Overview
Nile tilapia (O. niloticus) is a cornerstone of aquaculture in many countries, providing a reliable protein source. In Ethiopia, rapid growth of aquaculture has been accompanied by concerns about bacterial pathogens, particularly Pseudomonas spp., which can threaten yield and raise public health considerations due to antimicrobial resistance. A cross‑sectional study conducted from December 2022 to November 2023 across three Ethiopian aquaculture sites—Sebeta, Hawassa, and Batu—assessed the presence, characteristics, and antimicrobial susceptibility of Pseudomonas species isolated from organ samples of diseased Nile tilapia.
Study design and methods
Researchers collected 240 Nile tilapia (80 from each site) with suggestive clinical signs, then performed postmortem examinations. Tissue samples (muscle, liver, spleen, kidney) were processed for culture and identification. Initial bacterial isolation used enrichment in TSB followed by plating on Pseudomonas Agar Base with cetriNix, and colonies were subjected to biochemical tests, Gram staining, and oxidase/catalase reactions. Confirmatory species identification employed MALDI‑TOF analysis. Phenotypic traits, including hemolysis on 5% sheep blood, were recorded. For molecular detection, genomic DNA was extracted and conventional PCR amplified Pseudomonas 16S rDNA targeting species‑level regions, with primers designed from Spilker et al. (2004).
Key findings: prevalence and species distribution
From 81 presumptive Pseudomonas isolates, three species were confirmed: P. aeruginosa, P. putida, and P. fluorescens. On biochemical grounds, isolates matched typical Pseudomonas profiles (green‑blue pigment, oxidase positive, Gram‑negative rods; alpha or beta hemolysis observed on blood agar). Molecular detection revealed that 92.6% of isolates harbored the targeted adhesin virulence gene, underscoring pathogenic potential across species. The distribution varied by site: Ziway ponds showed the highest overall prevalence (44 isolates; 23.8%), followed by Hawassa (8; 11%) and Sebeta (29; 7.6%).
Tissue distribution and virulence indicators
Organ tropism differed among species. P. putida predominated in muscle (14 isolates, 50%), with substantial presence in kidney (8, 28.6%), liver (5, 17.9%), and spleen (1, 3.6%). P. fluorescens was most common in muscle (16, 35%), then kidney (11, 27.6%), spleen (9, 20%), and liver (3, 9.4%). P. aeruginosa was more frequent in the spleen (3, 35.5%), with the remainder spread across kidney and muscle. Hemolysis patterns indicated virulence: β‑hemolysis dominated (69 of 81 isolates, 85.2%), with the rest showing α hemolysis (12 isolates, 14.8%). These virulence traits align with the capacity of Pseudomonas spp. to cause tissue damage and disseminate in fish hosts.
Molecular confirmation and antibiotic resistance
Conventional PCR confirmed Pseudomonas spp. in the samples, using species‑level 16S rDNA targets. Among PCR‑positive samples, 8 (10.7%) were P. aeruginosa, 28 (37.6%) were P. putida, and 39 (52%) were P. fluorescens. The data indicate a predominance of P. fluorescens in this setting, consistent with several regional reports that identify Pseudomonas spp. as common fish pathogens. The antibiotic susceptibility profile showed high sensitivity to Ceftriaxone, Ciprofloxacin, Streptomycin, and Tetracycline, with Gentamicin also showing strong effectiveness (92.9% susceptible). All isolates were resistant to Amoxicillin‑clavulanate and Ampicillin; complete resistance to Amoxicillin and Penicillin was observed. The calculated Multiple Antibiotic Resistance (MAR) index was 0.018, suggesting a relatively low overall burden of multi‑drug resistance among the tested isolates in this study context.
Clinical and postmortem relevance
Clinical signs in infected tilapia included body depigmentation, corneal opacity, fin erosion, exophthalmia, abdominal distention, hemorrhages, and ulcers. Postmortem findings frequently revealed splenomegaly, gill congestion, enlarged kidneys, and hepatobiliary changes. These observations corroborate the role of Pseudomonas spp. as significant fish pathogens that can compromise production and raise zoonotic considerations in the handling and processing of fish products.
Implications for Ethiopian aquaculture and public health
This study confirms the presence and pathogenic potential of Pseudomonas spp. in Nile tilapia from Ethiopian ponds and highlights site‑specific differences in prevalence. The predominance of P. fluorescens and the presence of virulence genes emphasize the need for ongoing surveillance, improved biosecurity, and prudent antibiotic use to mitigate disease impact and curb antimicrobial resistance. The relatively low MAR index is encouraging, but continuous monitoring is essential to prevent the emergence and spread of resistance, particularly given expanding aquaculture and potential zoonotic transmission.
Conclusion
In Ethiopian aquaculture settings, Pseudomonas spp. pose a measurable threat to Nile tilapia health and productivity. Integrating phenotypic, molecular, and antibiogram data provides a robust framework for diagnosing, monitoring, and managing these infections, ultimately supporting sustainable growth of the sector and safeguarding public health.
