Multidrug-Resistant Salmonella Found in Broiler Chickens at Tanzanian Live-Bird Markets

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The finding that all three Salmonella enterica isolates in this study came from broiler chickens, and specifically from a single market site, deserves careful interpretation in light of how modern broiler production is organized in Tanzania. Broilers are raised on short production cycles, often three to six weeks from hatch to slaughter, and are frequently moved through dense trading networks that concentrate birds from many farms at a single point of sale. This turnover creates repeated opportunities for Salmonella introduction and amplification, because each new batch of birds may arrive carrying different microbial loads, and the market environment itself can act as a persistent reservoir between trading days. Layers, by contrast, tend to remain on farms for longer periods under more stable management, and indigenous chickens are typically raised in small free-range flocks with lower stocking densities, both of which can reduce the intensity of pathogen circulation. The clustering of positive samples in broilers at Bigwa market is therefore consistent with the broader epidemiological understanding that commercial meat poultry systems, with their rapid turnover and high throughput, represent a particularly important interface where antimicrobial-resistant organisms can move between animals, environments, and people.

The laboratory workflow used in the study reflects a layered confirmation strategy that is worth appreciating for what each step contributes. Selective enrichment in Selenite F broth increases the relative abundance of Salmonella against a background of competing enteric organisms, which is essential when the target bacterium may be present at low levels in fecal material. Culture on Salmonella-Shigella agar and Xylose Lysine Deoxycholate agar exploits characteristic biochemical reactions, notably the production of hydrogen sulfide, which produces the distinctive black-centered colonies that alert microbiologists to presumptive Salmonella. However, culture and biochemistry alone cannot always distinguish closely related species or detect atypical strains, which is why the study’s use of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry represents a meaningful methodological strength. This technique generates a protein fingerprint from each isolate and matches it against a reference database, providing rapid and highly accurate species-level identification, as reflected in the 99.9% confidence scores reported for all three isolates. The subsequent polymerase chain reaction targeting the invA gene adds a third, independent line of confirmation. The invA gene encodes a component of the type III secretion system needle complex that Salmonella uses to invade host cells, and it is highly conserved across Salmonella enterica serovars, making it a widely accepted molecular marker for confirming the genus and detecting false-negative culture results. The consistent 796 base pair amplicon obtained from all three isolates indicates that the genetic target was intact and detectable, reinforcing confidence that the cultured organisms were genuinely Salmonella enterica rather than biochemically similar commensals.

The antimicrobial susceptibility pattern observed in these isolates tells a coherent story about the selective pressures operating in poultry production. Tetracyclines and sulfonamide-potentiator combinations such as trimethoprim-sulfamethoxazole are among the oldest and most inexpensive antimicrobial classes, and they have historically been used extensively in Tanzanian poultry farming for growth promotion, prophylaxis, and treatment. Prolonged and widespread exposure to these drugs creates strong selection for resistance determinants, which often accumulate on mobile genetic elements such as plasmids, integrons, and transposons. These elements can carry multiple resistance genes simultaneously, which helps explain why resistance to tetracycline and trimethoprim-sulfamethoxazole appeared in every isolate. The finding that two-thirds of isolates were resistant to ampicillin and azithromycin is also notable. Ampicillin resistance is common in enteric bacteria from livestock settings and often reflects the dissemination of beta-lactamase enzymes, while reduced susceptibility to azithromycin is of particular concern because macrolides are increasingly important for treating invasive salmonellosis in settings where fluoroquinolone resistance limits other options. The observation that one isolate was resistant to ciprofloxacin, a fluoroquinolone that is frequently used to treat severe human salmonellosis, adds to a growing body of evidence that fluoroquinolone resistance is emerging in food animal reservoirs in sub-Saharan Africa, mirroring patterns documented in Asia and elsewhere where fluoroquinolone use in poultry has been widespread.

Equally significant is what did not appear in the susceptibility results. All three isolates remained susceptible to ceftriaxone, a third-generation cephalosporin that is a mainstay for treating invasive non-typhoidal Salmonella infections in children and immunocompromised patients. Cephalosporin resistance in Salmonella is typically mediated by extended-spectrum beta-lactamase enzymes or AmpC beta-lactamases, genes that have spread alarmingly through Enterobacteriaceae in many regions. The absence of ceftriaxone resistance in this small sample suggests that these particular determinants had not yet established themselves in the sampled population, though the very small number of isolates means this observation carries limited statistical weight and should not be interpreted as reassurance about the broader situation. Similarly, the complete susceptibility profile of any single isolate cannot be assumed from the aggregate data, and the study does not report minimum inhibitory concentrations, which would provide more precise quantification of resistance levels than disk diffusion zones alone.

The multidrug resistance observed in all three isolates, defined as non-susceptibility to at least one agent in three or more antimicrobial classes, is a pattern that public health authorities consider especially problematic. Multidrug-resistant Salmonella infections are associated with longer illness duration, higher rates of hospitalization, increased treatment failure, and greater healthcare costs compared with infections caused by susceptible strains. When resistance spans multiple drug classes, clinicians face progressively narrower treatment options, and in severe invasive disease the therapeutic margin can shrink to a handful of last-line agents. The mechanisms underlying multidrug resistance in Salmonella frequently involve the acquisition of resistance genes on conjugative plasmids, which can transfer horizontally not only between Salmonella strains but also across bacterial species and genera within the gut environment. This means that resistant organisms or their resistance genes circulating in poultry can potentially colonize or transfer determinants to human commensal and pathogenic bacteria through the food chain, direct contact with live birds, or environmental contamination in markets and households.

The live-bird market setting itself warrants attention as an ecological niche for antimicrobial resistance. Markets where birds from multiple sources are held together, often with shared cages, water sources, and handling equipment, create conditions for cross-contamination and for resistant organisms to spread between flocks. Fecal matter accumulates in market environments, and Salmonella can survive for extended periods in organic material, particularly when temperatures are moderate and moisture is retained. Workers, traders, and consumers who handle live birds or carcasses in these settings face direct exposure risks, and the practice of transporting birds home for slaughter can extend contamination into domestic kitchens. The cloacal swabbing approach used in this study captures intestinal carriage, which is the most relevant indicator of both food contamination risk and environmental shedding, but it does not distinguish between transient passage of organisms and persistent colonization, a distinction that would require repeated sampling or molecular typing to resolve.

The overall isolation rate of 1% in this study is low compared with prevalence figures reported in some other studies of poultry in low- and middle-income countries, where Salmonella detection rates in cloacal samples or carcasses have ranged from several percent to more than twenty percent depending on the production system, sampling strategy, and diagnostic methods. Several factors could contribute to the low recovery rate observed here. The sensitivity of culture-based detection depends heavily on sample handling, the interval between collection and laboratory processing, the adequacy of enrichment conditions, and the number of colonies examined per plate. Sampling from live birds at markets rather than from flocks on farms introduces additional variability, because stress during transport and holding can alter gut microbiology. It is also possible that the true prevalence in the source flocks was genuinely low at the time of sampling, or that the three positive samples represent a cluster from a single contaminated batch rather than three independent introductions. The authors’ transparent acknowledgment that logistical constraints reduced the sample size below the calculated minimum of 385, widening the margin of error to approximately 5.6%, is an important caveat for interpreting the precision of the prevalence estimate.

From a surveillance perspective, the study illustrates both the value and the limitations of cross-sectional market-based sampling. Such studies provide snapshots of what is circulating at the point of sale, which is directly relevant to consumer exposure, but they cannot trace resistant organisms back to specific farms, hatcheries, feed mills, or transport chains. Achieving that traceability would require integrated sampling along the production continuum, combined with molecular methods such as whole-genome sequencing, which can identify specific resistance genes, plasmid types, and phylogenetic relationships between isolates from animals, food, and humans. Whole-genome sequencing has become the reference standard for antimicrobial resistance surveillance in high-income settings because it reveals not only which resistance determinants are present but also how they are related to one another across time and geography. Building such capacity in Tanzania and neighboring countries would substantially strengthen the evidence base for targeted interventions.

The policy implications suggested by these findings align with the One Health framework, which recognizes that human, animal, and environmental health are interconnected and that antimicrobial resistance must be addressed across all three domains. In the poultry sector specifically, interventions with demonstrated potential include enforcing regulations on antimicrobial sales so that veterinary oversight precedes drug use, phasing out the use of medically important antimicrobials for growth promotion, improving biosecurity and vaccination to reduce the underlying disease burden that drives antibiotic demand, and training farmers and market traders in hygiene practices that limit pathogen transmission. Tanzania’s participation in regional and global antimicrobial resistance initiatives provides a foundation for such measures, but implementation at the level of smallholder and commercial poultry producers remains uneven. The detection of multidrug-resistant Salmonella enterica in a market serving urban consumers underscores that the consequences of antimicrobial practices on farms do not remain on farms; they travel with the birds, the meat, and the resistant bacteria themselves into kitchens, hospitals, and communities, making sustained surveillance and stewardship in food animal systems a matter of direct public health protection.

Subject of Research: …

Article Title: Detection of AMR and identification of Salmonella enterica in indigenous, broiler and layers poultry in some major markets of Morogoro Municipal, Tanzania

Article References: Peter, B., Mzula, A., & Hoza, A. S. (2026). Detection of AMR and identification of Salmonella enterica in indigenous, broiler and layers poultry in some major markets of Morogoro Municipal, Tanzania. Frontiers in Bacteriology, 5, Article 1815759. https://doi.org/10.3389/fbrio.2026.1815759

Image Credits: AI Generated

DOI: 10.3389/fbrio.2026.1815759

Keywords: Detection, identification, Salmonella, enterica, indigenous, broiler, layers, poultry, some, major, markets, Morogoro