Antimicrobial resistance profiling and molecular detection of the blaTEM-1 gene in Salmonella spp. isolated from layer birds in Bangladesh

Antimicrobial resistance in Salmonella spp. is a growing concern for poultry production and public health, particularly in settings where antimicrobial use is inadequately regulated. This study investigated the antimicrobial susceptibility profile and occurrence of the blaTEM-1 gene among Salmonella spp. isolated from layer birds in Dinajpur district, Bangladesh. A total of 87 blood samples were collected from 15 layer farms between July 2024 and June 2025. Samples were screened using the rapid serum plate agglutination test (RSPAT), and suspected isolates were characterized by cultural and biochemical methods and confirmed by polymerase chain reaction (PCR) targeting the invA gene. Of the 87 samples, 83 (95.4%) were serologically positive, while 53 isolates were confirmed as Salmonella spp. by the combined identification procedures. Antimicrobial susceptibility was assessed using the Kirby–Bauer disk diffusion method. All 53 isolates were resistant to amoxicillin, ampicillin, tetracycline, and ciprofloxacin, whereas susceptibility was highest to co-trimoxazole (100%), followed by levofloxacin (71.7%) and ceftriaxone (64.15%). PCR screening of the 53 aminopenicillin-resistant isolates detected the blaTEM-1 gene in 47 isolates (88.68%), with the expected amplicon size of 643 bp. These findings demonstrate substantial antimicrobial resistance among Salmonella isolates from layer farms and a high occurrence of blaTEM-1 among aminopenicillin-resistant isolates. Practically, the findings support veterinary-supervised antimicrobial use, routine susceptibility testing before treatment, strengthened farm biosecurity, and molecular surveillance of resistance determinants across poultry production chains to guide treatment decisions and reduce the potential transmission of resistant Salmonella to humans.