The emergence of diseases and the indiscriminate use of antibiotics have had deleterious consequences in the aquaculture industry. Consequently, the growing emphasis on eco-friendly alternatives has taken center stage, and probiotics have emerged as a notable solution. This investigation aimed to identify and characterize potential probiotic bacteria present in the gills and intestine of rainbow trout ( Oncorhynchus mykiss (Walbaum)). A total of 29 isolates were selected for in vitro evaluation. The assays included morphological evaluation, catalase, oxidase, hemolysis, pathogen antagonism, antimicrobial susceptibility, protease, and amylase tests. Seven isolates exhibited antagonism against one of the three pathogens tested ( Aeromonas hydrophila , Streptococcus agalactiae , and Lactococcus garvieae ). None of the isolates showed resistance to the antibiotics ampicillin, florfenicol, or tetracycline. Furthermore, all isolates showed proteolytic but not amylolytic activity. The isolates with the best characteristics were identified as Lactococcus lactis (n=7) by 16S rRNA gene sequencing.

Autochthonous probiotics derived from the host’s native microbiota can enhance gastrointestinal colonization, feed efficiency, disease resistance, and stress tolerance in aquaculture species. This study evaluated bacteria isolated from the intestine and gills of gamitana (Colossoma macropomum) based on key probiotic selection criteria, including tolerance to acidic conditions and bile salts, antagonistic activity against pathogens, antibiotic susceptibility, biofilm formation, and enzymatic activity. A total of 69 bacterial isolates were obtained using Man, Rogosa, and Sharpe (MRS) broth and Trypticase Soy Broth (TSB). Five isolates (7.2%) met all selection criteria and were molecularly identified as Lactobacillus brevis (IPIFAB2, IPIFAC1, BRALA1) and Lactococcus garvieae (G25, G23). L. brevis isolates showed moderate acid tolerance, with survival rates between 31 ± 2.04% and 36.4 ± 0.17% at pH 4. All selected strains tolerated bile salts; however, L. brevis maintained high viability (≥78% at 0.8%), while L. garvieae exhibited marked reductions at higher concentrations. Antagonistic activity was significantly greater in L. brevis, producing inhibition halos of 2.3 ± 0.1 to 3.9 ± 0.1 mm, compared with 1.2 ± 0.21 to 1.9 ± 0.12 mm for L. garvieae. Proteolytic activity was observed in three L. brevis strains, and six isolates demonstrated biofilm-forming capacity. All selected strains were susceptible to the antibiotics tested. Among them, L. brevis IPIFAB2 exhibited the strongest probiotic profile. Overall, L. brevis strains show considerable potential as autochthonous probiotics for C. macropomum aquaculture, contributing to improved fish health, disease prevention, and sustainable production.

This review synthesizes the available evidence on the effects of Hermetia illucens meal inclusion on the intestinal microbiota of fish. A literature search was conducted in Scopus, Web of Science, PubMed, and Embase from inception to 23 April 2025, including in vivo experimental studies reporting changes in fish intestinal microbiota. Of 2525 records identified, 39 studies met the predefined PICOS-based eligibility criteria and were included in a PRISMA 2020-guided synthesis; risk of bias was assessed using the SYRCLE tool. Given the substantial methodological heterogeneity among studies (fish species, dietary formulations, sequencing platforms, and analytical pipelines), no quantitative meta-analysis was performed, and results were synthesized semi-quantitatively based on the direction of reported effects. At the phylum level, most studies reported significant increases in Bacillota (41% vs. 16% decreases; n = 32) and Actinomycetota (65% increases; n = 26), while Pseudomonadota showed a recurrent, though not universal, reduction (50%; n = 28), with enrichment of genera such as Bacillus (70%; n = 20), Enterococcus (81%; n = 16), and Oceanobacillus (93%; n = 15). These changes have been hypothesized to involve prebiotic, metabolic, and antimicrobial mechanisms related to chitin, lauric acid, and antimicrobial peptides. Overall, H. illucens meal inclusion (~10–30%) appears to be associated with a predominant, though not universal, modulation of the fish intestinal microbiota toward profiles with greater fermentative and chitinolytic potential; an inference based on the ecological attributes of the enriched taxa rather than on functional measurements. The magnitude and consistency of this modulation depend on dose, species, habitat, and meal processing type.