Experimental infection of <i>Enterococcus faecalis</i> in red tilapia (<i>Oreochromis</i> hybrid) revealed low pathogenicity to cause streptococcosis

Abstract

Background: Streptococcosis, as a bacterial disease with broad tropism in fish and one of the causes of septicemia. Enterococcus faecalis is one of the causative agents of streptococcosis that can be isolated in tilapia.
Aim: This study was undertaken to complete the reporting gap on the pathogenicity profile and clinical symptoms of E. faecalis bacterial infection in red tilapia (Oreochromis hybrid). The study is expected to provide enriching information regarding recognizable clinical signs in the field that can lead to the diagnosis of streptococcosis caused by E. faecalis, especially in the Indonesian aquaculture environment.
Methods: The method used in this artificial infection study using red tilapia, which were divided into two types of route groups infection, namely intraperitoneal (IP) and peroral (PO) with bacterial concentrations given for each route of infection to be 2.1 × 108 CFU ml⁻¹; 2.1 × 10⁷ CFU ml⁻¹; and 2.1 × 10⁶ CFU ml⁻¹. One group was given brain heart infusion broth media sterile as a non-infectious control. Clinical symptoms, changes in swimming habits and consuming feed, external and internal organ lesion, and leukocytes profile changes were observed during the observation period along 14 days to evaluate the infectious effect of each treated fish group. The lethal dose 50 (LD₅₀) was estimated with the Spearman–Kärber method. The evaluation of the leukocyte profile was performed to find leukocytosis as the clinical sign of infection.
Results: The results showed variations in clinical symptoms inflicted on fish through death or the moribund stage. The highest mortality occurred in the treatment group of 2.1 × 108 CFU ml−1 with the PO route. The bacterial concentration of 2.1 × 10⁷ CFU ml⁻¹ given either as PO or IP can cause mild infection symptoms but did not cause mortality. The LD₅₀ of the PO and IP route was obtained at 1.99 × 10⁸ CFU ml⁻¹ and 0.79 × 108 CFU ml⁻¹, respectively. The total leukocytes in the infected fish group increased significantly (p < 0.05) by twofold when compared with the non-infectious group. The bacteria’s discovery on the blood smear examination was taken from fresh dead fish or moribund fish in the treatment group of 2.1 × 10⁸ CFU ml⁻¹, for both PO and IP.
Conclusion: Enterococcus faecalis with low pathogenicity can lead to septicemia, characterized by a total increase in leukocytes, bacteria’s discovery on the blood smear examination, and various clinical symptoms systemically found in the treated fish.