Nutritional improvements in intensive aquaculture production systems is necessary for the reduction of stress, maximum utilization of nutritional components, and expression of the genetic potential of fish. The objective of this study was to evaluate the hemato-immunological, and histological parameters and gut microbiota of Nile tilapia fed with the microalga Schizochytrium sp. Males of Nile tilapia were distributed among eight net cages (6 m 3 ), and fed for 105 days with two diets: control (CON), without Schizochytrium sp., and supplemented (SUP), with 1.2% Schizochytrium sp. in the diet. The final weight, mortality, hematocrit, total erythrocyte count (RBC), hemoglobin, hematimetric indices, white blood cell count (WBC), total protein, and serum lysozyme were measured. Alterations in intestinal morphology were evaluated. The gut microbiota was evaluated with next-generation sequencing. No significant differences (p>0.05) were found in the final weight and mortality between diets. Regarding the hematological parameters, a difference (p<0.05) was detected only in RBC, with there being lower values in the SUP, although this group also showed a tendency toward having an increased mean corpuscular hemoglobin level. There were no differences (p>0.05) in total protein and serum lysozyme concentrations or in WBCs between diets, except for lymphocytes, which presented lower values (p<0.05) in the SUP, suggesting immunomodulation by the polyunsaturated fatty acids present in the microalga. There was no difference (p>0.05) in the intestinal morphology between diets. Metagenomic data indicated greater richness (represented by the Chao index) and a higher abundance of the bacterial phylum Firmicutes in the gut microbiota of the tilapia fed with the SUP diet, demonstrating that the digestion and use of the components of the microalga could influence the microbial community. The results indicated that the microalga had modulatory PLOS ONE | https://doi.

This work evaluated the effects of dietary supplementation of A-Live (phytogenic) either individually or in combination with Aquaform (potassium diformate, acidifier) on juvenile Nile tilapia ( Oreochromis niloticus ) growth performance, innate immune parameters, gut microbiome, and resistance against Francisella noatunensis subsp. orientalis challenge. Each experimental group contained 140 fishes (34.3 ± 0.33) in two 150L tanks. The experimental design consisted of five groups: a negative control; treated groups (G1, G2, G3) supplemented with different concentrations of A-Live and Aquaform in the feed; and a positive control (PC) for pathogen infection. Groups G1, G2, G3, and PC were challenged with Francisella spp. after 15 days. After infection, the mortality was significantly lower in groups G1, G2, and G3 (p < 0.01). Furthermore, these groups showed significant increase (p < 0.05) in daily weight gain, feed conversion rate, and specific growth rate. The PC group presented increase (p < 0.05) in the leukocytes and neutrophils number. Innate immunity parameters showed no difference between treatments after infection. Microbiome analysis revealed an increased number of bacteria belonging to the Vibrionaceae family after pathogen infection suggesting a secondary pathogen function of these bacteria. These results validate the beneficial effects of these products in tilapia farming.

Francisellosis is a disease responsible for severe economic losses in aquaculture. Currently, this disease is controlled via biosecurity measures and treatment of affected batches. In this study, tilapia juveniles were challenged with Francisella orientalis via immersion and fed with oxytetracycline‐medicated feed, at 100 mg kg fish−1 and 200 mg kg fish−1, under prophylactic and therapeutic strategies for 16 and 15 days respectively. All doses and treatment strategies reduced the fish mortality. The detection of the pathogen from the spleen was lower in the treated groups. Histopathology analysis showed that challenged fish that received the highest dosage presented fewer granuloma in their spleens and less inflammatory infiltrate in their livers than untreated fish. Fish that received the prophylactic treatment, regardless of the dose, presented fewer granuloma in their livers than untreated fish. Fish treated with 100 mg kg fish−1 had better scores for hepatocyte accumulations than fish treated with the higher dosage. Furthermore, 20% of the unchallenged fish that received the higher dosage presented mild inflammatory infiltrate in their livers. No differences were observed in the serum hepatocyte lesion enzyme quantifications. The results indicate that oxytetracycline‐medicated feed is safe, even at the higher dose, and effective at controlling francisellosis in tilapia.

Streptococcus agalactiae is an important pathogen to world aquaculture due to its high mortality rates in fish farms and consequent economic losses. Our study presents the complete genome sequence of strain S13, isolated from a tilapia farm outbreak in southern Brazil.

No abstract