Bacterial Pathogens in Chickens: Salmonella, Escherichia coli, and Necrotic Enteritis

Introduction

Bacterial infections represent a major constraint to poultry production worldwide, causing significant economic losses through mortality, reduced performance, and carcass condemnation [1, 2]. Among the diverse array of bacterial agents affecting chickens, three groups stand out for their clinical and economic impact: Salmonella spp., avian pathogenic Escherichia coli (APEC), and Clostridium perfringens, the causative agent of necrotic enteritis [3, 4]. These pathogens are central to discussions of chicken bacteria news, as they are frequently implicated in both clinical disease outbreaks and food safety incidents [5, 6]. Understanding the biology, epidemiology, and control of these agents is essential for veterinary practitioners and poultry health professionals [7, 8]. This article provides a detailed reference on these three major bacterial pathogen groups, covering their etiology, pathogenesis, clinical presentation, diagnostic approaches, and management strategies.

Salmonella in Chickens

Etiology and Serovar Diversity

Salmonella enterica subspecies enterica is the primary cause of salmonellosis in poultry [9, 10]. The species encompasses over 2,500 serovars, many of which are capable of infecting chickens [11]. Serovars are classified based on the Kauffmann-White scheme, which differentiates them by somatic (O) and flagellar (H) antigens [12]. In poultry, the most clinically relevant serovars include Salmonella Pullorum, Salmonella Gallinarum, Salmonella Enteritidis, and Salmonella Typhimurium [13, 14]. Salmonella Pullorum and Salmonella Gallinarum are host-adapted to poultry and cause pullorum disease and fowl typhoid, respectively, which are systemic, often fatal infections [15]. Salmonella Enteritidis and Salmonella Typhimurium are zoonotic serovars that typically cause subclinical intestinal colonization in adult chickens but can contaminate eggs and meat, posing a public health risk [16, 17].

Epidemiology and Transmission

Salmonella is transmitted horizontally through the fecal-oral route and vertically through transovarian transmission [18, 19]. Contaminated feed, water, litter, and equipment serve as major sources of infection [20]. Rodents, wild birds, and insects can act as mechanical vectors [21]. The prevalence of Salmonella in poultry flocks varies widely by region and production system [22]. A prospective study of small poultry flocks in Ontario, Canada, detected Salmonella spp. in 3% of tested submissions [11]. In contrast, a global review reported median prevalence values of 40.5% in broiler chickens, 30% in raw chicken meat, and 40% in eggs and laying hens [32]. The question of why does chicken have salmonella but not beef is rooted in differences in animal physiology, production systems, and slaughter processes. Chickens have a higher body temperature and a shorter gastrointestinal tract, which can influence Salmonella colonization dynamics [23]. Additionally, the processing of chicken carcasses, particularly the scalding and evisceration steps, can spread bacteria from the intestinal tract to the meat surface [24].

Pathogenesis and Host Interaction

Salmonella infection begins with oral ingestion of the bacteria, followed by colonization of the intestinal tract [25]. The bacteria adhere to and invade intestinal epithelial cells, particularly in the ceca and ileum [26]. Invasion is mediated by a type III secretion system (T3SS) encoded on Salmonella pathogenicity island 1 (SPI-1), which injects effector proteins into host cells, triggering cytoskeletal rearrangements and bacterial uptake [27]. Once inside the host cell, Salmonella resides within a Salmonella-containing vacuole (SCV), where it can survive and replicate [28]. The bacteria then disseminate to internal organs, including the liver, spleen, and reproductive tract, via infected macrophages [29]. The host immune response involves both innate and adaptive components. Toll-like receptors (TLRs) recognize Salmonella pathogen-associated molecular patterns (PAMPs), initiating a signaling cascade that leads to the production of pro-inflammatory cytokines [33]. The expression of avian beta-defensins (gallinacins) and interleukins is modulated during infection [12]. Genetic resistance to Salmonella in chickens is a polygenic trait involving major histocompatibility complex (MHC) molecules, natural resistance-associated macrophage protein 1 (Nramp-1), and various cytokines [3]. Transcriptome analysis of cecal tonsils from resistant and susceptible chickens has revealed differential expression of genes involved in focal adhesion, extracellular matrix-receptor interaction, and the intestinal immune network for IgA production [30].

Clinical Signs and Pathology

Clinical signs of salmonellosis vary depending on the serovar, age of the bird, and immune status [31]. In chicks infected with Salmonella Pullorum or Salmonella Gallinarum, signs include depression, anorexia, white diarrhea, pasted vents, and high mortality [32]. Surviving birds may become chronic carriers [33]. In older birds infected with Salmonella Enteritidis or Salmonella Typhimurium, infection is often subclinical, but birds may exhibit reduced feed intake, decreased egg production, and intermittent diarrhea [34]. Postmortem lesions in acute cases include hepatomegaly, splenomegaly, necrotic foci in the liver and spleen, and caseous cecal cores [35]. In chronic carriers, the primary lesion is ovarian regression and peritonitis [1].

Diagnostics

Diagnosis of Salmonella infection relies on bacterial culture, serology, and molecular methods [2]. Conventional culture involves pre-enrichment in buffered peptone water, selective enrichment in Rappaport-Vassiliadis or tetrathionate broth, and plating on selective agar such as xylose lysine deoxycholate (XLD) or brilliant green agar [3]. Suspect colonies are confirmed by biochemical tests (e.g., API-20E) and serotyping [20]. Real-time PCR assays targeting the invA gene or other Salmonella-specific sequences allow direct detection and quantification from cecal samples, with a reported detection limit of approximately 4.3 log10 cells/g [9]. An indirect enzyme-linked immunosorbent assay (ELISA) based on the detection of PagC antibodies in sera has been developed for serological surveillance [17]. Phage-based magnetoelastic biosensors have been explored for rapid detection of Salmonella on chicken meat surfaces [28].

Treatment and Control

Antibiotic therapy for salmonellosis is complicated by widespread antimicrobial resistance [20, 32]. A global review found the highest resistance levels to nalidixic acid and ampicillin among Salmonella isolates from poultry [32]. In Zambia, Salmonella isolates from broiler chickens showed resistance to ampicillin, amoxicillin/clavulanic acid, and cefotaxime [20]. Control strategies focus on biosecurity, vaccination, and the use of competitive exclusion products [34]. Live attenuated Salmonella vaccines and killed bacterins are available for commercial use [34]. Competitive exclusion cultures, consisting of defined bacterial strains from the adult chicken gut, can be administered to day-old chicks to prevent Salmonella colonization [10]. The use of a nitrate reductase enzyme pathway (NREP) inhibitor, an experimental chlorate product, has been shown to significantly reduce Salmonella levels in the ceca of treated chickens [10]. Probiotic approaches, such as Lactobacillus casei overexpressing myosin-cross-reactive antigen, have demonstrated efficacy in reducing Salmonella enterica colonization in the chicken gut [8]. Bifidobacterium bifidum postbiotics have been shown to prevent Salmonella Pullorum infection by modulating pyroptosis, restoring intestinal barrier function, and improving cecal microbiota diversity [7]. Phage therapy using a Salmonella phage cocktail has been evaluated at a commercial farm scale and did not adversely affect the normal development of the cecal microbiota [18]. The concept of a chicken without salmonella is a goal of these integrated control programs, though complete eradication from commercial flocks remains challenging.

Escherichia coli in Chickens

Etiology and Pathotypes

Avian pathogenic Escherichia coli (APEC) is the causative agent of colibacillosis, a complex of diseases including septicemia, airsacculitis, pericarditis, perihepatitis, and cellulitis [13, 35]. APEC strains belong to a subset of extraintestinal pathogenic E. coli (ExPEC) that possess specific virulence factors enabling them to cause disease in poultry [13]. Key virulence genes include iss (increased serum survival), ompT (outer membrane protease), iutA (aerobactin receptor), iroN (siderophore receptor), and hly (hemolysin) [13]. Serogrouping of APEC isolates has identified O78, O1, O2, and O125 as common serogroups associated with disease [2, 25]. A study of cellulitis in broiler chickens found that E. coli was the most prevalent bacterial isolate, accounting for 80.3% of recovered isolates, with serogroup O125 being the most common [2].

Epidemiology and Transmission

E. coli is a ubiquitous component of the normal intestinal microbiota of chickens [21]. APEC strains are transmitted horizontally through the fecal-oral route, via contaminated litter, feed, and water [22]. Respiratory infection, often initiated by viral or environmental stressors, can lead to systemic dissemination of E. coli [23]. The prevalence of E. coli in poultry flocks is high. In a study of septicemic chickens in India, 96.1% of flocks tested positive for E. coli [13]. In Zambia, 280 E. coli isolates were recovered from 470 samples collected from broiler chickens at farms, abattoirs, and open markets [20]. The question of does chicken have e coli or salmonella is frequently asked; the answer is that both pathogens can be present, but E. coli is more commonly isolated from the intestinal tract of healthy chickens, while Salmonella is more frequently associated with foodborne outbreaks [16, 24]. The presence of chicken e coli poop is a normal finding, as E. coli is a commensal inhabitant of the avian gut, but pathogenic strains can be shed in feces and contaminate the environment [21].

Pathogenesis and Host Interaction

APEC pathogenesis involves initial colonization of the respiratory tract, followed by invasion of the bloodstream and dissemination to internal organs [13]. The bacteria adhere to epithelial cells using fimbriae, such as type 1 fimbriae and P fimbriae [25]. The aerobactin iron acquisition system allows APEC to scavenge iron in the iron-limited environment of the host [13]. The iss gene product confers resistance to complement-mediated killing, enabling survival in serum [13]. The host response to APEC infection involves the recruitment of heterophils and macrophages, and the production of pro-inflammatory cytokines [14]. The expression of toll-like receptors (TLRs) in the gastrointestinal tract is modulated during E. coli infection [33]. Genetic resistance to colibacillosis has been linked to MHC haplotypes and other immune-related genes [3].

Clinical Signs and Pathology

Colibacillosis presents in several forms. In acute septicemic colibacillosis, birds exhibit depression, anorexia, ruffled feathers, and sudden death [13]. Postmortem lesions include fibrinous pericarditis, perihepatitis, and airsacculitis, often described as a "glazed" appearance of the liver and heart [13]. In chronic cases, lesions may include polyserositis, arthritis, and salpingitis [1]. Cellulitis, a subcutaneous infection of the thigh and abdomen, is characterized by thickened, discolored skin and a caseous exudate [2]. E. coli is also a major cause of bacterial chondronecrosis with osteomyelitis (BCO), a leading cause of lameness in broiler chickens [26, 27]. Genome analysis of E. coli isolates from BCO lesions has revealed frequent host shifts, suggesting that E. coli can readily adapt to different host species [26].

Diagnostics

Diagnosis of colibacillosis is based on isolation of E. coli from typical lesions in pure culture [13]. Samples are plated on MacConkey agar, where E. coli appears as pink, lactose-fermenting colonies [20]. Confirmation is achieved through biochemical testing (e.g., API-20E) or 16S rDNA sequencing [20]. Molecular detection of virulence genes by PCR can differentiate APEC from commensal E. coli [13]. Antimicrobial susceptibility testing is critical due to high levels of resistance [20, 35]. A study of E. coli isolates from diseased chickens in China found 100% resistance to trimethoprim-sulfamethoxazole and oxytetracycline, and 83% resistance to ampicillin and enrofloxacin [35]. In Spain, E. coli isolates from septicemic chickens showed high resistance to quinolones [25]. Multidrug resistance (MDR) was observed in 75.7% of E. coli isolates from broiler chickens in Zambia, and 11.4% exhibited extensive drug resistance (XDR) [20].

Treatment and Control

Antibiotic therapy for colibacillosis is guided by antimicrobial susceptibility testing [2]. However, the emergence of MDR and XDR strains limits treatment options [20]. An e coli chicken vaccine is available for breeder flocks to provide passive immunity to progeny through maternal antibodies [14]. Autogenous vaccines, prepared from farm-specific APEC isolates, are also used [14]. Control measures include improved biosecurity, litter management, and reduction of environmental stressors [21]. The use of probiotics and prebiotics, such as Jerusalem artichokes, has been shown to stimulate growth and protect against endotoxins and potential cecal pathogens [22]. Essential oils have demonstrated antimicrobial activity against Gram-negative bacterial pathogens isolated from diseased broiler chickens [4].

Necrotic Enteritis

Etiology

Necrotic enteritis (NE) is an enteric disease of chickens caused by Clostridium perfringens, a Gram-positive, spore-forming, anaerobic bacillus [15, 19]. C. perfringens is classified into five toxinotypes (A, B, C, D, E) based on the production of four major toxins (alpha, beta, epsilon, iota) [31]. In chickens, NE is primarily caused by type A and, to a lesser extent, type C strains [31]. The major virulence factor in type A strains is NetB, a pore-forming toxin that causes necrosis of intestinal epithelial cells [19]. The complete genome sequence of C. perfringens Del1, a field strain isolated from chickens with NE, revealed a single circular chromosome of 3,559,163 bp and four plasmids, harboring numerous genes for pathogenesis and virulence factors, including six for antibiotic and antimicrobial resistance [31].

Epidemiology and Transmission

C. perfringens is a normal inhabitant of the chicken intestinal tract, but disease occurs when predisposing factors allow the bacterium to proliferate to high numbers [15]. The most important predisposing factor is coccidiosis, caused by Eimeria spp., which damages the intestinal mucosa and provides a protein-rich environment for C. perfringens growth [15]. Other risk factors include high dietary levels of non-starch polysaccharides (NSPs), which increase intestinal viscosity, and the use of antibiotic growth promoters (AGPs) [19]. The withdrawal of AGPs in many regions has been associated with an increased incidence of NE [31]. A meta-analysis of microbiota studies found that coccidia infection affects the microbiota more than C. perfringens infection alone, and that the combined infection alters the relative abundance of 29 bacterial families [15]. The prevalence of C. perfringens in broiler cecal samples collected at slaughter was 71.1% in a Brazilian study, with a mean concentration of 5.5 log10 cells/g [9].

Pathogenesis and Host Interaction

The pathogenesis of NE involves the overgrowth of C. perfringens in the small intestine, followed by the production of NetB toxin [19]. NetB forms pores in the membranes of intestinal epithelial cells, leading to cell death and necrosis [19]. The resulting tissue damage allows the bacteria to access the underlying mucosa and further proliferate [19]. Transcriptome analysis of the small intestine of broiler chickens infected with C. perfringens revealed significant downregulation of genes involved in energy production, MHC Class I antigen presentation, and amino acid and nucleotide metabolism [19]. Upregulated genes were primarily engaged in innate and adaptive immunity, cellular processes, and genetic information processing [19]. The transcriptional levels of four other foodborne pathogens were significantly elevated in a synergistic relationship with C. perfringens infection [19].

Clinical Signs and Pathology

Clinical NE typically occurs in broiler chickens between 2 and 6 weeks of age [19]. Affected birds exhibit depression, anorexia, diarrhea, and a marked decrease in feed intake [19]. Mortality can be high, reaching 30-50% in untreated flocks [19]. The term chicken necrosis is often used to describe the characteristic postmortem lesions. Grossly, the small intestine, particularly the jejunum and ileum, is distended, friable, and filled with a dark, foul-smelling, necrotic material [19]. The intestinal mucosa is covered by a thick, pseudomembranous layer of necrotic tissue [19]. Microscopically, there is severe necrosis of the villi, with fibrinoid necrosis of the lamina propria and infiltration of heterophils [19].

Diagnostics

Diagnosis of NE is based on clinical signs, gross pathology, and histopathology [19]. Confirmation requires the isolation of C. perfringens from the intestinal contents or lesions [9]. Anaerobic culture on blood agar or tryptose sulfite cycloserine (TSC) agar is used [9]. Real-time PCR assays targeting the cpa (alpha toxin) or netB genes can be used for direct detection and quantification from cecal samples [9]. The detection of C. perfringens by real-time PCR from broiler cecal samples collected in the slaughter line has been demonstrated [9].

Treatment and Control

Treatment of NE involves the use of antibiotics effective against C. perfringens, such as bacitracin, virginiamycin, and lincomycin [19]. However, the emergence of antibiotic resistance is a growing concern [31]. Control strategies focus on preventing predisposing factors, particularly coccidiosis [15]. Coccidiosis control is achieved through vaccination or the use of anticoccidial drugs [15]. Dietary modifications, such as reducing the level of NSPs and using feed enzymes, can help reduce intestinal viscosity and limit C. perfringens proliferation [19]. Probiotics, including Lactobacillus spp. and Bacillus spp., have been shown to reduce the severity of NE [8]. The use of Bifidobacterium bifidum postbiotics has been shown to improve gut health and reduce inflammation in chickens infected with Salmonella, and similar approaches may be applicable to NE [7]. The development of vaccines against NetB toxin is an area of active research [31].

Diagnostic Workflow

The following Mermaid diagram illustrates a general diagnostic workflow for bacterial pathogens in chickens, integrating clinical, microbiological, and molecular approaches.

flowchart TD
    A[Clinical Signs: Depression, Diarrhea, Mortality], > B[Postmortem Examination]
    B, > C{Lesions Consistent with Bacterial Infection?}
    C, >|Yes| D[Sample Collection: Liver, Spleen, Intestine, Cecal Contents]
    D, > E[Microbiological Culture]
    E, > F[Selective & Differential Agar: MacConkey, XLD, Blood Agar]
    F, > G[Biochemical Confirmation: API-20E, 16S rDNA Sequencing]
    G, > H[Antimicrobial Susceptibility Testing: Kirby-Bauer, MIC]
    H, > I[Treatment Decision: Antibiotic Selection]
    C, >|Yes| J[Molecular Detection: Real-Time PCR, Multiplex PCR]
    J, > K[Pathogen Identification & Quantification: Salmonella, E. coli, C. perfringens]
    K, > L[Virulence Gene Profiling: iss, netB, invA]
    L, > M[Epidemiological Typing: Serotyping, MLST, WGS]
    M, > N[Control Strategy: Vaccination, Biosecurity, Probiotics]
    C, >|No| O[Consider Viral, Parasitic, or Metabolic Causes]
    O, > P[Further Diagnostic Testing: Serology, Histopathology, Fecal Floatation]

Integrated Control Strategies

Effective control of bacterial pathogens in chickens requires a multifaceted approach [3, 14]. Biosecurity measures, including all-in/all-out production, cleaning and disinfection of facilities, and control of rodents and wild birds, are fundamental [11]. Vaccination plays a key role for Salmonella and E. coli [14, 34]. The use of probiotics, prebiotics, and postbiotics is gaining traction as an alternative to antibiotic growth promoters [7, 8, 22]. Immunomodulation through targeted dietary supplementation can enhance the bird's natural immune response [14]. Genetic selection for disease resistance, using tools such as whole-genome sequencing and high-density SNP genotyping, offers a long-term strategy for reducing susceptibility to bacterial infections [3]. The question of what bacteria can you get from chicken is answered by the three groups discussed here, along with Campylobacter and other less common agents [9, 11]. The chicken salmonella usda regulatory framework sets standards for Salmonella reduction in processing plants, and these standards drive many of the control measures implemented in commercial production [32]. The question of undercooked chicken e coli is a common food safety concern; proper cooking to an internal temperature of 165 degrees Fahrenheit (74 degrees Celsius) kills both E. coli and Salmonella [24]. The broader topic of chicken and bacteria encompasses the complex microbial ecology of the avian gut and the constant challenge of managing pathogenic populations within the context of commercial production [15, 18].

References

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[6] Foster, N. (2021). Immunity to bacterial pathogens of pigs and chickens. Advancements and Technologies in Pig and Poultry Bacterial Disease Control. https://www.semanticscholar.org/paper/ea8f2ccd6f9121acd72c54f0a8b2fd7c0250d419

[7] Chen, Y., Zhu, F., Yu, G., et al. (2025). Bifidobacterium bifidum postbiotics prevent Salmonella Pullorum infection in chickens by modulating pyroptosis and enhancing gut health. Poultry Science. https://www.semanticscholar.org/paper/9179abe463d885e2983e818ba0c84b079ed09bea

[8] Tabashsum, Z., Peng, M., Alvarado-Martínez, Z., et al. (2020). Competitive reduction of poultry-borne enteric bacterial pathogens in chicken gut with bioactive Lactobacillus casei. Scientific Reports. https://www.semanticscholar.org/paper/f4003a43f848a87ab38b8f6da1fb09c02b4a39d2

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