Overview
Nile tilapia (Oreochromis niloticus) is a cornerstone of freshwater aquaculture worldwide thanks to its fast growth and adaptability. In Ethiopia, rapid expansion of tilapia farming in semi‑intensive and intensive systems has brought attention to fish health and food safety. This study investigates the presence, phenotypic traits, molecular detection, and antibiotic susceptibility of Pseudomonas spp. isolated from Nile tilapia across three ponds in Ethiopia, highlighting potential risks to aquaculture productivity and public health.
Why Pseudomonas spp. matter in tilapia farming
Pseudomonas species are ubiquitous Gram-negative bacteria that can cause disease in fish and pose zoonotic risks. P. aeruginosa, P. putida, and P. fluorescens have been linked to morbidity and mortality in aquaculture, complicating treatment due to rising antimicrobial resistance. In the Ethiopian context, monitoring these pathogens helps safeguard fish health, farm productivity, and consumer safety amid expanding aquaculture projects and expanding water resources.
Study design and methods
From December 2022 to November 2023, researchers collected 240 Nile tilapia across three study sites: Sebeta, Hawassa (CARE), and Batu. Fishes with external lesions or clinical signs were selected for sampling. Internal organs (muscle, liver, spleen, kidney) were aseptically processed and enriched for bacterial isolation. Pseudomonas spp. were presumptively identified through biochemical tests, confirmed by MALDI-TOF, and further characterized using phenotypic assays and molecular methods.
Phenotypic characterization
Isolates displayed classic Pseudomonas traits: Gram-negative, oxidase- and catalase-positive, with green-blue pigmentation on selective media or creamy-white colonies on nutrient agar. Biochemical tests showed motility, citrate utilization, and mixed sugar metabolism with variable MR-VP results. Hemolysis was assessed on blood agar, revealing predominantly beta-hemolysis, a marker of virulence potential in several isolates.
Molecular detection
DNA was extracted from pure cultures, and conventional PCR targeted a 618 bp fragment of the 16S rRNA gene using primers designed to detect Pseudomonas spp. PCR amplification confirmed the presence of Pseudomonas spp. in the isolates, with a high proportion of samples testing positive. Specifically, among PCR-positive samples, 8 were identified as P. aeruginosa, 28 as P. putida, and 39 as P. fluorescens, underscoring the dominance of P. fluorescens in this setting. A notable finding was the detection of an adhesin-associated gene in a majority of Pseudomonas isolates, suggesting enhanced binding and tissue colonization capabilities and potential as a vaccine target in future work.
Organ distribution and tissue tropism
Molecular and phenotypic data revealed tissue-specific prevalence: P. putida was most frequent in muscle tissue, whereas P. fluorescens predominated in kidney samples. P. aeruginosa tended to be more common in spleen tissue. These patterns reflect pathogen-host interactions that vary by organ and environment, and they help explain observed clinical signs such as fin erosion, hemorrhages, and abdominal distention.
Antibiogram and antimicrobial resistance
Antibiotic susceptibility testing used the Kirby‑Bauer disc diffusion method against ten antibiotics, including amoxicillin-clavulanate, ampicillin, ceftriaxone, ciprofloxacin, tetracycline, and gentamicin. Findings showed high resistance to amoxicillin-clavulanate and ampicillin across isolates, with better activity observed for ceftriaxone, ciprofloxacin, streptomycin, and tetracycline. The observed multidrug resistance index (MARI) remained low (0.018), suggesting relatively limited multi-antibiotic resistance within the dataset, though vigilance is still warranted given broader AMR trends in aquaculture worldwide.
Implications for Ethiopian aquaculture
The study confirms that Pseudomonas spp. are present in tilapia farms in Ethiopia, with P. fluorescens emerging as a dominant pathogen in pond systems. The combination of phenotypic virulence traits, molecular confirmation, and antibiotic susceptibility profiles provides a practical basis for targeted surveillance, biosecurity improvements, and prudent antimicrobial use. As Ethiopian aquaculture continues to scale up, integrating routine diagnostic screening for Pseudomonas infections will help maintain productivity, protect public health, and support sustainable fish production.
Conclusions
Comprehensive characterization of Pseudomonas spp. in Nile tilapia from Ethiopian ponds demonstrates a clear presence of P. aeruginosa, P. putida, and P. fluorescens, with organ-specific patterns and notable yet manageable antibiotic resistance profiles. The combination of phenotypic, molecular, and antibiogram analyses offers a robust framework for ongoing disease monitoring and informed decision-making in aquaculture management.
