Tehran University of Medical Sciences Journal of Medical Bacteriology 2251-8649 14 1 2026 02 11 37 43 Mona Hamedi Department of immunopathology, Faculty of veterinary medicine, Islamic Azad University, Science and research branch, Tehran, iran Parimah BourBour Veterinary Medicine Faculty, SR.C., Islamic Azad University, Tehran, Iran. Alireza Mortazavinia Veterinary Medicine Faculty, SR.C., Islamic Azad University, Tehran, Iran. Ghazal Aftab Department of Small Animal Internal Medicine, SR.C., Islamic Azad University, Tehran, Iran. 2025 12 02 2025 12 25 Background:   The health of breeders is essential for the next generation of chicks. Bacterial agents, such as Escherichia coli, Salmonella enterica and Mycoplasma gallisepticum or Mycoplasma synoviae, are the leading causes of disease and mortality in breeder poultry. The widespread usage of antimicrobials in poultry, has raised concerns about the potential for spread of antimicrobial resistance. The present study aimed to identify bacterial infections and antibiotic resistance patterns in the case of breeder’s mortality. Methods:   Samples were collected from tissues and bone marrow of sixteen broiler breeders. The specimens were cultured onto blood agar, MacConkey agar and confirmatory biochemical media for the bacteriological examination. Sabouraud dextrose agar was used for fungal isolation. The specimens were cultured on Mueller-Hinton agar to assess their antibiotic resistance to gentamicin, ceftriaxone, tetracycline, penicillin, erythromycin, streptomycin, lincomycin, and amoxicillin Results:   The bacteria including Escherichia coli (93.8%), Staphylococcus (31.3%), Streptococcus (12.5%), Bacillus (12.5%) and Clostridium (6.3%) were isolated. Co-infection was observed in 50% of carcasses, most frequently involving E. coli and Staphylococcus. Isolates from 62.5% were resistant to all the antibiotics tested. Gentamicin was the most effective antibiotic. Conclusion:   Escherichia coli was the dominant isolate, and its detection in the bone marrow of 31.3% of cases confirms systemic dissemination. Consequently, enhanced biosecurity protocols are essential to prevent the spread of pathogens such as E. coli. Antimicrobial susceptibility testing revealed a multidrug-resistant pattern. Therefore, poultry farms should base antibiotic selection on specific laboratory results rather than relying on empirical broad-spectrum therapy. https://jmb.tums.ac.ir/index.php/jmb/article/view/640