Bacterial Pathogens in Chickens: Salmonella and Escherichia coli Infections
By Dr. Zubair Khalid, DVM, MS, PhD ·
Key Takeaways
- Salmonella enterica subspecies enterica serovars, particularly S. enterica serovar Gallinarum (fowl typhoid), S. enterica serovar Pullorum (pullorum disease), and broad-host-range serovars like S. enterica serovar Typhimurium and S. enterica serovar Enteritidis, are significant poultry pathogens. These can cause high mortality, subclinical carriage, and egg contamination, with multidrug-resistant (MDR) and extensively drug-resistant (XDR) strains posing a critical concern due to plasmid-mediated resistance genes.
- Avian pathogenic Escherichia coli (APEC) causes colibacillosis, characterized by airsacculitis, pericarditis, and perihepatitis, driven by virulence factors such as adhesins, iron acquisition systems, and toxins. Atypical enteropathogenic E. coli (aEPEC) and commensal E. coli also contribute to antimicrobial resistance, particularly through the carriage of extended-spectrum beta-lactamase (ESBL) genes.
- Transmission of Salmonella occurs via vertical (transovarian) and horizontal (fecal-oral, contaminated feed/water/litter) routes, with hatcheries serving as critical control points for XDR strains. APEC spreads primarily through inhalation of contaminated dust and feces, often exacerbated by immunosuppression or other infections.
- Molecular diagnostics, including multiplex PCR for differentiating Salmonella biovars and whole-genome sequencing for characterizing APEC virulence and resistance genes, are crucial for accurate detection and surveillance.
- Treatment of salmonellosis and colibacillosis is challenged by widespread antimicrobial resistance; alternative strategies like lytic bacteriophages, antimicrobial peptides, probiotics, and synbiotics are being investigated.
- Control strategies encompass stringent biosecurity measures, hatchery hygiene, feed/water treatment with organic acids, competitive exclusion products, and vaccination programs utilizing killed whole-cell or conjugate vaccines for Salmonella and outer membrane protein (OMP) or whole-cell vaccines for APEC.
Introduction
Salmonella and Escherichia coli represent the two most significant bacterial pathogen groups affecting commercial poultry worldwide. These organisms cause substantial economic losses through mortality, reduced performance, and carcass condemnations. They also serve as major reservoirs for foodborne zoonoses. Understanding the complex biology of these pathogens, their interactions with the avian host, and the factors driving antimicrobial resistance is critical for effective control programs. This article provides a detailed review of the etiology, epidemiology, pathogenesis, diagnostics, treatment, and control of Salmonella and avian pathogenic Escherichia coli (APEC) infections in chickens, with an emphasis on recent molecular and genomic insights.
Etiology and Taxonomy of Salmonella in Chickens
Salmonella enterica subspecies enterica encompasses over 2,500 serovars, many of which are capable of infecting poultry [<a href="#ref-1">1</a>]. Serovars are classified based on somatic (O) and flagellar (H) antigens. In chickens, the most clinically relevant serovars include the host-adapted S. enterica serovar Gallinarum (which causes fowl typhoid), S. enterica serovar Pullorum (which causes pullorum disease), and the broad-host-range serovars S. enterica serovar Typhimurium and S. enterica serovar Enteritidis, which are frequently associated with subclinical intestinal carriage and egg contamination [<a href="#ref-2">2</a>]. The emergence of multidrug-resistant (MDR) and extensively drug-resistant (XDR) strains has become a pressing concern. Molecular profiling of XDR Salmonella isolates from hatchery environments has revealed the presence of class 1 integron gene cassettes linked to resistance determinants, including those encoding beta-lactamases and aminoglycoside-modifying enzymes [<a href="#ref-3">3</a>]. Whole-genome sequencing efforts have further characterized the distribution of plasmid-mediated cephalosporin resistance genes such as blaCTX-M and blaCMY among Salmonella serovars in poultry [<a href="#ref-4">4</a>].
Pathogenesis of Salmonella Infection
Salmonella infection in chickens begins with oral ingestion of the organism. Following transit through the proventriculus and gizzard, bacteria adhere to and invade the epithelial cells of the cecum and ileum. The type III secretion system 1 (T3SS-1), encoded by pathogenicity island 1 (SPI-1), is essential for injecting effector proteins into host cells that trigger cytoskeletal rearrangements and bacterial internalization [<a href="#ref-5">5</a>]. Once internalized, Salmonella resides within a modified phagosome known as the Salmonella-containing vacuole (SCV). The type III secretion system 2 (T3SS-2), encoded by SPI-2, is required for intracellular survival and replication. Intracellular replication within macrophages is a hallmark of systemic infection. The host protein SIRT1 has been shown to negatively regulate immune responses to Salmonella, thereby attenuating host resistance [<a href="#ref-6">6</a>]. Organic acids such as butyrate can impede Salmonella infection in chicken macrophage-like HD11 cells by modulating the expression of itaconate-related genes, thereby enhancing antimicrobial activity [<a href="#ref-7">7</a>].
Following systemic dissemination, Salmonella can colonize the liver, spleen, bone marrow, and reproductive tract. In laying hens, colonization of the ovarian follicles and oviduct leads to transovarian transmission and the production of contaminated eggs. The ability of certain serovars to persist in the reproductive tract without causing clinical signs is a critical factor in vertical transmission.
Etiology and Pathotypes of Escherichia coli in Chickens
Escherichia coli is a commensal inhabitant of the chicken intestinal tract, but specific pathotypes cause significant disease. Avian pathogenic E. coli (APEC) is the primary causative agent of colibacillosis, a syndrome encompassing airsacculitis, pericarditis, perihepatitis, salpingitis, and coligranuloma [<a href="#ref-8">8</a>, <a href="#ref-9">9</a>, <a href="#ref-10">10</a>]. APEC strains are characterized by the presence of specific virulence genes encoding adhesins (e.g., type 1 fimbriae, P fimbriae), iron acquisition systems (e.g., aerobactin, salmochelin), protectins (e.g., lipopolysaccharide, capsular polysaccharide K1), and toxins (e.g., hemolysin HlyE, vacuolating autotransporter toxin) [<a href="#ref-11">11</a>, <a href="#ref-12">12</a>, <a href="#ref-13">13</a>]. The virulence protein Hcp2a, a component of the type VI secretion system (T6SS), induces incomplete autophagy in chicken HD11 macrophages, facilitating intracellular survival [<a href="#ref-14">14</a>]. Extracellular vesicles (EVs) carrying the HlyE variant (HlyE-V) from APEC induce mitochondria-dependent apoptosis in HD11 cells, mediated through caspase-3 and caspase-9 activation [<a href="#ref-13">13</a>]. The ecnAB toxin-antitoxin system modulates APEC virulence by regulating the capsular sialic acid biosynthesis pathway [<a href="#ref-11">11</a>]. Small non-coding RNAs such as RyfA and TimR play a role in stress resistance and virulence in APEC [<a href="#ref-10">10</a>]. The zoonotic potential of some APEC serotypes is linked to the specificity of P-like fimbrial (PLF) adhesins [<a href="#ref-12">12</a>].
Atypical enteropathogenic E. coli (aEPEC) strains, which lack the bundle-forming pilus (BFP) but carry the intimin gene eae, have been detected at high prevalence in retail chicken meat. These strains harbor diverse sequence types and antimicrobial resistance profiles [<a href="#ref-15">15</a>]. Commensal E. coli from poultry also serve as reservoirs for extended-spectrum beta-lactamase (ESBL) genes [<a href="#ref-16">16</a>].
Coinfections and Host Interactions
Salmonella and APEC infections can be exacerbated by concurrent viral infections. Direct interaction between APEC and H9N2 avian influenza virus promotes bacterial adhesion during coinfection, likely mediated by alterations in host cell surface receptor expression [<a href="#ref-17">17</a>]. The presence of gut microbiota signatures is associated with the development of Salmonella Typhimurium super-shedders in broiler chickens; certain microbial community structures correlate with high levels of fecal shedding [<a href="#ref-18">18</a>].
Epidemiology and Transmission
Salmonella
The epidemiology of Salmonella in poultry involves both vertical and horizontal transmission routes. Vertical transmission via the egg is a key pathway for S. Enteritidis and S. Pullorum [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>]. Horizontal transmission occurs through the fecal-oral route, contaminated feed, water, litter, and fomites. Hatchery environments are a critical control point, as dead-in-shell eggs and hatchery debris can harbor XDR Salmonella strains [<a href="#ref-3">3</a>]. A hierarchical Bayesian modeling approach has been used to estimate the most probable number (MPN) concentration of Salmonella in raw chicken from qualitative data, providing refined risk assessments [<a href="#ref-19">19</a>]. Prevalence surveys in broiler flocks in Northern Algeria have identified a high occurrence of Salmonella, with significant antimicrobial resistance detected [<a href="#ref-20">20</a>]. The landscape of antimicrobial resistance in S. Gallinarum-Pullorum in Bangladesh reveals a combination of phenotypic and genotypic resistance [<a href="#ref-1">1</a>].
Escherichia coli
APEC is ubiquitous in poultry environments. Transmission occurs primarily through inhalation of contaminated dust and feces, leading to respiratory tract colonization that can progress to systemic infection. Colibacillosis is often secondary to immunosuppressive conditions, mycoplasma infections, or environmental stressors. Commensal E. coli can acquire virulence genes through horizontal gene transfer. Retail chicken meat can serve as a reservoir for emerging antibiotic-resistant pathotypes of E. coli, even in pristine areas free from agricultural activity, highlighting the role of imported meat in disseminating resistance [<a href="#ref-21">21</a>].
Clinical Signs and Pathology
Salmonellosis
Acute infections with S. Gallinarum or S. Pullorum cause septicemia, leading to depression, anorexia, diarrhea, and high mortality in young chicks. In older birds, fowl typhoid presents with anemia, pallor of the comb and wattles, and hepatomegaly with bronze discoloration of the liver. Mortality can be variable but may exceed 50% in susceptible flocks. Chronic carriers exhibit no overt signs but shed Salmonella intermittently in feces and eggs.
Colibacillosis
Colibacillosis in broilers typically presents as an acute to subacute disease. Clinical signs include depression, ruffled feathers, respiratory distress (dyspnea, rales), and increased flock mortality. Airsacculitis is the most common lesion, with thickened, cloudy, and caseous air sacs. Pericarditis and perihepatitis are hallmark fibrinous lesions. In layers, salpingitis is common, characterized by inflammation and accumulation of caseous exudate in the oviduct. Coligranuloma (Hjarre's disease) presents as granulomatous nodules in the liver, ceca, and duodenum. Avian pathogenic E. coli can also cause cellulitis and swollen head syndrome. Virulent APEC strains induce characteristic pathological changes, including the formation of a fibrinous exudate, indicative of a robust but ineffective host inflammatory response. Chicken e coli poop may be observed as watery or mucoid diarrhea in affected birds. The presence of chicken necrosis, or necrotic lesions in internal organs, is a frequent post-mortem finding in severe colibacillosis.
Molecular Diagnostics
Accurate and rapid detection of Salmonella and APEC is essential for effective control. Multiple molecular diagnostic platforms have been developed.
A one-step multiplex PCR has been established for the accurate detection and differentiation of Salmonella Gallinarum biovars Pullorum and Gallinarum [<a href="#ref-2">2</a>]. This assay targets specific genomic regions and can differentiate the two biovars from other Salmonella serovars. An indirect ELISA based on the Sptp protein has been developed for detecting Salmonella infection in poultry, offering a serological surveillance tool [<a href="#ref-22">22</a>]. The use of a PCR-enhanced fluorescence biosensor employing upconversion nanoparticles (UCNPs) coupled with tungsten disulfide nanosheets (WS2) enables rapid detection of Salmonella Typhimurium based on a dual quenching-dual recovery mechanism [<a href="#ref-23">23</a>]. This biosensor can achieve detection limits suitable for food safety applications.
Genomic characterization of APEC has highlighted its potential as a marker organism for antimicrobial resistance surveillance. Whole-genome sequence analysis can identify resistance genes, virulence determinants, and phylogenetic relationships, informing control strategies [<a href="#ref-24">24</a>]. Comparative genomic analysis of XDR APEC strains has identified novel mechanisms of resistance [<a href="#ref-9">9</a>].
Treatment and Antimicrobial Resistance
Antimicrobial therapy for salmonellosis and colibacillosis has been severely compromised by the emergence and spread of multidrug-resistant strains. High levels of resistance to fluoroquinolones, cephalosporins, and aminoglycosides have been reported globally [<a href="#ref-1">1</a>, <a href="#ref-4">4</a>]. Fluoroquinolone-resistant APEC has been isolated from asymptomatic broiler chickens at slaughter [<a href="#ref-25">25</a>]. Whole-genome analysis of ESBL-producing E. coli from poultry in Turkey reveals a high prevalence of blaCTX-M genes and diverse plasmid replicon types [<a href="#ref-16">16</a>]. The global prevalence and distribution of cephalosporin resistance mechanisms, including blaCTX-M, blaCMY, and blaSHV, in Salmonella have been characterized through genome-wide association studies [<a href="#ref-4">4</a>].
A number of alternative strategies are under investigation. Lytic bacteriophages have shown efficacy against Salmonella in different food matrices and in broilers, reducing colonization and contamination [<a href="#ref-26">26</a>, <a href="#ref-27">27</a>]. Phage therapy has also been evaluated against Salmonella Pullorum, demonstrating potential to combat intracellular infection [<a href="#ref-28">28</a>]. Antimicrobial peptides identified through artificial intelligence (AI) screening have been evaluated for safety and efficacy against APEC in broilers [<a href="#ref-29">29</a>]. Probiotics and synbiotic supplementation provide protective effects against Salmonella Typhimurium infection in young broilers, reducing colonization and modulating the gut microbiota [<a href="#ref-30">30</a>]. Dietary supplementation with Bacillus subtilis can reduce general infection with Salmonella Pullorum [<a href="#ref-31">31</a>].
Vaccination and Immunoprophylaxis
Vaccination remains a cornerstone of Salmonella and E. coli control. Killed whole-cell vaccines and conjugate vaccines have been prepared against Salmonella Typhimurium, with conjugate vaccines inducing superior antibody responses in chickens [<a href="#ref-32">32</a>]. For APEC, outer membrane protein (OMP) vaccines and whole-cell killed vaccines have been compared, with OMP-based vaccines providing better protection against heterologous challenge [<a href="#ref-33">33</a>].
A novel vaccine platform uses APEC-derived bacterial biomimetic vesicles displaying the H9 subtype avian influenza virus HA1 protein, offering a dual-protection approach against both colibacillosis and avian influenza [<a href="#ref-34">34</a>]. The combination of Ilex rotunda Thunb. and Cyperus rotundus L. herb pair extract has demonstrated preventive effects against avian colibacillosis, reducing clinical signs and pathological lesions [<a href="#ref-8">8</a>]. Houttuynia cordata extract protects against Salmonella infection by targeting T3SS-1, thereby inhibiting bacterial invasion [<a href="#ref-5">5</a>].
Control Strategies
Effective control of Salmonella and E. coli in commercial poultry requires a multi-faceted approach. Biosecurity measures are paramount, including strict sanitation of housing, equipment, and footwear; control of rodents, wild birds, and insects; and the use of all-in/all-out production systems. Hatchery hygiene is critical to prevent vertical transmission [<a href="#ref-3">3</a>]. Feed and water can be treated with organic acids to reduce bacterial contamination. The use of competitive exclusion products, which introduce beneficial gut microbiota to newly hatched chicks, can reduce Salmonella colonization.
In the context of why does chicken have salmonella but not beef, the primary explanation lies in processing differences and the nature of carcass contamination. Chickens are processed through a series of common tanks (scalding, chilling) that can facilitate cross-contamination between carcasses. Additionally, the common practice of pooling eggs from multiple flocks in hatcheries creates opportunities for widespread dissemination. The chicken salmonella USDA refers to the regulatory framework administered by the United States Department of Agriculture's Food Safety and Inspection Service (FSIS) for Salmonella surveillance and control in poultry products.
Consumer awareness regarding undercooked chicken e coli and the question does chicken have e coli or salmonella is vital for food safety. Both organisms can contaminate raw poultry, and proper cooking to an internal temperature of 165°F (74 degrees Celsius) is necessary to kill these pathogens. Intestinal colonization means that chicken e coli poop is a major source of environmental contamination on farms.
Conclusions
Salmonella and Escherichia coli remain the most significant bacterial pathogens affecting modern poultry production. The evolution of multidrug resistance, the emergence of hypervirulent clones, and the complexity of host-pathogen interactions present ongoing challenges. A comprehensive understanding of the biology of these organisms, combined with robust molecular diagnostics, biosecurity, vaccination, and antimicrobial stewardship, is essential for sustaining flock health and reducing the risk of foodborne illness. Continued chicken bacteria news will likely focus on the spread of resistance genes, the development of novel intervention technologies such as bacteriophages and AI-identified antimicrobial peptides, and the refinement of detection and surveillance systems.