Abstract
Background:
Salmonella has become one of the hazards prevalent foodborne pathogens causing different diseases in chickens. However, Salmonella Typhimurium (ST), a non-host-specific serovar, is a major avian agent that causes severe disturbance in young chicken wellness.
Aim:
The occurrence of Salmonella in chickens and their antimicrobial resistance were explored in this study. Additionally, the immune response of one-day-old broiler chicks, against multi-drug resistant (MDR) Salmonella typhimurium infection, was also assessed at 4 and 24 h post-infection in the cecum and spleen, representing their mucosal and systemic immune responses, respectively.
Methods:
A total of 375 samples from 130 diseased and apparently healthy broiler and layer chickens were randomly collected for Salmonella isolation, identification, and resistance profile evaluation, from farms and different clinical laboratories. The immune response of one-day-old broiler chicks, Ross 308, against in-vivo ST infection was ascertained through the evaluation of heterophile phagocytosis and mRNA expression of cytokines, immunoglobulin A and other immune-regulating genes in the cecum and spleen. Twenty-four, one-day-old non-vaccinated broiler chicks were used and divided into two groups. The chicks in the infected group were orally inoculated with 0.5 mL of 2 × 108 colony forming units (CFU)/mL of MDR ST suspension, while those in the control group were taken nutrient broth.
Results:
Seven out of 130 (5.38%) examined chickens were positive for Salmonella. All isolates (100%) were resistant to amoxicillin-clavulanic acid, cefazolin, cefoxitin, ciprofloxacin, nalidixic acid, tetracycline, fosfomycin, and colistin with multiple antimicrobial resistances (MAR) index range of 0.72-0.83, where none of them was resistant to meropenem. The results of immune response revealed that chicks infected with ST showed significantly different phagocytosis percentages and index values compared to controls. According to the real-time quantitative polymerase chain reaction (RT-qPCR) results, the transcription of IL-8, iNOS, IL-18, IgA, and IFN-γ for chicks infected by ST showed a significantly increased trend (P < 0.01) with increasing chicken age and was higher in the cecum than spleen compared to controls (P < 0.05) during 24 h after infection.
Conclusion:
The findings indicated a strong mucosal immune response in the chicks after the ST challenge, which reflects humoral and cellular responses. Our insight recommended the occurrence of a natural immune response stimulator at one day age to face the infection, and this can prevent the resistance transfer, with efficient control measures.

Key words: Salmonella typhimurium, Cytokines, Challenge, Phagocytosis, Real-time PCR