Bacterial Contamination in Chicken Eggs: Risks, Pathogens, and Mitigation Strategies
1. Introduction
Bacterial contamination of chicken eggs represents a significant challenge to food safety and veterinary public health. The egg, as a biological product, can become contaminated through two primary routes: transovarian (internal) contamination, where pathogens are deposited into the egg contents prior to shell formation, and trans-shell (external) contamination, where bacteria penetrate the eggshell after oviposition [49, 51]. The primary pathogens of concern include Salmonella enterica serovars, particularly Salmonella Enteritidis and Salmonella Typhimurium, as well as Escherichia coli, Staphylococcus aureus, and Yersinia enterocolitica [1, 2, 42]. This review examines the biological mechanisms of contamination, the prevalence and diversity of bacterial pathogens, the role of antimicrobial residues, and the current and emerging mitigation strategies.
2. Routes of Contamination and Eggshell Biology
2.1. Transovarian Contamination
Salmonella Enteritidis possesses a unique ability to colonize the reproductive tissues of laying hens, including the ovary and oviduct, without causing overt clinical disease [49, 50]. This allows for the direct deposition of bacteria into the egg albumen or yolk prior to shell formation. The lipopolysaccharide biosynthesis gene rfbH is essential for survival of Salmonella Enteritidis in egg albumen, as it confers resistance to the antimicrobial properties of this matrix [43]. The magnitude of the specific antibody response in laying hens correlates with the probability of producing contaminated eggs, indicating that systemic infection is a key determinant of internal contamination [50].
2.2. Trans-Shell Contamination
The eggshell is a porous structure covered by a cuticle layer, which acts as a primary physical barrier against bacterial penetration [3]. The cuticle is composed of proteins, polysaccharides, and lipids that seal the pores. Cuticle deposition varies through the laying cycle and is influenced by hen age and nutrition [3]. Once the cuticle is compromised, bacteria can penetrate through the shell pores and reach the inner shell membranes. Salmonella Enteritidis can multiply on the yolk membrane and subsequently penetrate into the yolk contents at temperatures as low as 30 degrees Celsius [44]. The vitelline membrane itself is a site of bacterial multiplication; inoculation of Salmonella on the outside of this membrane leads to rapid penetration into the yolk [4].
2.3. Environmental and Management Risk Factors
Environmental contamination of the laying hen house is a major risk factor for egg contamination. Fecal shedding of Salmonella by hens is influenced by flock management practices, including molting procedures and housing density [5, 46]. In free-range systems, the dynamics of Salmonella shedding are complex, with environmental exposure playing a larger role than in conventional cage systems [6, 5]. Management risk factors for Salmonella Enteritidis contamination include flock size, cage cleanliness, and the presence of rodents or other pests [51]. The role of the poultry red mite Dermanyssus gallinae as a vector for Salmonella is also recognized, as these ectoparasites can carry bacteria between hens and into the egg production environment (see Ectoparasites of Poultry).
3. Pathogen Prevalence and Serotype Diversity
3.1. Global Prevalence of Salmonella in Eggs
A meta-analysis of Salmonella prevalence in eggs from China reported a pooled prevalence estimate of 4.2% (95% CI: 2.8-5.6%) in retail eggs, with Salmonella Enteritidis being the dominant serotype [7]. In Ethiopia, a study of raw chicken eggs from commercial farms and retail shops found a prevalence of 8.3% in farm samples and 11.7% in retail samples, with Salmonella Typhimurium and Salmonella Enteritidis being the most common [8]. In Ghana, genomic characterization of Salmonella isolates from traditional market eggs revealed a high diversity of serovars, including Salmonella Kentucky, Salmonella Infantis, and Salmonella Dublin [9]. In Iran, a study of hen eggs and quail eggs reported a prevalence of 6.7% for Salmonella Enteritidis and 4.0% for Salmonella Typhimurium in hen eggs [10]. In Colombia, Salmonella Paratyphi B var. Java was the predominant serovar isolated from commercial egg-laying farms [11].
3.2. Other Gram-Negative Pathogens
Escherichia coli is a frequent contaminant of eggshells and egg contents. In Bangladesh, multidrug-resistant E. coli was isolated from 45% of egg samples, with high rates of resistance to tetracycline and ciprofloxacin [1]. Extended-spectrum beta-lactamase (ESBL) and plasmid-mediated AmpC beta-lactamase-producing E. coli have been isolated from commercial layer farms in Korea, indicating a reservoir of resistance genes in the egg production environment [12]. Coliform bacteria, including E. coli, are commonly found on hatching broiler eggs, and the presence of ESBL/AmpC-producing strains is a concern for vertical transmission to chicks [13]. Yersinia enterocolitica can survive on the eggshell surface and penetrate into the egg contents, particularly when the cuticle is damaged [42].
3.3. Gram-Positive Pathogens
Staphylococcus aureus is a common contaminant of eggshells. In Italy, a high prevalence of clonally diverse S. aureus spa type t026 was found contaminating rural eggshells [2]. Staphylococcus aureus is also a cause of bumblefoot and osteomyelitis in broilers, and can contaminate eggs through contact with infected skin or environmental sources (see Staphylococcus aureus Bumblefoot and Osteomyelitis in Broilers). Streptococcus spp., including Streptococcus zooepidemicus, have been isolated from eggs, though their prevalence is lower than that of Salmonella or E. coli [1].
4. Antimicrobial Residues and Resistance
4.1. Residue Levels in Eggs
Antimicrobial residues in eggs are a consequence of therapeutic or prophylactic administration to laying hens, as well as cross-contamination of feed. A meta-synthesis of research on drug residues in livestock and poultry identified tetracyclines and quinolones as the most frequently detected antibiotic classes in eggs [14]. In Shanghai, quinolone and tetracycline residues were detected in 12.4% and 8.7% of fresh egg samples, respectively [15]. In Haryana, India, tetracycline residues were found in 18% of layer eggs, with levels exceeding the maximum residue limit (MRL) in some samples [16]. In Nepal, quinolone residues were detected in 22% of poultry meat and egg samples, representing a public health concern [17]. In Croatia, an LC-MS/MS multi-residue method detected residues of nine antibiotic classes, including tetracyclines, sulfonamides, and fluoroquinolones, in eggs from commercial poultry farms [18]. In Nigeria, chloramphenicol residues, a banned antibiotic in food-producing animals, were detected in 14% of commercial chicken eggs [19]. In Trinidad, antimicrobial residues were found in 8% of table eggs, with sulfonamides being the most common [47].
4.2. Impact on Bacterial Resistance
The presence of sub-inhibitory concentrations of antimicrobials in eggs selects for resistant bacteria. A study of the farm-to-fork changes in poultry microbiomes in Mozambique demonstrated that the resistome (the collection of resistance genes) in poultry products, including eggs, becomes enriched with tetracycline and beta-lactam resistance genes along the production chain [20]. Salmonella isolates from eggs in Japan showed high rates of resistance to streptomycin and sulfamethoxazole, with multidrug-resistant (MDR) strains being common [21]. In Bangladesh, Salmonella and E. coli isolates from eggs were resistant to multiple antibiotics, including ciprofloxacin and ceftriaxone [1]. The presence of ESBL-producing E. coli in eggs is a direct indicator of the selective pressure exerted by cephalosporin use in poultry [12, 13].
5. Diagnostic and Detection Methods
5.1. Culture-Based Methods
Traditional culture methods for Salmonella detection in eggs involve pre-enrichment in buffered peptone water, followed by selective enrichment in Rappaport-Vassiliadis broth or tetrathionate broth, and plating on selective agar such as xylose lysine deoxycholate (XLD) agar or brilliant green agar [8, 10]. For E. coli, MacConkey agar and eosin methylene blue (EMB) agar are used [1]. These methods are sensitive but require 3-5 days for definitive results.
5.2. Molecular Methods
Real-time PCR (qPCR) assays targeting the invA gene of Salmonella are widely used for rapid detection in eggs and poultry meat [45]. A PCR-enhanced fluorescence biosensor using upconversion nanoparticles (UCNPs) and tungsten disulfide (WS2) nanosheets has been developed for the detection of Salmonella Typhimurium, achieving a limit of detection of 10 CFU/mL [22]. This dual quenching-dual recovery mechanism allows for signal amplification without the need for enrichment. Suspension array analysis, using microspheres conjugated with specific antibodies, has been used for the detection of Salmonella Enteritidis-specific yolk antibodies in eggs [39]. This method provides a high-throughput serological screening tool for flock-level monitoring.
5.3. Biosensor and Electrochemical Methods
Portable electrochemical sensors based on gold nanostars (STAB-GN) have been developed for the simultaneous detection of chloramphenicol and enrofloxacin in foods, including eggs [23]. These sensors provide a rapid, field-deployable alternative to laboratory-based LC-MS/MS methods. The use of automated impedance analyzers for bacterial detection in liquid egg products is also under investigation.
5.4. Liquid Chromatography-Mass Spectrometry (LC-MS/MS)
LC-MS/MS is the gold standard for the confirmation and quantification of antimicrobial residues in eggs. A validated multi-residue method for nine antibiotic classes in eggs, compliant with EU regulation 2021/808, has been developed [18]. This method uses a simple extraction with acetonitrile and water, followed by analysis on a triple quadrupole mass spectrometer. The method is capable of detecting residues at levels below the MRL for all regulated antibiotics.
6. Mitigation Strategies
6.1. Physical and Chemical Decontamination
Eggshell surface sanitization is a critical step in reducing the bacterial load on eggs. Technologies include hot water washing, chlorine dioxide, and ozone. A comparison of eggshell surface sanitization technologies found that hot water (50 degrees Celsius) and chlorine dioxide (100 ppm) were effective at reducing Salmonella Enteritidis on the shell surface without affecting consumer acceptability [24]. Non-thermal plasma (NTP) treatment using double rotary plasma jets has been shown to inactivate Salmonella Enteritidis on shell eggs by 4.5 log CFU per egg, with no adverse effects on sensory properties [25].
6.2. Photodynamic Inactivation
Photodynamic inactivation (PDI) uses a photosensitizer, such as curcumin or carvacrol, and light to generate reactive oxygen species (ROS) that kill bacteria. A novel PDI strategy for Salmonella Enteritidis PT4 on eggshells using curcumin and carvacrol achieved a 3.5 log reduction in bacterial counts [26]. This method is non-toxic and does not leave chemical residues on the eggshell.
6.3. Bacteriophage Therapy
Bacteriophages are viruses that specifically infect and lyse bacteria. A bacteriophage emulsion applied to eggshells has been shown to prevent Salmonella contamination by reducing bacterial attachment and penetration [27]. The phages are applied as a spray or dip, and they remain viable on the eggshell surface for up to 14 days. This approach is highly specific and does not affect the normal egg microflora.
6.4. Cuticle Deposition and Biosecurity
Improving cuticle deposition through genetic selection or nutritional supplementation enhances the natural barrier function of the eggshell. Cuticle deposition can be measured on hatching eggs without compromising embryonic development, allowing for the selection of hens with superior cuticle quality [3]. Biosecurity measures, including rodent control, cleaning and disinfection of laying houses, and the use of dedicated egg collection equipment, are essential for reducing the environmental load of Salmonella [51].
6.5. Antimicrobial Stewardship
The reduction of antimicrobial use in laying hens is a key strategy for minimizing the selection of resistant bacteria. Monitoring of antibiotic residues in eggs, as conducted in the Umbria and Marche regions of Italy, provides data for risk assessment and for the implementation of withdrawal periods [28]. The use of alternative therapies, such as probiotics, prebiotics, and organic acids, can reduce the need for antibiotics in the control of Salmonella and E. coli.
7. Workflow for Egg Contamination Risk Assessment
The following Mermaid diagram illustrates the decision tree for assessing and managing bacterial contamination in chicken eggs.
graph TD
A[Egg Production Flock] --> B{Salmonella Shedding?}
B -->|Yes| C[Fecal Contamination of Environment]
B -->|No| D[Low Risk of Internal Contamination]
C --> E[Eggshell Contamination]
E --> F{Cuticle Integrity?}
F -->|Intact| G[Low Penetration Risk]
F -->|Damaged| H[Bacterial Penetration]
H --> I{Storage Temperature?}
I -->|7 degrees C| J[Reduced Bacterial Growth]
I -->|18 degrees C| K[Increased Bacterial Growth]
K --> L[Risk of Yolk Contamination]
L --> M[Consumer Exposure]
D --> N[Antimicrobial Residue Monitoring]
N --> O{Residues > MRL?}
O -->|Yes| P[Withdrawal Period Enforcement]
O -->|No| Q[Safe for Consumption]
G --> R[Eggshell Sanitization]
R --> S[Phage or PDI Treatment]
S --> T[Reduced Pathogen Load]
8. Conclusion
Bacterial contamination of chicken eggs is a multifactorial problem involving pathogen biology, host physiology, and environmental management. Salmonella Enteritidis remains the most significant pathogen due to its ability to cause transovarian contamination. The emergence of ESBL-producing E. coli and MDR Salmonella serovars in eggs highlights the need for integrated antimicrobial stewardship programs. Mitigation strategies, including bacteriophage therapy, photodynamic inactivation, and improved cuticle quality, offer promising alternatives to traditional chemical decontamination. Continued surveillance using molecular and biosensor-based methods is essential for the early detection of contamination events and for the protection of the food supply.
References
[1] Rafiq K, Sani AA, Hossain MT et al. Assessment of the presence of multidrug-resistant Escherichia coli, Salmonella and Staphylococcus in chicken meat, eggs and faeces in Mymensingh division of Bangladesh. Heliyon. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/39281621/
[2] Bencardino D, Vitali LA, Petrelli D. High prevalence of clonally diverse spa type t026 Staphylococcus aureus contaminating rural eggshells. J Med Microbiol. 2017. URL: https://pubmed.ncbi.nlm.nih.gov/28786785/
[3] Bain MM, Zheng J, Zigler M et al. Cuticle deposition improves the biosecurity of eggs through the laying cycle and can be measured on hatching eggs without compromising embryonic development. Poult Sci. 2019. URL: https://pubmed.ncbi.nlm.nih.gov/30476345/
[4] Gast RK, Guraya R, Guard J et al. Multiplication of Salmonella enteritidis in egg yolks after inoculation outside, on, and inside vitelline membranes and storage at different temperatures. J Food Prot. 2010. URL: https://pubmed.ncbi.nlm.nih.gov/21067679/
[5] Gole VC, Woodhouse R, Caraguel C et al. Dynamics of Salmonella Shedding and Welfare of Hens in Free-Range Egg Production Systems. Appl Environ Microbiol. 2017. URL: https://pubmed.ncbi.nlm.nih.gov/28039133/
[6] McWhorter AR, Chousalkar KK. From hatch to egg grading: monitoring of Salmonella shedding in free-range egg production systems. Vet Res. 2019. URL: https://pubmed.ncbi.nlm.nih.gov/31362780/
[7] Zhang S, Ma J, Chen T et al. Meta-analysis of the prevalence, serotype distribution, and antimicrobial susceptibility of Salmonella spp. from eggs in China. Int J Food Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41806722/
[8] Adane P, Tadesse T, Alemayehu H et al. Prevalence, Risk Factors and Antimicrobial Susceptibility Profile of Salmonella in Raw Chicken Eggs Sourced from Commercial Poultry Farms and Retail Shops and In-contact Humans in Addis Ababa, Ethiopia. J Food Prot. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40383270/
[9] Archer EW, Chisnall T, Tano-Debrah K et al. Prevalence and genomic characterization of Salmonella isolates from commercial chicken eggs retailed in traditional markets in Ghana. Front Microbiol. 2023. URL: https://pubmed.ncbi.nlm.nih.gov/38029182/
[10] Yousefi Amin A, Oshaghi M, Habibi S et al. Prevalence and antimicrobial susceptibility of Salmonella enteritidis and Salmonella typhimurium isolated from hen eggs and quail eggs in Karaj, Iran. Vet Med Sci. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/38739101/
[11] Rodríguez R, Fandiño C, Donado P et al. Characterization of Salmonella from Commercial Egg-Laying Hen Farms in a Central Region of Colombia. Avian Dis. 2015. URL: https://pubmed.ncbi.nlm.nih.gov/26292535/
[12] Shim JB, Seo KW, Kim YB et al. Molecular characteristics of extended-spectrum and plasmid-mediated AmpC beta-lactamase-producing Escherichia coli isolated from commercial layer in Korea. Poult Sci. 2019. URL: https://pubmed.ncbi.nlm.nih.gov/30239888/
[13] Mezhoud H, Chantziaras I, Iguer-Ouada M et al. Presence of antimicrobial resistance in coliform bacteria from hatching broiler eggs with emphasis on ESBL/AmpC-producing bacteria. Avian Pathol. 2016. URL: https://pubmed.ncbi.nlm.nih.gov/27011291/
[14] Onohuean H, Igere BE, Apollo E et al. Meta-synthesis of research advances on drug residues associated with antihelmintic, antibiotics in livestock and poultry: Implications for public health. Food Res Int. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41819887/
[15] Qiu D, Yang J, Sun C et al. Residue levels and health risks of quinolones and tetracyclines in fresh eggs sold in Shanghai. Wei Sheng Yan Jiu. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41820007/
[16] Bishnoi K, Moudgil P, Soni D et al. Occurrence and Risk Assessment of Tetracycline Residues in Layer Eggs in Haryana, India. J Food Prot. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/39778680/
[17] Shrestha N, Layalu S, Amatya S et al. Quinolones residue in poultry meat and eggs; an alarming public health issue in Nepal. BMC Res Notes. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41495852/
[18] Varenina I, Bilandžić N, Kaurinović J et al. Validation of an LC-MS/MS multi-residue method for nine antibiotic classes in eggs according to EU regulation 2021/808 and assessment of their occurrence on Croatian poultry farms. Food Addit Contam Part A Chem Anal Control Expo Risk Assess. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41637638/
[19] Mbodi FE, Nguku P, Okolocha E et al. Determination of chloramphenicol residues in commercial chicken eggs in the Federal Capital Territory, Abuja, Nigeria. Food Addit Contam Part A Chem Anal Control Expo Risk Assess. 2014. URL: https://pubmed.ncbi.nlm.nih.gov/25208093/
[20] Olson N, Lamar F, Mucache H et al. Farm-to-fork changes in poultry microbiomes and resistomes in Maputo City, Mozambique. mSystems. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/39699181/
[21] Sasaki Y, Ohya K, Momose Y et al. Serovars and Antimicrobial Resistance of Salmonella in Food Workers and Livestock Products: Insights into Foodborne Transmission Pathways in Eastern Japan. Pathogens. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/41156569/
[22] Han L, Chen M, Huang Q et al. A PCR-enhanced UCNPs-WS(2) fluorescence biosensor via dual quenching-dual recovery mechanism for rapid detection of Salmonella typhimurium. Food Res Int. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42083239/
[23] Gao R, Chen Y, Su Y et al. Development of a portable electrochemical sensor based on STAB-GN for simultaneous detection of chloramphenicol and enrofloxacin in foods. Anal Chim Acta. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41813350/
[24] Al-Ajeeli MN, Taylor TM, Alvarado CZ et al. Comparison of eggshell surface sanitization technologies and impacts on consumer acceptability. Poult Sci. 2016. URL: https://pubmed.ncbi.nlm.nih.gov/26944973/
[25] Lin CM, Herianto S, Chen HL et al. The application of a novel non-thermal plasma device with double rotary plasma jets for inactivation of Salmonella Enteritidis on shell eggs and its effects on sensory properties. Int J Food Microbiol. 2021. URL: https://pubmed.ncbi.nlm.nih.gov/34358812/
[26] Keyvan E, Donmez S, Kahraman HA et al. Novel Photodynamic Inactivation Strategy for Salmonella Enteritidis PT4 on Eggshells: Exploiting the Antimicrobial Potential of Curcumin and Carvacrol. Vet Med Sci. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/39821598/
[27] Jo SJ, Kim SG, Giri SS et al. Bacteriophage emulsion for prevention of Salmonella contamination in eggshells. Int J Food Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41075770/
[28] Pagano C, Puccetti M, Perioli L et al. Monitoring of antibiotic residues in muscles, milk and eggs of food-producing animals in Umbria and Marche regions (Central Italy) during the period time 2012-2021. Ann Ist Super Sanita. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40106322/
[29] Fang L, Pan S, Lu B et al. Contamination status and the health risk evaluation of dietary exposure of quinolone and tetracycline antibiotics in animal derived foods in Ningbo City from 2018 to 2020. Wei Sheng Yan Jiu. 2022. URL: https://pubmed.ncbi.nlm.nih.gov/35341506/
[30] Dahlawi S, Berekaa M, Salama K et al. Profiling of Essential Mineral Content, Heavy Metals, and Bacterial Contaminants in Conventional and Organic Eggs Available in the Hypermarkets of the Eastern Province of Saudi Arabia. Recent Pat Food Nutr Agric. 2021. URL: https://pubmed.ncbi.nlm.nih.gov/35026976/
[31] Ji X, Yang H, Wang J et al. Evaluation of Tilmicosin Contamination in Eggs Following Its Administration to Laying Hens and Subsequent Assessment of Dietary Risks to Chinese Consumers. J Food Sci. 2019. URL: https://pubmed.ncbi.nlm.nih.gov/31524951/
[32] Taddese D, Tolosa T, Deresa B et al. Antibiograms and risk factors of Salmonella isolates from laying hens and eggs in Jimma Town, South Western Ethiopia. BMC Res Notes. 2019. URL: https://pubmed.ncbi.nlm.nih.gov/31370868/
[33] Lee M, Seo DJ, Jeon SB et al. Detection of Foodborne Pathogens and Mycotoxins in Eggs and Chicken Feeds from Farms to Retail Markets. Korean J Food Sci Anim Resour. 2016. URL: https://pubmed.ncbi.nlm.nih.gov/27621686/
[34] Hamamoto K, Akama R, Mizuno Y. Measurement of ampicillin residue levels in chicken eggs during and after medicated feed administration by LC-MS/MS. Food Addit Contam Part A Chem Anal Control Expo Risk Assess. 2015. URL: https://pubmed.ncbi.nlm.nih.gov/26156099/
[35] Im MC, Jeong SJ, Kwon YK et al. Prevalence and characteristics of Salmonella spp. isolated from commercial layer farms in Korea. Poult Sci. 2015. URL: https://pubmed.ncbi.nlm.nih.gov/26015591/
[36] Pouillot R, Hoelzer K, Ramirez GA et al. Assessment of the risk of salmonellosis from internally contaminated shell eggs following initial storage at 18 degrees C (65 degrees F), compared with 7 degrees C (45 degrees F). Food Microbiol. 2014. URL: https://pubmed.ncbi.nlm.nih.gov/24929877/
[37] Vandenberge V, Delezie E, Huyghebaert G et al. Residues of sulfadiazine and doxycycline in egg matrices due to cross-contamination in the feed of laying hens and the possible correlation with physicochemical, pharmacokinetic and physiological parameters. Food Addit Contam Part A Chem Anal Control Expo Risk Assess. 2012. URL: https://pubmed.ncbi.nlm.nih.gov/22380936/
[38] Stepień-Pyśniak D. Occurrence of gram-negative bacteria in hens'
Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.