Mechanism of the Potential Therapeutic Candidate Bacillus subtilis BSXE-1601 Against Shrimp Pathogenic Vibrios and Multifunctional Metabolites Biosynthetic Capability of the Strain as Predicted by Genome Analysis

Evaluation of Potential Probiotic Properties of a Strain of Lactobacillus plantarum for Shrimp Farming: From Beneficial Functions to Safety Assessment

In recent years the safety of probiotics has received increasing attention due to the possible transfer and spread of virulence factors (VFs) and antibiotic resistance genes (ARGs) among microorganisms. The safety of a strain of Lactobacillus plantarum named W2 was evaluated in phenotype and genotype in the present study. Its probiotic properties were also evaluated both in vivo and in vitro, including adherence properties, antibacterial properties and beneficial effects on the growth and immunity of Pacific white shrimp, Penaeus vannamei. Hemolysis tests, antibiotic resistance tests and whole genome sequence analysis showed that W2 had no significant virulence effects and did not carry high virulence factors. W2 was found to be sensitive to chloramphenicol, clindamycin, gentamicin, kanamycin and tetracycline, and to be resistant to ampicillin and erythromycin. Most ARGs have no transfer risk and a few have transfer risk but no significant enrichment in human-associated environments. The autoaggregation of W2 was 82.6% and the hydrophobicity was 81.0%. Coaggregation rate with Vibrio parahaemolyticus (24.9%) was significantly higher than Vibrio's autoaggregation rate (17.8%). This suggested that W2 had adhesion potential to mucosal/intestinal surfaces and was able to attenuate the adherence of V. parahaemolyticus. In addition, several adhesion-related protein genes, including 1 S-layer protein, 1 collagen-binding protein and 9 mucus-binding proteins were identified in the W2 genome. W2 had efficiently antagonistic activity against 7 aquatic pathogenic strains. Antagonistic components analysis indicated that active antibacterial substances might be organic acids. W2 can significantly promote the growth of shrimp when supplemented with 1 × 10¹⁰ cfu/kg live cells. Levels of 7 serological immune indicators and expression levels of 12 hepatopancreatic immune-related genes were up-regulated, and the mortality of shrimp exposed to V. parahaemolyticus was significantly reduced. Based on the above, L. plantarum W2 can be applied safely as a potential probiotic to enhance the growth performance, immunity capacity and disease resistance of P. vannamei.

Antimicrobial Bacillus: Metabolites and Their Mode of Action

The agricultural industry utilizes antibiotic growth promoters to promote livestock growth and health. However, the World Health Organization has raised concerns over the ongoing spread of antibiotic resistance transmission in the populace, leading to its subsequent ban in several countries, especially in the European Union. These restrictions have translated into an increase in pathogenic outbreaks in the agricultural industry, highlighting the need for an economically viable, non-toxic, and renewable alternative to antibiotics in livestock. Probiotics inhibit pathogen growth, promote a beneficial microbiota, regulate the immune response of its host, enhance feed conversion to nutrients, and form biofilms that block further infection. Commonly used lactic acid bacteria probiotics are vulnerable to the harsh conditions of the upper gastrointestinal system, leading to novel research using spore-forming bacteria from the genus Bacillus. However, the exact mechanisms behind Bacillus probiotics remain unexplored. This review tackles this issue, by reporting antimicrobial compounds produced from Bacillus strains, their proposed mechanisms of action, and any gaps in the mechanism studies of these compounds. Lastly, this paper explores omics approaches to clarify the mechanisms behind Bacillus probiotics.

Biocontrol Ability and Mechanism of a Broad-Spectrum Antifungal Strain Bacillus safensis sp. QN1NO-4 Against Strawberry Anthracnose Caused by Colletotrichum fragariae

Strawberry is a very popular fruit with a special taste, color, and nutritional value. Anthracnose caused by Colletotrichum fragariae severely limits fruit shelf life during post-harvest storage. Use of traditional chemical fungicides leads to serious environment pollution and threatens food safety. Biocontrol is considered as a promising strategy to manage the post-harvest fruit diseases. Here, strain QN1NO-4 isolated from noni (Morinda citrifolia L.) fruit exhibited a high antifungal activity against C. fragariae. Based on its physicochemical profiles and phylogenetic tree of the 16S rRNA sequence, strain QN1NO-4 belonged to the genus Bacillus. The average nucleotide identity (ANI) calculated by comparing two standard strain genomes was below 95-96%, suggesting that the strain might be a novel species of the genus Bacillus and named as Bacillus safensis sp. QN1NO-4. Its extract effectively reduced the incidence of strawberry anthracnose of harvested fruit. Fruit weight and TSS contents were also maintained significantly. The antifungal mechanism assays indicated that the extract of the test antagonist inhibited mycelial growth and spore germination of C. fragariae in vitro. Cells of strain QN1NO-4 demonstrated the cytoplasmic heterogeneity, disappeared organelles, and ruptured ultrastructure. Notably, the strain extract also had a broad-spectrum antifungal activity. Compared with the whole genome of strain QN1NO-4, several functional gene clusters involved in the biosynthesis of active secondary metabolites were observed. Fifteen compounds were identified by gas chromatography-mass spectrometry (GC-MS). Hence, the fruit endophyte B. safensis sp. QN1NO-4 is a potential bio-agent identified for the management of post-harvest disease of strawberry fruit.

Acute Hepatopancreatic Necrosis Disease (AHPND): Virulence, Pathogenesis and Mitigation Strategies in Shrimp Aquaculture

Shrimp, as a high-protein animal food commodity, are one of the fastest growing food producing sectors in the world. It has emerged as a highly traded seafood product, currently exceeding 8 MT of high value. However, disease outbreaks, which are considered as the primary cause of production loss in shrimp farming, have moved to the forefront in recent years and brought socio-economic and environmental unsustainability to the shrimp aquaculture industry. Acute hepatopancreatic necrosis disease (AHPND), caused by Vibrio spp., is a relatively new farmed penaeid shrimp bacterial disease. The shrimp production in AHPND affected regions has dropped to ~60%, and the disease has caused a global loss of USD 43 billion to the shrimp farming industry. The conventional approaches, such as antibiotics and disinfectants, often applied for the mitigation or cure of AHPND, have had limited success. Additionally, their usage has been associated with alteration of host gut microbiota and immunity and development of antibiotic resistance in bacterial pathogens. For example, the Mexico AHPND-causing V. parahaemolyticus strain (13-306D/4 and 13-511/A1) were reported to carry tetB gene coding for tetracycline resistance gene, and V. campbellii from China was found to carry multiple antibiotic resistance genes. As a consequence, there is an urgent need to thoroughly understand the virulence mechanism of AHPND-causing Vibrio spp. and develop novel management strategies to control AHPND in shrimp aquaculture, that will be crucially important to ensure food security in the future and offer economic stability to farmers. In this review, the most important findings of AHPND are highlighted, discussed and put in perspective, and some directions for future research are presented.