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Isolation and Pathogenicity of Streptococcus iniae in Cultured Red Hybrid Tilapia in Malaysia
M Rahmatullah1, M Ariff1, M Kahieshesfandiari1
1a Faculty of Agriculture, Universiti Putra Malaysia , Serdang , Selangor 43400 , Malaysia.
Abstract:
This study describes the isolation and pathogenicity of Streptococcus iniae in cultured red hybrid tilapia (Nile Tilapia Oreochromis niloticus × Mozambique Tilapia O. mossambicus) in Malaysia. The isolated gram-positive S. iniae appeared punctiform, transparently white, catalase and oxidase negative and produced complete β-hemolysis on blood agar, while a PCR assay resulted in the amplification of the 16 S rRNA gene and lactate oxidase encoded genes. The isolate was sensitive to tetracycline, vancomycin, and bacitracin but was resistant to streptomycin, ampicillin, penicillin, and erythromycin. Pathogenicity trials conducted in local red hybrid tilapia (mean ± SE = 20.00 ± 0.45 g) showed 90.0, 96.7, and 100.0% mortality within 14 d postinfection following intraperitoneal exposure to 104, 106, and 108 CFU/mL of the pathogen, respectively. The clinical signs included erratic swimming, lethargy, and inappetance at 6 h postinfection, while mortality was recorded at less than 24 h postinfection in all infected groups. The LD50-336 h of S. iniae against the red hybrid tilapia was 102 CFU/mL. The post mortem examinations revealed congested livers, kidneys, and spleens of the infected fish. This is the first report of S. iniae experimental infection in cultured red hybrid tilapia in Malaysia. Received January 20, 2017; accepted July 16, 2017.
Insights
This study details the first report of Streptococcus iniae infection in Malaysian red hybrid tilapia. The pathogen caused high mortality rates and specific clinical signs, highlighting its pathogenicity in aquaculture.
Area of Science:
- Aquatic Animal Health
- Fish Pathology
- Microbiology
Background:
- Streptococcus iniae is a significant pathogen affecting various fish species globally.
- Tilapia aquaculture, particularly red hybrid tilapia, faces economic losses due to bacterial diseases.
- Understanding the pathogenicity of S. iniae is crucial for disease management in aquaculture.
Purpose of the Study:
- To isolate and identify Streptococcus iniae from diseased red hybrid tilapia in Malaysia.
- To characterize the microbiological and genetic features of the isolated S. iniae.
- To determine the pathogenicity and lethal dose (LD50) of S. iniae in red hybrid tilapia.
Main Methods:
- Bacterial isolation and identification using morphological, biochemical, and PCR assays (16S rRNA, lactate oxidase genes).
- Antimicrobial sensitivity testing against various antibiotics.
- Experimental infection trials via intraperitoneal injection to determine mortality rates and LD50.
- Clinical sign observation and post-mortem examination.
Main Results:
- Gram-positive S. iniae was isolated and identified, exhibiting specific growth and hemolytic characteristics.
- The isolate showed sensitivity to tetracycline, vancomycin, and bacitracin, but resistance to streptomycin, ampicillin, penicillin, and erythromycin.
- Pathogenicity trials resulted in high mortality (90-100%) at different bacterial concentrations, with an LD50 of 10^2 CFU/mL.
- Clinical signs included erratic swimming and lethargy; post-mortem revealed congested organs.
Conclusions:
- This study confirms Streptococcus iniae as a pathogenic agent in cultured red hybrid tilapia in Malaysia.
- The findings provide essential data on S. iniae virulence and antibiotic resistance patterns in the region.
- This research is the first report of experimental S. iniae infection in this specific tilapia hybrid in Malaysia, aiding future disease control strategies.
