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Bacillus aquimaris  SH6 spores produce carotenoids that are beneficial to white-leg shrimp ( Litopenaeus vannamei ) health. However, the optimal dose and mechanisms behind these effects are not well understood. We investigated the fate of SH6 spores in the gut of  L .  vannamei . Shrimp were divided into six groups administrated with either feed only (negative control) or SH6 spores at 5 × 10 6  CFU/g pellet (high dose, SH6 spore-H group), 1 × 10 6  CFU/g pellet (medium dose, SH6 spore-M group), 2 × 10 5  CFU/g pellet (low dose, SH6 spore-L group), astaxanthin at 0.5 mg/g pellet (Carophyll group), or carotenoids from SH6 vegetative cells at 5 μg/g pellet (SH6 carotenoid group). The growth rate was highest in SH6 spore-H (3.38%/day), followed by SH6 spore-M (2.84%/day) and SH6 spore-L (2.25%/day), which was significantly higher than the control (1.45%/day), Carophyll (1.53%/day) or SH6 carotenoid (1.57%/day) groups. The astaxanthin levels (1.9–2.0 μg/g shrimp) and red-colour scores (21–22) in SH6 spore-H/M were higher than the control (astaxanthin: 1.2 μg/g shrimp; red score: 20) or SH6 spore-L, but lower than the Carophyll and SH6 carotenoids. Feeding with medium and high doses of SH6 spores after 28 days resulted in respective 1.3-2-fold increases in phenol oxidase activity and 8–9 fold increases in  Rho  mRNA expression compared to the control and low dose group. The live-counts of SH6 in the gut gradually increased during the 28-day feeding period with SH6 spores at different concentrations, starting from 4.1, 8.2, and 5.4 × 10 4  CFU/g gut at day 1 and reaching 5.3, 5.1, and 4.4 × 10 5  CFU/g gut in the SH6-H/M/L groups, respectively, at day 28. Gut microbiota became more diversified, resulting in a 2-8-fold increase in total bacterial live-counts compared to the controls. SH6 spore germination was detected by measuring the mRNA expression of a specific sequence coding for SH6 amylase at 4 h, reaching saturation at 24 h. Our results confirm that SH6 spores colonize and germinate in the gut to improve the microbial diversity and boost the immune system of shrimp, exhibiting beneficial effects at >1 × 10 6  CFU/g pellet.

Fate of carotenoid-producing Bacillus aquimaris SH6 colour spores in shrimp gut and their dose-dependent probiotic activities