E. coli and Salmonella on Raw Chicken: Comparative Pathogenesis and Food Safety

Etiology and Taxonomy

The bacterial genera Escherichia and Salmonella (family Enterobacteriaceae) are facultatively anaerobic, Gram-negative rods that share a common ecological niche in the gastrointestinal tract of poultry. Escherichia coli is a diverse species that includes both commensal strains and pathogenic pathotypes. In avian medicine, the most clinically relevant pathotype is avian pathogenic E. coli (APEC), which causes colibacillosis. APEC strains typically possess virulence-associated genes encoding adhesins (e.g. type 1 fimbriae, P fimbriae), iron acquisition systems (e.g. aerobactin), and toxins (e.g. hemolysins) [1, 2]. Salmonella enterica subspecies enterica includes over 2,500 serovars, with host-restricted serovars (e.g. Salmonella Gallinarum, Salmonella Pullorum) causing systemic disease in poultry and broad-host-range serovars (e.g. Salmonella Typhimurium, Salmonella Enteritidis) colonizing the intestinal tract without necessarily causing clinical disease in birds. Both e coli on raw chicken and Salmonella can be detected as contaminants on poultry carcasses, with prevalence rates influenced by farming practices, slaughter hygiene, and cold-chain integrity [3, 4].

Epidemiology and Occurrence on Raw Chicken Carcasses

The presence of e coli on raw chicken carcasses is nearly universal due to fecal contamination during slaughter and processing. E. coli is a routine indicator of fecal contamination, and its detection on raw poultry products is expected. However, the proportion of E. coli isolates that carry antimicrobial resistance (AMR) determinants, including extended-spectrum beta-lactamase (ESBL) and AmpC beta-lactamase genes, has increased substantially in poultry production systems worldwide [1, 2]. Surveillance data indicate that chicken e coli or salmonella can be co-isolated from the same carcass, reflecting concurrent contamination events. Salmonella carriage in broiler flocks is often subclinical, and the organism can persist in the ceca and crop, leading to fecal shedding during transport and slaughter [3, 4]. The prevalence of Salmonella on raw chicken varies by serovar and geographic region, with S. Enteritidis and S. Typhimurium being the most commonly reported serovars in many countries.

Cross-contamination within processing plants, from scalding tanks to chillers, can disseminate both E. coli and Salmonella across carcass lots [3]. Biofilm formation on stainless steel surfaces in processing environments further contributes to persistent contamination. The ability of both organisms to survive on refrigerated raw chicken for several days to weeks underscores the importance of cold-chain management and consumer handling.

Comparative Pathogenesis in the Avian Host

Avian Pathogenic E. coli (APEC)

APEC strains colonize the respiratory tract following inhalation of contaminated dust or litter. Adhesion to respiratory epithelium is mediated by type 1 and P fimbriae. After breaching the mucosal barrier, APEC enters the bloodstream, causing bacteremia and systemic dissemination to the pericardium, air sacs, liver, and spleen [1, 2]. The hallmark lesion of avian colibacillosis is fibrinous polyserositis, characterized by pericarditis, perihepatitis, and airsacculitis. Virulence factors such as the colicin V plasmid (encoding aerobactin for iron acquisition) and O-antigen serotypes (e.g. O1, O2, O78) are strongly associated with systemic pathogenicity. The bacterial lipopolysaccharide (LPS) and capsular polysaccharides (K antigens) contribute to resistance against serum killing and phagocytosis [1, 2].

Salmonella Serovars in Poultry

Pathogenesis of Salmonella in poultry is serovar-dependent. Host-restricted serovars such as S. Gallinarum and S. Pullorum cause fowl typhoid and pullorum disease, respectively, with systemic invasion via the gut-associated lymphoid tissue (GALT) and subsequent septicemia [3, 4]. These serovars produce severe clinical signs including depression, anorexia, diarrhea, high mortality in young birds, and vertical transmission through the egg. In contrast, broad-host-range serovars like S. Enteritidis and S. Typhimurium typically colonize the cecal tonsils without inducing overt disease in adult birds. The bacteria invade M cells overlying Peyer's patches and translocate to the lamina propria, but the avian immune system often contains the infection to the intestinal lumen [3, 4]. However, stressed or immunosuppressed birds may develop bacteremia. Salmonella LPS and type III secretion systems (T3SS) encoded on Salmonella pathogenicity islands (SPI-1 and SPI-2) are critical for invasion and intracellular survival within macrophages.

Comparative Summary of Virulence Factors

Clinical Signs in Poultry Flocks

Avian colibacillosis presents with acute or subacute signs. In broilers, affected birds display reduced feed intake, huddling, drooping wings, and labored breathing. Mortality peaks at 2 to 6 weeks of age, often precipitated by concurrent viral infections (e.g. infectious bronchitis virus) or environmental stress [1, 2]. Pericarditis and perihepatitis are observed at necropsy. Salmonellosis in poultry varies by serovar. Pullorum disease causes high mortality in chicks, with white diarrhea, pasty vents, and septicemia. Fowl typhoid results in acute septicemia with liver necrosis and splenomegaly in older birds. In contrast, paratyphoid infections (e.g. S. Enteritidis) are typically subclinical in adult birds but can cause occasional diarrhea and decreased egg production [3, 4]. Both chicken e coli or salmonella can be shed in feces, perpetuating environmental contamination.

Pathology and Lesions

Macroscopic Lesions

APEC infection produces characteristic fibrinous exudates. The pericardium is thickened and opaque with fibrin deposition. The liver surface is covered with a fibrinous inflammatory film (perihepatitis), and the air sacs contain fibrinous caseous material. In acute colisepticemia, the spleen and kidneys may be enlarged and congested [1, 2]. Salmonella Gallinarum infection results in hepatomegaly with bronze discoloration, splenomegaly, and hemorrhagic follicles in layers. S. Pullorum produces necrotic foci in the liver, lungs, and myocardium, as well as necrotic cecal cores. Paratyphoid Salmonella infections rarely produce gross lesions in adult poultry, though the ceca may appear normal or contain fluid contents [3, 4].

Histopathology

Microscopically, APEC-induced airsacculitis shows heterophilic infiltration, fibrin deposition, and edema. Perihepatitis involves a mixed inflammatory cell infiltrate with fibrin and bacterial colonies on the serosal surface. Salmonella Gallinarum infection reveals multifocal hepatic necrosis with heterophilic and mononuclear infiltration. Intracellular bacteria within macrophages are identifiable via modified acid-fast or immunohistochemical staining. In paratyphoid infections, the lamina propria of the cecum contains infiltrates of lymphocytes and plasma cells with occasional heterophilic microabscesses [3, 4].

Diagnostic Approaches

Bacteriological Culture and Isolation

Traditional culture methods remain the gold standard for detecting e coli on raw chicken and Salmonella. E. coli is isolated on MacConkey agar, appearing as lactose-fermenting (pink) colonies, and confirmed by biochemical tests (indole positive, methyl red positive, Voges-Proskauer negative, citrate negative). Salmonella requires selective enrichment in tetrathionate or selenite broth followed by plating on selective agars (e.g. xylose lysine deoxycholate agar, brilliant green agar). Typical Salmonella colonies produce black centers (H2S production) on XLD agar. Serological confirmation using somatic (O) and flagellar (H) antisera is performed for serovar identification [3, 4].

Phenotypic Detection of Antimicrobial Resistance

Antimicrobial susceptibility testing by disk diffusion or broth microdilution is essential for surveillance. ESBL production is confirmed by the double-disk synergy test using cephalosporin and clavulanic acid disks. AmpC beta-lactamase production is detected using cloxacillin inhibition tests [1, 2, 3]. Resistance to third-generation cephalosporins and carbapenems in E. coli and Salmonella from poultry is of increasing concern.

Molecular Diagnostics

Polymerase chain reaction (PCR) assays targeting species-specific genes (e.g. uidA for E. coli, invA for Salmonella) provide rapid detection. Quantitative real-time PCR enables enumeration of bacterial load on raw chicken samples. Virulence genotyping of APEC isolates targets genes such as fimC, papC, iucD, tsh, and iss. For Salmonella, serovar-specific PCR assays targeting SPI regions or O-antigen gene clusters differentiate common serovars. Whole-genome sequencing, increasingly used in research and surveillance, provides high-resolution genotyping, AMR gene profiling, and phylogenetic analysis [1, 2, 3, 4].

Differential Diagnosis

Clinical differentiation between chicken e coli or salmonella infection based on signs alone is unreliable. Laboratory confirmation is mandatory. APEC colibacillosis must be differentiated from Pasteurella multocida (fowl cholera), Mycoplasma gallisepticum (chronic respiratory disease), and Gallibacterium anatis infection. Salmonella systemic disease is differentiated from Pasteurella multocida and Riemerella anatipestifer infections. Co-infections with immunosuppressive viruses (e.g. infectious bursal disease virus, chicken anemia virus) complicate the clinical picture.

Treatment and Antimicrobial Stewardship

Antimicrobial Therapy

Treatment of avian colibacillosis relies on antibiotics effective against Gram-negative bacteria. Historically used compounds include tetracyclines, sulfonamides, aminoglycosides, fluoroquinolones, and beta-lactams [1, 2]. However, the widespread emergence of multidrug-resistant (MDR) APEC strains, including ESBL and carbapenemase producers, has severely reduced therapeutic options. Culture and susceptibility testing are critical before administering therapy. For Salmonella, treatment is generally discouraged in commercial poultry, as antibiotic pressure can select for resistance and does not eliminate the carrier state. In valuable breeding stock, fluoroquinolone or ceftiofur therapy may be attempted under veterinary oversight, with strict withdrawal periods for meat and eggs [3, 4].

Limitations of Antibiotic Therapy

Bacteriostatic antibiotics are often ineffective in acute septicemic disease, and delayed treatment leads to high mortality. The inability to eliminate Salmonella cecal carriage with antibiotics has led to a focus on preventive strategies. Phage therapy, bacteriocins, and probiotic formulations are under investigation as alternatives.

Control and Food Safety Measures

On-Farm Biosecurity

Control of both e coli on raw chicken and Salmonella begins at the farm level. Strict biosecurity protocols including all-in/all-out flock management, rodent and insect control, disinfection of footwear and equipment, and chlorination of drinking water are essential. Litter management to reduce moisture and ammonia levels minimizes respiratory epithelial damage that predisposes to APEC infection [1, 2]. Vaccination against Salmonella using live attenuated or killed bacterins is practiced in some breeding flocks and layers to reduce shedding and egg contamination. No broadly effective commercial APEC vaccine is currently available, though autogenous bacterins are used in problem flocks.

Processing Plant Interventions

During slaughter, interventions to reduce e coli on raw chicken include carcass washing with organic acids (e.g. lactic acid, peroxyacetic acid), chlorinated water immersion, and air chilling systems that reduce surface moisture. Steam pasteurization and UV light treatment have also been evaluated. Salmonella reduction is targeted by similar methods, with additional emphasis on prevention of fecal spillage during evisceration and transfer of carcasses between conveyors [3, 4].

Consumer Handling and Cooking

Proper cooking of raw chicken to an internal temperature of 74 degrees Celsius (165 degrees Fahrenheit) inactivates both E. coli and Salmonella. Cross-contamination of ready-to-eat foods via cutting boards, utensils, and hands is the primary risk factor for foodborne illness from raw poultry. Refrigeration at 4 degrees Celsius slows bacterial growth but does not eliminate viable organisms.

Decision Workflow for Differential Diagnosis and Control

flowchart TD
    A["Clinical signs in flock: respiratory or enteric distress, mortality"], > B["Necropsy and gross pathology"]
    B, > C{"Fibrinous polyserositis\n(pericarditis, perihepatitis, airsacculitis)?"}
    C, Yes, > D["Suspect APEC colibacillosis"]
    D, > E["Culture liver, air sac, pericardium\non MacConkey agar; isolate E. coli"]
    E, > F["Confirm virulence genes by PCR\n(fimC, papC, iucD, iss)"]
    F, > G["Antimicrobial susceptibility testing"]
    G, > H["Therapeutic antibiotic selection\nunder veterinary guidance"]
    C, No, > I{"Systemic disease with liver/spleen enlargement,\ncecal cores, or high mortality?"}
    I, Yes, > J["Suspect host-restricted Salmonella\n(S. Gallinarum, S. Pullorum)"]
    J, > K["Culture liver, spleen, cecal tonsils\non selective media (XLD, BG)"]
    K, > L["Serovar identification\nby antiserum agglutination or PCR"]
    L, > M["Quarantine, depopulation if necessary"]
    I, No, > N{"Asymptomatic cecal carriage\nin adult birds?"}
    N, Yes, > O["Suspect paratyphoid Salmonella\n(S. Enteritidis, S. Typhimurium)"]
    O, > P["Cecal culture or PCR\nfor prevalence monitoring"]
    P, > Q["Biosecurity reinforcement,\nvaccination of breeders"]

Antimicrobial Resistance Considerations

The dissemination of ESBL and AmpC genes between E. coli and Salmonella in poultry environments presents a significant veterinary and food safety challenge. Plasmid-mediated resistance genes (e.g. blaCMY-2, blaCTX-M-15) are transferable between enteric bacteria in the avian gut. The detection of carbapenemase-producing Enterobacteriaceae from retail food sources, including vegetables, indicates the expanding resistance reservoir [1, 2, 3]. Surveillance of chicken e coli or salmonella for AMR markers is critical for guiding veterinary treatment decisions and informing regulatory policies on antibiotic use in poultry production.

Conclusions

E. coli and Salmonella represent two major bacterial hazards associated with raw chicken, each with distinct pathogenic mechanisms, clinical presentations, and diagnostic requirements. APEC causes fibrinous polyserositis in poultry, while Salmonella serovars range from highly virulent, systemic agents to subclinical colonizers that nonetheless pose food safety risks. The control of e coli on raw chicken and chicken e coli or salmonella contamination requires integrated strategies spanning on-farm biosecurity, processing interventions, and consumer education. The continued emergence of antimicrobial resistance underscores the need for robust molecular surveillance and prudent antibiotic use in veterinary practice.

References

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[2] Aiddi A, Zerdani I, Khazaz A, et al. Global High-Risk Escherichia coli Clones in Moroccan Aquatic Settings: Genomic Evidence of Environmental Dissemination. Int J Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42327605/

[3] Akinola OT, Oluranti OO, Elutade OO, et al. Phenotypic detection of ESBL, AmpC, and carbapenemase-producing Enterobacteriaceae from retail fresh vegetables in Iwo, Nigeria. BMC Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42323570/

[4] Raineri S, Gazzola A, Dilio G, et al. Occurrence of third-generation cephalosporin-resistant Escherichia coli in European hedgehogs (Erinaceus europaeus) from a wildlife rescue centre in Lombardy, Northern Italy. Vet Res Commun. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42322447/ *** 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.