Escherichia coli in Chickens and Poultry Products: Bacterial Pathogenesis, Contamination Routes, Clinical Signs in Flocks, and Public Health Risks

Introduction

Escherichia coli is a facultative anaerobic Gram-negative bacillus that is a ubiquitous member of the chicken intestinal microbiota. While most strains are commensal, a subset designated avian pathogenic Escherichia coli (APEC) causes colibacillosis, a complex of extraintestinal infections that represents one of the most economically significant bacterial diseases in poultry production worldwide [20, 84]. APEC infections result in elevated mortality, reduced growth performance, carcass condemnation, and increased antimicrobial use [25, 90]. The disease affects broilers, layers, and breeders, with higher incidence in young chicks during the first week of life. Beyond the direct impact on poultry health, APEC strains and other E. coli lineages from chickens constitute a potential zoonotic reservoir for extraintestinal pathogenic E. coli (ExPEC) in humans [29, 49, 58]. The emergence of multidrug-resistant (MDR) and extended-spectrum beta-lactamase (ESBL)-producing E. coli in poultry products has intensified public health concerns, particularly with the global dissemination of plasmid-mediated colistin resistance genes (mcr) and carbapenemase genes [1, 36, 50]. This review provides an exhaustive examination of the pathogenesis, contamination routes, clinical manifestations, and public health implications of E. coli in chickens and poultry products, with a focus on molecular mechanisms and epidemiological evidence.

Bacterial Pathogenesis of Avian Pathogenic Escherichia coli

Virulence Factors and Molecular Determinants

APEC strains harbor a repertoire of virulence-associated genes (VAGs) that facilitate colonization, immune evasion, and systemic dissemination. A systematic compilation of frequently reported VAGs includes ten key determinants: iss (increased serum survival), tsh (temperature-sensitive hemagglutinin), iroN (catecholate siderophore receptor), ompT (outer membrane protease), iutA (ferric aerobactin receptor), cvaC (colicin V), hlyF (hemolysin F), iucD (aerobactin biosynthesis), papG (P fimbriae adhesin), and papC (P fimbriae assembly). These genes are predominantly plasmid-borne, often located on large ColV or ColBM virulence plasmids, facilitating horizontal transfer [46, 97]. The presence of five or more of these VAGs is commonly used as a molecular criterion to define APEC [32, 53].

Iron acquisition systems are critical for APEC survival in the iron-limited environment of the host. The aerobactin operon (iucABCD-iutA) and the salmochelin system (iroBCDEN) enable efficient iron scavenging from transferrin and lactoferrin [46, 100]. In a chicken lung infection model, APEC infection co-upregulated chicken surfactant protein A (cSP-A) and chicken lung lectin (cLL), indicating recognition of bacterial surface carbohydrates by host innate lectins.

Adhesins mediate attachment to respiratory and intestinal epithelium. The tsh gene encodes a temperature-sensitive hemagglutinin that binds to extracellular matrix components. Type 1 fimbriae and P fimbriae (Pap) facilitate adherence to epithelial surfaces. Invasion of host cells is assisted by the ibeA and gimB genes, though these are less prevalent in APEC compared to human ExPEC.

Toxins contribute to tissue damage and inflammation. Alpha-hemolysin (hlyA) is present in some APEC strains. The vat gene encodes a vacuolating autotransporter toxin, and the astA gene produces heat-stable enterotoxin 1 (EAST1) [28, 40]. APEC strains also possess a lipopolysaccharide (LPS) that triggers strong inflammatory responses via Toll-like receptor 4 (TLR4) signaling [13, 18].

Host Immune Response and Immunopathology

Upon respiratory or oral entry, APEC is phagocytosed by heterophils and macrophages in the lung. In a CSF1R-reporter transgenic chicken model, both APEC O1 and O2 strains were predominantly detected in CSF1R-negative and CSF1R-low cells identified as heterophils, while a smaller fraction localized to CSF1R-high macrophages and dendritic cells. The O2 strain induced a more pronounced repression of immune pathways, including aryl hydrocarbon receptor signaling and IL-17 pathways, compared to O1.

LPS from E. coli activates the TLR4/MyD88/NF-κB pathway, leading to upregulation of pro-inflammatory cytokines such as IL-1β, IL-6, and IL-8 [13, 30]. In heat-stressed Ma chickens infected with E. coli O157:H7, TLR4-NF-κB signaling was amplified, aggravating intestinal inflammation. Dihydromyricetin attenuates LPS-induced ileum injury by inhibiting the NLRP3 inflammasome and TLR4/NF-κB signaling. Similarly, isoquinoline alkaloids from Macleaya cordata co-regulate TLR4/MyD88/NF-κB and Nrf2 pathways in broiler chickens.

Intestinal barrier disruption is a hallmark of E. coli infection. LPS decreases expression of tight junction proteins occludin, claudin-1, and ZO-1, leading to increased intestinal permeability [13, 34]. Probiotics such as Lactobacillus acidophilus and Clostridium butyricum restore barrier integrity by upregulating occludin and ZO-1 expression [34, 79]. The probiotic Lacticaseibacillus rhamnosus GG reduces E. coli adhesion to chicken intestinal epithelial cells by 75.7% and enhances innate immune responses via Toll-like receptor upregulation [3].

Phylogenetic Groups and Pathotype Classification

APEC strains mostly belong to phylogroups B2 and D, although phylogroups A, B1, and F are also represented. In a study of colibacillosis-related isolates, phylogroup F was significantly more prevalent among disease isolates, and 87% of phylogroup F isolates met the APEC molecular criterion. Pangenome-wide association studies have identified 143 genes enriched in APEC compared to commensal E. coli, including genes for metabolism, LPS synthesis, heat shock response, and antimicrobial resistance. ST117 is an emerging poultry-associated lineage with high pathogenic potential.

Sequence types (STs) commonly found in chicken E. coli include ST10, ST48, ST58, ST69, ST88, ST93, ST117, ST131, ST155, ST410, and ST515 [1, 37, 49, 65]. Many of these STs overlap with human ExPEC, suggesting potential zoonotic transmission. For instance, ST515 and ST69 were the most prevalent among NDM-positive E. coli from retail chickens in China [1].

Contamination Routes

On-Farm Transmission

E. coli enters chicken flocks via multiple routes. Day-old chicks may acquire infection from contaminated eggshells or hatchery equipment. Enterococcus faecalis and E. coli co-infection in embryonating eggs increases neonatal mortality and upregulates IL-1 and CXCR4 expression in embryos. Horizontal transmission occurs through the fecal-oral route, contaminated feed, water, litter, and equipment. A seeder-bird model showed that inoculation of only 10¹ CFU of ESBL/AmpC-producing E. coli led to persistent colonization of all sentinel birds within three days, demonstrating rapid transmission even at low doses.

Rodents, insects, and farmworkers act as mechanical vectors. Multidrug-resistant E. coli has been isolated from rodents and soil in chicken farm environments, with 27.7% of chicken isolates and 10.8% of rodent isolates exhibiting MDR in Tanzania [4]. In Bangladesh, broiler chickens (86% prevalence) and farm sewage (78%) showed similar resistance patterns, indicating environmental contamination [5]. Risk factors for MDR infection include winter season, absence of specific footwear for staff, and antibiotic use without veterinary prescription [5].

Post-Slaughter Contamination

Contamination of poultry products occurs during processing. Cross-contamination from intestinal contents to carcasses, equipment, and chill water is well documented. In Hong Kong wet markets, 88.8% of fresh chicken meat samples harbored ESBL-producing E. coli, with bla_CTX-M-1 as the predominant beta-lactamase gene. A study in Peru found higher MDR rates in non-organic chickens than in humans, with florfenicol resistance significantly more prevalent in chicken vendors, suggesting transmission via meat handling [6].

In South Korea, chicken carcass isolates showed high resistance to nalidixic acid and ciprofloxacin, with MDR rates exceeding 80%. Genomic surveillance of ESBL-carrying E. coli from poultry in the UK revealed that bla_CTX-M-1 dominated in 2016 and 2018, but was largely replaced by bla_CTX-M-55 by 2020, indicating dynamic plasmid dissemination.

Role of Plasmid-Mediated Resistance Genes

Plasmids are central to the dissemination of resistance genes among chicken E. coli populations. IncFIB and IncI1 are common replicon types. The mcr-1 gene is frequently located on IncHI2, IncI2, or IncX4 plasmids [33, 78]. In chickens from Egypt, tet(X7) conferring high-level tigecycline resistance was found on IncHI2 plasmids co-carrying mcr-1.1. Fosfomycin resistance gene fosA3 is disseminated by F33:A-:B- and IncHI2/ST3 plasmids in China. The co-occurrence of mcr-1 and bla_NDM on the same plasmid has been reported in healthy chicken ExPEC isolates [29, 77].

The following table summarizes key plasmid-mediated resistance genes found in chicken E. coli:

Clinical Signs in Flocks

Acute Septicemic Form

The acute septicemic form typically occurs in broilers aged 2-6 weeks. Clinical signs include sudden death, depression, ruffled feathers, reduced feed and water intake, and cyanosis of comb and wattles [20, 26]. Mortality can reach 20-50% in untreated flocks. Gross lesions include fibrinous pericarditis, perihepatitis, and airsacculitis (collectively termed polyserositis). Fibrinous exudate covers the liver and heart, and the spleen may be enlarged. In a study with APEC strain HB2016, intraperitoneal injection of 2 × 10⁶ CFU reproduced classic colibacillosis lesions.

Respiratory Form

Respiratory colibacillosis often follows primary viral (infectious bronchitis virus, Newcastle disease virus) or mycoplasma infections. APEC invades through damaged respiratory epithelium, causing airsacculitis, pneumonia, and tracheitis [2, 88]. In CSF1R-reporter transgenic chickens, APEC O1 and O2 strains were found within heterophils and macrophages in the lung at 6 hours post-infection, inducing strong inflammatory gene expression. Experimental infection with APEC O78 and O26 serotypes showed significant upregulation of IL-6 and IL-8 in the ileum, while IL-10 was downregulated.

Yolk Sac Infection (Omphalitis)

In neonatal chicks, E. coli is a major cause of yolk sac infection (omphalitis), characterized by unabsorbed yolk sac, peritonitis, and septicemia. This leads to elevated first-week mortality. Large-scale surveys indicate that E. coli is the most common bacterial isolate from dead embryos and early chick mortalities [27, 90].

Chronic and Subclinical Forms

Chronic colibacillosis presents with localized infections such as salpingitis (in laying hens), synovitis, and osteomyelitis. Subclinical infections impair growth performance and feed conversion, even without overt mortality. Dual infections with Eimeria tenella or Mycoplasma gallisepticum exacerbate pathology. Co-infection with M. gallisepticum and E. coli impairs Newcastle disease virus vaccine performance [63, 75].

A decision tree for diagnosing colibacillosis in broiler flocks is presented below.

flowchart TD
 A[Flock with elevated mortality or morbidity] --> B[Post-mortem examination]
 B --> C{Typical lesions?}
 C -->|Pericarditis, perihepatitis, airsacculitis| D[Collect liver/heart swabs]
 C -->|Yolk sac infection| E[Collect yolk sac/viscera]
 D --> F[Isolation on MacConkey agar]
 E --> F
 F --> G[Gram-negative, lactose-fermenting colonies]
 G --> H["'Biochemical confirmation (IMViC')"]
 H --> I[Serotyping / PCR for VAGs]
 I --> J{APEC molecular criteria met?}
 J -->|≥5 VAGs positive| K[Confirm APEC]
 J -->|<5 VAGs positive| L[Consider other pathogens]
 K --> M[Antimicrobial susceptibility testing]
 M --> N[Select therapy / implement control]

The table below lists the most commonly reported APEC serogroups and their association with disease.

Public Health Risks

Antimicrobial Resistance as a One Health Threat

Chickens serve as a major reservoir of antimicrobial-resistant E. coli that can be transmitted to humans through direct contact, environmental contamination, or food consumption. A meta-analysis of mcr-mediated colistin resistance found a global prevalence of 15.8% in chickens, 14.9% in pigs, 7.4% in healthy humans, and 4.2% in clinical isolates. In Shandong, China, 11.3% of retail chicken E. coli isolates were NDM-positive, with phylogenomic clustering indicating close relatedness to human clinical isolates [1].

ESBL-producing E. coli are highly prevalent in poultry worldwide. In Zambia, 20.1% of market-ready chickens harbored ESBL-producing E. coli. In the United Arab Emirates, 95% of E. coli from broiler cecal samples were ESBL producers, with bla_CTX-M-15 and bla_CTX-M-55 as dominant genotypes. The co-occurrence of ESBL genes with mcr-1 in B2 phylogroup isolates from Tunisia highlights the convergence of resistance and virulence.

Foodborne Transmission and ExPEC Link

E. coli from chicken meat can cause human extraintestinal infections, including urinary tract infections (UTI), bloodstream infections, and neonatal meningitis. In Brazil, 23% of human ExPEC isolates shared sequence types (ST10, ST48, ST58, ST88, ST93, ST131, ST602, ST617, ST1018) with poultry E. coli, and several isolates exhibited identical antimicrobial resistance profiles. A study from Peru showed that florfenicol resistance (a poultry-exclusive antibiotic) was significantly higher in chicken vendors than in non-vendors, suggesting transmission of resistant strains through occupational exposure [6].

The presence of mcr-1 and bla_NDM in healthy chicken ExPEC isolates is alarming, as these genes compromise the last-resort antibiotics colistin and carbapenems [29, 77]. In Bangladesh, an mcr-1-carrying IncHI2 plasmid from a chicken isolate shared 93% nucleotide identity with a plasmid from a Bangladeshi human E. coli isolate, indicating plasmid transfer across species.

Virulence Potential of Chicken E. coli in Humans

Experimental animal models demonstrate that chicken fecal E. coli meeting ExPEC molecular criteria can cause sepsis, meningitis, and UTI in mice, as well as colibacillosis in chickens. In Senegal, APEC isolates belonging to phylogroup F serogroup O25 (ST624) exhibited strong biofilm formation and carried ExPEC virulence profiles, suggesting a risk for human health. In Egypt, APEC strains carrying mcr-1 and tet(X7) have been identified, further narrowing therapeutic options.

Control Strategies

Vaccination

Live attenuated and inactivated vaccines have been developed to reduce colibacillosis. A live aroA gene-deleted vaccine decreased the number of E. coli isolates from internal organs and reduced antimicrobial use in commercial broilers. Outer membrane vesicle (OMV) vaccines derived from ompA/ompC/ompD mutant Salmonella Typhimurium induced cross-protection against heterologous APEC O78 and Salmonella Enteritidis in chickens. Aerosol vaccination with irradiated APEC stimulated local immune responses, including IFN-γ-producing CD8α+ and TCR-γδ+ cells in the lung. Selective breeding for high natural antibody levels has been shown to increase resistance to APEC challenge.

Probiotics and Competitive Exclusion

Probiotic administration reduces E. coli colonization and enhances immune function. Lactobacillus plantarum B1 lowered cecal E. coli counts and increased ileal secretory IgA in challenged broilers. Clostridium butyricum improved growth performance, immune response, and intestinal barrier function after E. coli K88 challenge [79, 94]. Competitive exclusion cultures of undefined composition reduced ESBL/AmpC-producing E. coli cecal colonization by about 4.0 log10 units in young chicks. Lacticaseibacillus rhamnosus GG demonstrated strong in vitro inhibition of E. coli by up to 75.7% reduction in adhesion to intestinal cells [3].

Antimicrobial Peptides and Phage Therapy

Antimicrobial peptides (AMPs) are promising alternatives to conventional antibiotics. The CATH-2-derived peptide C2-2 showed excellent activity against MDR APEC (MIC 2-8 µg/mL), improved survival, and reduced bacterial loads in heart, liver, and spleen of infected chickens [7]. Microcin J25 improved growth performance and reduced fecal E. coli counts in broilers co-infected with E. coli and Salmonella. Phage therapy using T4-like phage Bp7 effectively controlled drug-resistant E. coli in chickens [8]. A phage cocktail targeting Salmonella Kentucky and E. coli O119 reduced pathogen counts in treated groups.

Farm Management and Hygiene

Reducing antimicrobial selection pressure is critical. In Australia, the industry-wide policy of excluding critically important antimicrobials has resulted in 63.1% of poultry E. coli being fully susceptible, with only sporadic fluoroquinolone resistance. Biosecurity measures such as chlorinated drinking water, proper ventilation, and all-in/all-out production help limit E. coli transmission. Disinfectant resistance genes (qacED1, qacA/B) have been detected in chicken E. coli in Egypt, underscoring the need for rotation of disinfectants.

Conclusion

Escherichia coli in chickens presents a multifaceted challenge: it causes significant economic losses through colibacillosis, is a vehicle for the propagation of antimicrobial resistance, and poses a zoonotic risk via foodborne transmission. The molecular pathogenesis of APEC involves a complex interplay of plasmid-encoded virulence factors and host immune modulation. Contamination routes span from hatchery to retail, with environmental and occupational exposures contributing to dissemination. Clinical signs vary from acute septicemia to subclinical growth depression. Public health risks are driven by the global spread of MDR and ESBL/AmpC-producing strains, including those carrying mcr and carbapenemase genes. Effective control requires an integrated One Health approach combining vaccination, probiotic interventions, bacteriophage therapy, prudent antimicrobial use, and enhanced biosecurity. Surveillance using whole-genome sequencing is essential to monitor the evolution and dissemination of pathogenic and resistant lineages across the poultry production continuum.

References

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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.