmission [8] (Figure 1) and puts the poultry’s life in danger. Fonseca et al. [9] observed that, by contaminating the eggshell, Campylobacter jejuni bacteria can penetrate it, cross the albumen and reach the yolk sac, probably resulting in embryonic mortality. This led the authors to state that the immunity conferred by breeding hens to the egg/embryo may be insufficient and inefficient for certain infections. In addition, although the eggshell is an oval antimicrobial wall formed by the fusion of membranes and mineral layers equivalent to a vital organ of a living organism (it promotes the flow of nutrients, water, oxygen and carbon dioxide to keep the embryo alive) [10], it is not totally resistant to microbial entry. The eggshell is challenged when there are microorganisms trying to move from its surface to the main target (embryo). Oliveira et al. [6] reviewed the types of microorganisms that contaminate eggshells. Among the bacterial and fungal genera cited are Alcaligenes, Enterobacter, Escherichia, Klebsiella, Proteus, Providencia, Pseudomonas, Salmonella, Clostridium, Enterococcus, Staphylococcus, Streptococcus, Aspergillus, Candida and Penicillium.

Previously published studies have reported the adverse relationship of microorganisms with embryos and/or chicks. Weil and Volentine [11] reported that contamination of the yolk sac of the chicken embryo by Shigella dysenteriae can cause lethal infection. Embryos from chickens killed by contamination with avian pathogenic Escherichia coli and Salmonella enterica subsp. enterica serovar Enteritidis showed signs of congestion and diffuse redness throughout the skin, head and neck, as well as microscopic lesions in the yolk sac, including congestion, inflammation, damaged blood vessels and abnormal endodermal epithelial cells [1]. Fungi of the genus Aspergillus, which may be responsible for causing mycoses or mycotoxicosis, have been isolated from dead chicken embryos [12]. Saleemi et al. [13] reported that aflatoxigenic fungal extracts isolated from Aspergillus fungi caused high embryonic mortality, weight reduction and severe alterations in the liver (fatty alteration and cell necrosis) and kidneys (congestion and tubular necrosis) of chicks. Karunarathna et al. [14] demonstrated that multidrug-resistant Escherichia coli and Enterococcus were recovered from the yolk of non-viable chicken embryos at hatching. Contamination by Enterococcus spp. can trigger pulmonary hypertension syndrome in chicken embryos and chicks [15]. Mortality of chicken embryos associated with Enterococcus contamination was reported by Karunarathna et al. [16]. Multidrug-resistant bacteria that cause yolk sac infection, including Escherichia coli, Salmonella, and Staphylococcus, have been recovered from dead embryos and chicks [17,18]. Far et al. [19] observed that dead ostrich embryos were contaminated with Pseudomonas spp., Klebsiella spp., Bacillus spp., Citrobacter spp., Staphylococcus spp., Proteus spp., Aeromonas spp., Enterobacter spp., as well as Escherichia coli with antimicrobial resistance profile.

The findings mentioned above raise concerns, especially in relation to the health of poultry and humans, since multiresistant microorganisms can spread and cause massive irreversible damage. In addition, the undue, exacerbated use of sanitizers without proven scientific tests and without the prescription of trained professionals can contribute to even worse health and economic instability. Therefore, collective efforts within the poultry industry should focus on antimicrobial interventions that involve the controlled use of broad-spectrum sanitizers focused on hatching egg sanitization.

Essential oils are any aromatic, viscous and volatile oils belonging to plants. Syzygium aromaticum, Allium sativum, Ocimum basilicum, Thymus vulgaris, Lavandula angustifolia, Eucalyptus globulus, Citrus sinensis, Citrus aurantifolia, Cinnamomum cassia, Rosmarinus officinalis, Origanum vulgare, Allium cepa, Cymbopogon winterianus, Cymbopogon flexuosus, Piper nigrum, Zingiber officinale, Protium pallidum, Litsea citrata, Satureja hortensis, Salvia officinalis, Mentha piperita, Cedrus deodara, and Cuminum cymincum are examples of plant species that provide commercially available essential oils that may have promising futures in poultry nutrition and production such as egg coating additives and sanitizers for hatching eggs. This is because essential oils have a chemical configuration that triggers their biological properties. For example, hydrocarbons, esters, lactones, alcohols, oxides, phenols, ketones, and aldehydes are present in the chemical composition of essential oils with similar or distinct bioactive functions. Depending on the compound, these functions include antimicrobial, antiviral, antitumoral, antibacterial, stimulant, anesthetic, anti-inflammatory, anti-fungal, antipyretic, and spasmolytic [20]. The content, quality, and effectiveness of essential oil compounds depend on factors such as extraction, which can be by hydro distillation, steam distillation, supercritical CO₂ extraction, ultrasonic extraction, and cold pressing [21,22,23,24].

In vitro antimicrobial screenings initially detect the potential viability of essential oils before they are used as in vivo antimicrobial agents. These screenings demonstrated that essential oils are effective against standard Gram-negative and positive bacterial strains and avian isolates, as well as standard and avian-isolated fungi. Among the bacteria are Salmonella enterica subsp. enterica serovar Enteritidis, Salmonella enterica subsp. enterica serovar Typhimurium, Salmonella enterica subsp. enterica serovar Infantis and avian pathogenic Escherichia coli (APEC), which are important pathogenic bacteria for poultry and public health. The antimicrobial effectiveness of essential oils ranges from mild to very strong. In fact, some of them have been shown to be more effective than conventional antibiotics [25,26]. Thymol, eugenol, carvacrol, linalool, citral, limonene, trans-cinnamaldehyde, geraniol and citronellal are some compounds that are part of the composition of some essential oils that can act as protagonists in antimicrobial action. The main mechanisms responsible for making the bacterial [27] and fungal [28] cells unfeasible are listed below:

• Bacteria:

• Cell membrane alteration and increased permeability.
• Stops energy production.
• Blocks active transport.

• Fungi:

• Cell membrane disruption, alteration, and inhibition of cell wall formation.
• Dysfunction of fungal mitochondria.
• Inhibition of efflux pumps.

Essential oils can promote beneficial actions for human health by reducing pain and inflammation, protecting and healing wounds, neutralizing or stopping the development of carcinogens, neutralizing oxidative stress and possessing antiviral, antibacterial, antifungal, cardioprotective, antidiabetic, and insect-repellent properties; among other benefits, they can also potentially treat central-nervous-system-based disorders [29,30,31,32]. The safety of a stock of essential oils including Ocimum basilicum, Zingiber officinale, Lavandula officinalis, Cymbopogon citratus, Mentha piperita, Rosmarinus officinalis, Thymus vulgaris, Eugenia caryophyllata, and Allium sativum has been documented and they received the generally recognized as safe (GRAS) seal [33]. However, the intake of essential oils needs to be monitored, as they can, like any other edible food, cause an inappropriate effect.

4. Comparing Essential Oils and Formaldehyde for Sanitizing Hatching Eggs