RESUMEN
To understand the growth-promoting and disease-inhibiting activities of plant growth-promoting rhizobacteria (PGPR) strains, the genomes of 12 Bacillus subtilis group strains with PGPR activity were sequenced and analyzed. These B. subtilis strains exhibited high genomic diversity, whereas the genomes of B. amyloliquefaciens strains (a member of the B. subtilis group) are highly conserved. A pairwise BLASTp matrix revealed that gene family similarity among Bacillus genomes ranges from 32 to 90%, with 2839 genes within the core genome of B. amyloliquefaciens subsp. plantarum. Comparative genomic analyses of B. amyloliquefaciens strains identified genes that are linked with biological control and colonization of roots and/or leaves, including 73 genes uniquely associated with subsp. plantarum strains that have predicted functions related to signaling, transportation, secondary metabolite production, and carbon source utilization. Although B. amyloliquefaciens subsp. plantarum strains contain gene clusters that encode many different secondary metabolites, only polyketide biosynthetic clusters that encode difficidin and macrolactin are conserved within this subspecies. To evaluate their role in plant pathogen biocontrol, genes involved in secondary metabolite biosynthesis were deleted in a B. amyloliquefaciens subsp. plantarum strain, revealing that difficidin expression is critical in reducing the severity of disease, caused by Xanthomonas axonopodis pv. vesicatoria in tomato plants. This study defines genomic features of PGPR strains and links them with biocontrol activity and with host colonization.
RESUMEN
A new strain of Aeromonas hydrophila has been implicated in significant losses in farm-raised catfish. Outbreaks attributable to this new strain began in Alabama in the summer of 2009 and have spread to Arkansas and Mississippi in subsequent years. These outbreaks mostly afflicted market-sized fish and resulted in considerable losses in short periods of time. The present research was designed to develop an expeditious diagnostic procedure to detect the new strains of A. hydrophila due to the rapid onset and biosecurity concerns associated with this new disease. A discriminatory quantitative polymerase chain reaction assay was developed using gene sequences unique to the virulent strains identified in a related comparative genomic study. Using this assay, suspect colonies on a culture plate can be positively identified as the new strain within 2 hr. The assay is repeatable and reproducible with a linear dynamic range covering 8 orders of magnitude and a sensitivity of approximately 7 copies of target DNA in a 15-µl reaction. In addition, the assay is able to detect and quantify the virulent strain from catfish tissues (0.025 g), pond water (40 ml), and sediments (0.25 g) with a sensitivity limit of approximately 100 bacteria in a sample. This assay provides rapid discrimination between the new virulent strain and more common A. hydrophila and is useful for epidemiological studies involving the detection and quantification of the virulent strain in environmental samples and fish tissues.
Asunto(s)
Aeromonas hydrophila/aislamiento & purificación , Bagres , Brotes de Enfermedades/veterinaria , Enfermedades de los Peces/microbiología , Infecciones por Bacterias Gramnegativas/veterinaria , Aeromonas hydrophila/genética , Aeromonas hydrophila/patogenicidad , Alabama/epidemiología , Animales , Acuicultura , Recuento de Colonia Microbiana/veterinaria , ADN Bacteriano/química , ADN Bacteriano/genética , Enfermedades de los Peces/diagnóstico , Enfermedades de los Peces/epidemiología , Infecciones por Bacterias Gramnegativas/diagnóstico , Infecciones por Bacterias Gramnegativas/epidemiología , Infecciones por Bacterias Gramnegativas/microbiología , Reacción en Cadena de la Polimerasa/veterinaria , ARN Ribosómico 16S/química , ARN Ribosómico 16S/genética , Reproducibilidad de los Resultados , Sensibilidad y Especificidad , Virulencia , Microbiología del AguaRESUMEN
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. Twenty one strains were selected and their antagonistic activity against other aquatic pathogens was also tested. Each of 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 that showed good antimicrobial activity and intestinal survival were incorporated into feed in spore form at a dose of 8×10(7) 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). A similar challenge experiment conducted in Vietnam with four of the five Bacillus strains also showed protective effects against E. ictaluri in striped catfish. Safety of the four strains exhibiting the strongest biological control in vivo was also investigated in terms of whether the strains contain plasmids or express resistance to clinically important antibiotics. The Bacillus strains identified from this study have good potential to mediate disease control as probiotic feed additives for catfish aquaculture.
