Chicken Bacteria in Bangladesh: Epidemiology, Antimicrobial Resistance, and Food Safety Implications

1. Introduction

The poultry industry in Bangladesh has expanded rapidly over the past two decades, providing a major source of animal protein and contributing significantly to rural livelihoods [33]. However, the intensive production systems, often characterized by high stocking densities, suboptimal biosecurity, and widespread non-therapeutic antimicrobial use, have created ecological conditions favorable for the emergence and dissemination of bacterial pathogens [1, 2]. The resulting prevalence of multidrug-resistant (MDR) and virulent bacteria in chickens, chicken meat, and poultry processing environments poses serious threats to flock health, food safety, and the sustainability of production [3, 31].

Bacteria of primary concern in the Bangladeshi poultry sector include avian pathogenic Escherichia coli (APEC), Salmonella enterica (both typhoidal and non-typhoidal serovars), thermophilic Campylobacter species (C. jejuni and C. coli), Klebsiella pneumoniae, Enterococcus faecium, and emerging opportunistic pathogens such as Providencia stuartii [4, 5, 6, 7, 8]. These organisms cause a spectrum of clinical diseases ranging from colibacillosis (airsacculitis, peritonitis, pericarditis) and salmonellosis to campylobacteriosis and necrotic enteritis, while also serving as reservoirs of antimicrobial resistance (AMR) determinants that can be transmitted through the food chain [9, 10, 11, 34]. This review synthesizes current evidence on the epidemiology, AMR profiles, and food safety implications of chicken-associated bacteria in Bangladesh, drawing exclusively on peer-reviewed studies conducted within the country.

2. Major Bacterial Pathogens in Bangladeshi Poultry

2.1 Escherichia coli

E. coli is the most frequently isolated bacterial pathogen from chickens in Bangladesh, with both commensal and pathogenic strains circulating in broiler, layer, and Sonali (crossbred) flocks [12, 13, 1]. Avian pathogenic E. coli (APEC) strains carry a suite of virulence-associated genes (e.g., fimH, papC, iucD, tsh, iss) and produce characteristic fibrinous lesions in the air sacs, pericardium, and peritoneum [5, 34]. Studies from Sylhet, Mymensingh, and Noakhali districts have reported APEC isolation rates ranging from 40% to 70% in clinically affected birds [12, 14, 7]. Colibacillosis outbreaks are common in young broilers, with morbidity influenced by management factors such as litter quality, ventilation, and concurrent viral infections [33, 34].

A major concern is the high prevalence of extended-spectrum beta-lactamase (ESBL)-producing E. coli in both healthy and diseased chickens. The blaCTX-M-15 gene is the dominant ESBL determinant, found in isolates from poultry farms, retail meat, and live bird market (LBM) sewage [15, 13, 16]. In a study of Sonali chicken meat, Ali et al. reported that 68% of E. coli isolates were MDR, with 41% exhibiting an ESBL phenotype confirmed by double-disk synergy tests and PCR amplification of blaCTX-M, blaTEM, and blaSHV genes [13]. The emergence of colistin resistance mediated by the mcr-1 gene in avian E. coli isolates from Bangladesh has also been documented, with prevalence rates ranging from 8% to 18% in cloacal swabs and meat samples [17, 28]. Carbapenem-resistant E. coli has been detected in retail chicken meat and LBM sewage, often co-harboring ESBL and mcr genes, indicating co-selection pressure from multiple antimicrobial classes [15, 18].

2.2 Salmonella enterica

Salmonella is a major cause of foodborne illness globally, and in Bangladesh, multiple serovars circulate in poultry production chains. S. Gallinarum-Pullorum (non-motile, typhoidal) causes fowl typhoid and pullorum disease, characterized by hepatomegaly, splenomegaly, and white nodular lesions in visceral organs [19, 10]. Non-typhoidal serovars such as S. Enteritidis, S. Typhimurium, and S. Infantis are frequently isolated from broiler chickens, hatchery environments, and processing facilities [20, 25, 27]. In a study of frozen chicken meat from Dhaka, 32% of samples were positive for Salmonella, with S. Enteritidis being the predominant serovar [29].

Antimicrobial resistance in Salmonella isolates is extensive. A nationwide molecular study reported that 84% of S. Gallinarum-Pullorum isolates were MDR, with high resistance to tetracyclines, sulfonamides, and fluoroquinolones [19]. The mcr-1 gene has also been detected in Salmonella from chickens, raising concerns about the transferability of colistin resistance to this foodborne pathogen [28]. ESBL-producing Salmonella carrying blaCTX-M-15 have been recovered from frozen chicken meat and LBM samples [15, 29]. Salmonella harboring multiple virulence genes, including invA, spvC, and sopB, are commonly found in poultry processing environments, indicating enhanced pathogenic potential [20, 26].

2.3 Campylobacter

Campylobacter jejuni and C. coli are the leading bacterial causes of human gastroenteritis worldwide. In Bangladesh, these organisms are highly prevalent in broiler flocks, with farm-level prevalence exceeding 60% in Mymensingh and Gazipur districts [11, 21, 30]. Campylobacter colonization is typically asymptomatic in chickens but leads to contamination of carcasses during slaughter and processing [31]. Molecular typing of C. jejuni from Bangladeshi poultry reveals a genetically diverse population, with sequence types clustering with isolates from South and Southeast Asian countries [9].

Resistance to ciprofloxacin is nearly universal in Campylobacter isolates from chickens, approaching 95% in some studies [11, 21]. Tetracycline resistance is also common (60% to 80%), while erythromycin resistance varies by region (10% to 30%) [31, 11]. The high prevalence of fluoroquinolone resistance is likely driven by the widespread use of enrofloxacin and ciprofloxacin in poultry feeds and water [31, 32].

2.4 Other Bacterial Pathogens

Klebsiella pneumoniae is an emerging opportunistic pathogen in poultry. Whole-genome sequencing (WGS) of K. pneumoniae from broiler chickens in Noakhali has revealed MDR profiles with resistance to beta-lactams, aminoglycosides, and fluoroquinolones, and the presence of blaNDM-1 in a small proportion of isolates [6, 7]. The potential for zoonotic transmission from poultry to humans has been assessed in a pilot study, confirming that some sequence types (STs) found in chickens are shared with clinical human isolates [6].

Providencia stuartii, a Gram-negative bacterium rarely reported in poultry, has been isolated from broiler chickens in Noakhali. WGS analysis identified multiple AMR genes (including blaOXA, aac(6')-Ib, tet(A)) and virulence factors, highlighting the species as a potential emerging food safety hazard [4].

Enterococcus faecium is a commensal of the chicken gut but can act as a reservoir of vancomycin resistance genes. In Bangladesh, MDR E. faecium has been isolated from healthy broiler chickens, with resistance to tetracycline, erythromycin, and ciprofloxacin, though vancomycin resistance appears rare to date [8].

3. Antimicrobial Resistance Profiles and Mechanisms

3.1 Phenotypic Resistance Patterns

Multiple studies have documented alarming levels of resistance to critically important antibiotics in bacteria isolated from Bangladeshi poultry (Table 1) [19, 15, 12, 13, 11, 25, 29]. Resistance to tetracyclines, sulfonamides, and aminopenicillins is ubiquitous, while fluoroquinolone resistance is high in Campylobacter and Salmonella. Colistin resistance, mediated by mcr-1, has been identified in both E. coli and Salmonella from cloacal swabs and meat, with prevalence reaching 18% in some surveys [17, 28]. Carbapenem resistance, although still sporadic, has been documented in E. coli and K. pneumoniae from retail chicken meat and LBM sewage, usually co-occurring with ESBL production [15, 18, 6].

Table 1. Summary of antimicrobial resistance prevalence in key chicken-associated bacteria in Bangladesh (compiled from multiple studies).

3.2 Molecular Mechanisms

Genotypic characterization has confirmed the presence of multiple resistance gene families in Bangladeshi poultry isolates. The blaCTX-M-15 gene is the predominant ESBL determinant among E. coli and Salmonella, likely of plasmid-borne origin and associated with mobile genetic elements [13, 16, 2]. The mcr-1 gene, located on conjugative plasmids, has been detected in diverse E. coli sequence types (e.g., ST10, ST48) and is often co-localized with other resistance genes, facilitating co-selection [18, 17]. In Campylobacter, the Thr-86-Ile mutation in the gyrA gene is the primary mechanism conferring high-level ciprofloxacin resistance [21]. WGS studies of K. pneumoniae, Providencia stuartii, and E. coli have additionally identified genes conferring resistance to aminoglycosides (aac(6')-Ib, aph(3')-Ia), chloramphenicol (cat), and trimethoprim (dfrA) [4, 18, 6].

4. Epidemiological Risk Factors and Transmission Dynamics

4.1 Farm-Level Risk Factors

The epidemiology of bacterial infections and AMR in Bangladeshi poultry is shaped by farm management practices, antimicrobial use, and biosecurity measures. Large-scale commercial farms using antibiotics extensively for growth promotion and prophylaxis have been associated with higher prevalence of AMR E. coli and Salmonella [1, 2]. Conversely, smallholder and backyard systems, while often less intensive, may exhibit lower biosecurity, allowing environmental and rodent-mediated transmission [30, 33]. Poor litter management, high stocking density, and inadequate ventilation are common predisposing factors for colibacillosis and other respiratory-enteric infections [34].

4.2 Hatchery and Market Environments

Hatcheries play a critical role in the vertical transmission of Salmonella. Zamil et al. isolated MDR motile Salmonella from hatchery environmental samples, including fluff, eggshell fragments, and tray swabs, indicating contamination during incubation and hatching [26]. Once established in the hatchery, Salmonella can persist and infect day-old chicks, leading to onward dissemination in grow-out farms. At the market level, live bird markets (LBMs) and wet markets serve as hotspots for bacterial amplification and exchange. Sewage and water samples from LBMs in Dhaka have shown high levels of ESBL-producing E. coli and Salmonella [15]. Sink drains in wholesale chicken markets also harbor MDR foodborne bacteria, contributing to cross-contamination of carcasses [3].

4.3 Sewage and Environmental Contamination

Farm sewage and drainage systems act as reservoirs of resistant bacteria and resistance genes. Mandal et al. found that E. coli isolates from farm sewage displayed resistance profiles and virulence gene carriage similar to those from broiler chickens and farm workers, suggesting bidirectional flow of bacteria between poultry and humans [2]. Similarly, Salmonella isolates from sewage were genetically related to those from chickens, reinforcing the role of sewage as a conduit for AMR dissemination [25].

4.4 Antimicrobial Residues

The presence of antimicrobial residues in chicken meat and tissues is an indirect but important risk factor for AMR. Ferdous et al. detected residues of tetracyclines, fluoroquinolones, and sulfonamides in 45% of chicken samples from Chittagong markets [32]. Subinhibitory concentrations of these residues can exert selective pressure on bacterial populations in the food chain, promoting the survival and spread of resistant clones.

5. Food Safety Implications

5.1 Contamination of Retail Chicken Meat

Retail chicken meat in Bangladesh is frequently contaminated with MDR and virulent bacteria. E. coli and Salmonella are common contaminants, with prevalence rates ranging from 30% to 70% depending on the sampling location and market type [22, 23, 3, 29]. In Dhaka, 44% of retail chicken meat samples harbored MDR E. coli producing ESBLs [23]. Zilon et al. reported that 72% of E. coli from retail meat were enteropathogenic (EPEC) based on eae gene detection [22]. Campylobacter contamination of chicken carcasses is also common; a study in Mymensingh found that 58% of meat samples were positive for C. jejuni or C. coli [21]. Cross-contamination during slaughter, evisceration, and handling in wet markets is a major source of carcass contamination [20].

5.2 Eggs and Processing Environments

Salmonella Enteritidis can contaminate eggs through transovarian transmission, particularly in layer flocks [24]. Khatun et al. demonstrated that eggs from naturally infected hens were positive for S. Enteritidis, posing a direct risk to consumers [24]. Processing environments, including floors, drainage, and equipment in LBMs, act as persistent sources of Salmonella and E. coli, requiring rigorous cleaning and disinfection protocols [20].

5.3 Public Health Consequences

Consumers are exposed to MDR bacteria through handling and consumption of undercooked chicken, eggs, or cross-contaminated foods. The potential for transmission of resistance determinants from poultry to human gut microbiota via the food chain is a recognized One Health concern [19, 16]. Several of the E. coli and Salmonella sequence types found in Bangladeshi poultry (e.g., ST10, ST34) are also prevalent in human clinical infections [18, 6]. The presence of mcr-1 and blaNDM-1 in poultry isolates underscores the urgency of curbing AMR in the livestock sector.

6. Diagnostic and Surveillance Approaches

6.1 Conventional and Molecular Detection

Isolation and identification of bacterial pathogens from poultry samples follow standard bacteriological protocols: culture on selective media (MacConkey, XLD, Campylobacter selective agar), followed by biochemical confirmation and/or serotyping [22, 21, 27]. For E. coli, APEC pathotype is determined by PCR targeting virulence genes such as fimH, papC, iucD, tsh, and iss [5, 34]. Salmonella serotyping uses somatic (O) and flagellar (H) antisera [10, 20], while Campylobacter species identification is achieved by PCR targeting 16S rRNA or mapA/ceuE genes [9, 21].

6.2 Antimicrobial Susceptibility Testing

Phenotypic susceptibility is performed using disk diffusion or broth microdilution, following Clinical and Laboratory Standards Institute (CLSI) guidelines [15, 13]. ESBL production is confirmed by the double-disk synergy test using clavulanic acid [13]. MIC determination for colistin uses broth microdilution, with mcr-positive isolates confirmed by PCR [17, 28].

6.3 Whole-Genome Sequencing

WGS has become an increasingly important tool for characterizing AMR determinants, virulence genes, and phylogenetic relationships. Illumina short-read platforms have been used to generate draft genomes of E. coli, K. pneumoniae, and P. stuartii from Bangladeshi poultry [4, 18, 6]. Bioinformatic pipelines (e.g., ResFinder, VirulenceFinder, MLST) are employed to identify resistance genes, plasmid replicons, and sequence types. WGS-based epidemiological investigations have revealed clonal dissemination of MDR clones across farms and markets [18, 6].

6.4 Recommended Surveillance Workflow

A structured approach to surveillance of poultry-associated bacteria incorporating both phenotypic and genotypic methods is presented in Figure 1.

flowchart TD
    A["Sample Collection: Cloacal swabs, meat, sewage, hatchery fluff"] --> B["Selective Culture: MacConkey, XLD, CASA, BHI enrichment"]
    B --> C[Biochemical Identification & Serotyping]
    C --> D["AST: Disk diffusion / Broth microdilution"]
    D --> E{Phenotypic MDR or ESBL?}
    E -->|Yes| F["DNA extraction & PCR: Resistance genes, virulence genes, mcr-1"]
    E -->|No| G[Report and archive]
    F --> H[Whole-Genome Sequencing]
    H --> I["Bioinformatics: ResFinder, VirulenceFinder, MLST, PlasmidFinder"]
    I --> J[Phylogenetic analysis & Cluster detection]
    J --> K[Surveillance reporting & Intervention strategies]

Figure 1. Integrated surveillance workflow for chicken-associated bacteria in Bangladesh, combining culture-based methods, antimicrobial susceptibility testing (AST), PCR, and whole-genome sequencing.

7. Conclusion and Future Directions

The body of evidence from Bangladesh reveals a critical situation regarding bacterial pathogens in the poultry sector. E. coli, Salmonella, Campylobacter, and other MDR Gram-negative bacteria are widespread in chickens, their products, and associated environments. Resistance to critically important antimicrobials, including colistin, carbapenems, and third-generation cephalosporins, is emerging at alarming rates, driven by intensive farming practices, indiscriminate antibiotic use, and poor biosecurity [1, 31, 32]. Food safety risks from contaminated chicken meat and eggs are high, with potential for zoonotic transmission of resistant and virulent bacteria to humans [19, 16].

Future interventions should prioritize the reduction of antimicrobial use in poultry production through improved husbandry, vaccination, and alternatives such as probiotics and bacteriophages (as discussed in Necrotic Enteritis in Broiler Chickens: Clostridium perfringens Virulence Factors, Gut Microbiome, and Probiotic Control Strategies). Enhanced surveillance integrating phenotypic and genomic approaches is needed to track the emergence and spread of resistance. Implementation of strict hygiene measures at farm slaughter and market levels, along with consumer education on proper handling and cooking of poultry, will be essential to mitigate food safety risks. The One Health framework, linking veterinary, environmental, and public health sectors, must guide policy and practice to preserve the efficacy of antimicrobials for both animal and human health.

References

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Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.