Detection and antibiotic resistance of Salmonella isolates from selected poultry farms in Dar es Salaam, Tanzania.

Abstract

Introduction. Salmonella is one of the most prevalent foodborne bacteria, posing a significant global health concern and responsible for ~155,000 deaths and 93.8 million human foodborne illnesses annually. The rampant use of antibiotic agents to combat salmonellosis in poultry has contributed to the emergence of resistance against commonly used antibiotics. Methodology. The cross-sectional study was conducted between January and June 2023 across three districts in the Dar es Salaam region, Tanzania. Salmonella isolates were detected and confirmed by using standard microbiological conventional methods and molecular methods such as PCR and genomic sequencing. PCR was used for detecting the presence of the invA gene, and partial DNA sequencing was performed to identify species and their close relatedness. A Kirby-Bauer disc diffusion method was employed to evaluate Salmonella sensitivity to seven different commonly used antibiotics, namely ampicillin, azithromycin, chloramphenicol, ciprofloxacin, gentamycin, trimethoprim/sulfamethoxazole and tetracycline. Salmonella strain with reference number ATCC 8739 was used as a control. Results. The overall Salmonella isolates from faecal droppings were 6.04% (n=796). Of the PCR-confirmed isolates, 64.3% (n=28) were resistant to more than two classes of antibiotics and hence considered multidrug resistant. The highest resistance was observed with ampicillin (92.9%, n=26), followed by tetracycline (71.43%, n=20), ciprofloxacin (42.9%, n=12), sulphonamide (42.85%, n=12), gentamicin (35.7%, n=10) and azithromycin (28.9%, n=8). All the isolates were susceptible to chloramphenicol (100%, n=28). Twenty-eight isolates were sent for sequencing, out of which 16 sequences (OR021717-OR021739) met the criteria for phylogenetic analysis. All 16 sequences had a per cent identity to EU348369 strains Senftenberg isolated from China, OL581594 Salmonella Newport isolated from China and EU348368 Salmonella Pullorum isolates from China. Other sequences diverged more distantly; these are Salmonella Abony with accession number CP007541 and Salmonella Kentucky with accession number OL581592. The tree also included an outgroup species, Salmonella bongori, which was downloaded from GenBank with accession numbers NC015761 and NC021870 S. bongori. Conclusion. The high level of antibiotic resistance found in this study could be due to the misuse of antibiotics in poultry management, and/or, probably, there were circulating resistant Salmonella strains in the environment. To reverse the trend observed, immediate interventions such as advocating for the prudent use of antibiotics in poultry production systems by strengthening extension services to poultry farmers and the use of a farmer field school model to improve poultry management through improved farm biosecurity are required.