Evaluating Efficacy of Plant Growth Promoting Rhizobacteria for Promoting Growth and Preventing Disease in both Fish and Plants
Type of DegreePhD Dissertation
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Plant growth promoting rhizobacteria (PGPR), are bacteria residing within the rhizosphere of a plant, that elicit health benefits to the plant (Kloepper and Schroth, 1978; Kloepper et al., 2004). To understand the growth promoting and disease-inhibiting activities of PGPR strains, the genomes of 12 different PGPR strains affiliated with the B. subtilis group were sequenced. These B. subtilis strains exhibited high genomic diversity, whereas the genomes of Bacillus amyloliquefaciens strains (a member of the B. subtilis group) and B. velezensis strains (formerly B. amyloliquefaciens subsp. plantarum, now a part of the B. amyloliquefaciens clade (Fan et. al., 2017)) are highly conserved. A total of 2,839 genes were consistently present within the core genome of B. velezensis. Comparative genomic analyses of B. amyloliquefaciens and B. velezensis strains identified conserved genes that have been linked with biological control and colonization of roots or leaves. There were 73 genes uniquely associated with B. velezensis strains with predicted functions related to signaling, transportation, secondary metabolite production, and carbon source utilization. Genes involved in secondary metabolite biosynthesis were deleted in B. velezensis strain AP193 to evaluate their role in plant pathogen biocontrol, revealing that difficidin expression is critical and solely sufficient for reducing the severity of bacterial spot caused by Xanthomonas axonopodis pv. vesicatoria in tomato plants. A root extract medium was used as a growth substrate along with extended incubation times to obtain new diverse rhizobacteria isolates. The antagonistic potential of these isolates against a broad range of plant pathogenic bacteria, fungi and oomycetes was observed. Among the isolates tested, Burkholderia gladioli C101 produced the most potent and heat-stable active secondary metabolites that were 3 active against root or foliar fungal, oomycete or bacterial pathogens. A cell-free formulation of B. gladioli C101 was used as a foliar spray to suppress bacterial spot disease in tomato; resulting in a significant reduction in bacterial spot disease severity in greenhouse experiments when the cell-free supernatants were applied prior or after inoculation with Xanthomonas perforans (Xp). Bacillus strains isolated from soil or channel catfish intestine were screened for their antagonism against Edwardsiella ictaluri and Aeromonas hydrophila, the causative agents of enteric septicemia of catfish (ESC) and motile aeromonad septicaemia (MAS), respectively. The top 21 strains expressed antagonistic activity against multiple aquatic bacterial pathogens including Edwardsiella tarda, Streptococcus iniae, Yersinia ruckeri, Flavobacterium columnare, and/or the oomycete Saprolegnia ferax. Survival of the 21 Bacillus strains in the intestine of catfish was determined as Bacillus CFU/g of intestinal tissue of catfish after feeding Bacillus spore-supplemented feed for seven days followed by normal feed for three days. Five Bacillus strains were selected to incorporated into feed in spore form at a dose of 8 x 107 CFU/g and fed to channel catfish for 14 days before they were challenged by E. ictaluri in replicate. Two Bacillus subtilis strains conferred significant benefit in reducing catfish mortality (P < 0.05). The effects of probiotic-amended diets fed to juvenile Nile tilapia, Oreochromis niloticus on growth and susceptibility to Streptococcus iniae infection was determined. Fish (average weight 16.5±0.2 g) were fed five diets formulated with Bacillus subtilis group strains SB3086, SB3295, SB3615 or AP193 either individually or in combination for strains SB3086 and SB3615 at a targeted concentration of approximately 4 x 107 CFU/g of feed, or with a basal control diet with no additives for 21 days. After the 21-day growth trial, no significant difference in growth performance 4 was observed with any probiotic-amended diet. However, results from the challenge showed significantly lower mortalities between treatments as compared to the control (P < = 0.0001). An additional study was conducted to evaluate the individual and combined effects of long-term feeding of diets containing two probiotic Bacillus subtilis group strains (Aqua NZ and AP193) and the prebiotic Previda®, a commercial hemicellulose extract, on growth performance, immune parameters and Aeromonas hydrophila susceptibility of juvenile Nile tilapia, O. niloticus. Nile tilapia of average weight 7.47 ± 0.11 g were fed diets formulated with the probiotics and/or the prebiotic, or a control diet for 8 weeks and, subsequently, challenged with A. hydrophila by intragastric gavage at a dosage of 3.9 x 107 CFU/fish. Fish attained a mean weight of 59.5 ± 0.99 g at the end of the growth period. None of the diets significantly improved mean percent weight gain (P = 0.70), thermal growth coefficient (P=0.88) or feed conversion ratio (P = 0.87) of Nile tilapia. Except for the diet containing the prebiotic Previda® only (P = 0.17), all other diets resulted in significantly higher fish survival compared to the control (P < 0.05) when challenged A. hydrophila. The prebiotic and probiotic strains used in combination emerged as the most important diet with respect to mortality reduction. Four Bacillus velezensis strains were evaluated for their probiotic effects and disease reduction due to E. ictaluri in an aquarium study. After ten weeks feeding, fish fed with B. velezensis AP193 were observed to have the best growth performance (14% increase in mean growth) and best survival rates after E. ictaluri challenge, compared to control fish. B. velezensis AP193 was therefore selected for evaluation in a ten-week pond trial, with four replicate ponds per probiotic treatment or control group. Feed amended with B. velezensis AP193 induced a 40.4% or 32.6% increase in growth relative to control feed in fingerling 5 catfish that originated from aquaria or raceways, respectively. No significant differences were observed in the catfish intestinal microbiota or the pond microbiota due to probiotic-amended feed. The water quality was improved in ponds in which fish were fed with the probiotic-amended feed, as significant reductions were found in total phosphorus (19%), total nitrogen (43%) and nitrate (75%).