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Objective To screen potential probiotics with antagonistic effects against Nocardia seriolae from the intestine of healthy largemouth bass (Micropterus salmoides). Methods Intestinal samples were collected from healthy largemouth bass, and strains were isolated and purified via serial dilution and spreading on TSA plates. The hole-punch method was adopted to select strains with antagonistic effects against N. seriolae as test strains for subsequent experiments. The species of the test strains were identified through morphological characterization, physiological and biochemical tests, 16S rRNA gene sequence alignment, and phylogenetic analysis. Additionally, the growth characteristics, adhesion ability, antibacterial activity of cell-free fermentation supernatants, and biosafety of the test strains were determined and analyzed. Results Among the 40 bacterial strains isolated from the intestinal samples of healthy largemouth bass, two strains (XXLC06 and XXLC08) with stable and significant antagonistic effects against N. seriolae were selected. XXLC06 was identified as Lysinibacillus macroides and XXLC08 as Lysinibacillus fusiformis. Both strains grew well at 25-37 ℃, salinities of 5‰-30‰, and pH 5.5-9.0, with survival rates higher than 53% at 0.3% and 0.5% bile salt concentrations. Auto-aggregation assays showed that the auto-aggregation rates of XXLC06 and XXLC08 after 8 h were 68.09% and 63.16%, respectively. In the three organic solvents, both XXLC06 and XXLC08 exhibited hydrophobicity rates exceeding 50%. After co-incubation with N. seriolae for 8 h, the co-aggregation rates of the two strains reached 54.62% and 52.44%, respectively. The diameters of the inhibition zones of cell-free fermentation supernatants of XXLC06 and XXLC08 against N. seriolae were (28.15±0.44) mm and (22.63±0.52) mm, respectively. Neither strain showed hemolytic activity, and both were sensitive to 23 tested antibacterial agents. Acute toxicity tests confirmed that they were non-pathogenic to largemouth bass. Conclusion Strains XXLC06 and XXLC08 exhibited favorable growth adaptability, adhesion capability, biosafety, and stable antagonistic activity in vitro . This study provides potential probiotic resources and experimental evidence for the control of N. seriolae.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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