Potential Control of Potato Soft Rot Disease by the Obligate Predators Bdellovibrio and Like Organisms

Bacterial soft rot diseases caused by Pectobacterium spp. and Dickeya spp. affect a wide range of crops, including potatoes, a major food crop. As of today, farmers mostly rely on sanitary practices, water management, and plant nutrition for control. We tested the bacterial predators Bdellovibrio and like organisms (BALOs) to control potato soft rot. BALOs are small, motile predatory bacteria found in terrestrial and aquatic environments. They prey on a wide range of Gram-negative bacteria, including animal and plant pathogens. To this end, BALO strains HD100, 109J, and a ΔmerRNA derivative of HD100 were shown to efficiently prey on various rot-causing strains of Pectobacterium and Dickeya solani BALO control of maceration caused by a highly virulent strain of Pectobacterium carotovorum subsp. brasilense was then tested in situ using a potato slice assay. All BALO strains were highly effective at reducing disease, up to complete prevention. Effectivity was concentration dependent, and BALOs applied before P. carotovorum subsp. brasilense inoculation performed significantly better than those applied after the disease-causing agent, maybe due to in situ consumption of glucose by the prey, as glucose metabolism by live prey bacteria was shown to prevent predation. Dead predators and the supernatant of BALO cultures did not significantly prevent maceration, indicating that predation was the major mechanism for the prevention of the disease. Finally, plastic resistance to predation was affected by prey and predator population parameters, suggesting that population dynamics affect prey response to predation.IMPORTANCE Bacterial soft rot diseases caused by Pectobacterium spp. and Dickeya spp. are among the most important plant diseases caused by bacteria. Among other crops, they inflict large-scale damage to potatoes. As of today, farmers have few options to control them. The bacteria Bdellovibrio and like organisms (BALOs) are obligate predators of bacteria. We tested their potential to prey on Pectobacterium spp. and Dickeya spp. and to protect potato. We show that different BALOs can prey on soft rot-causing bacteria and prevent their growth in situ, precluding tissue maceration. Dead predators and the supernatant of BALO cultures did not significantly prevent maceration, showing that the effect is due to predation. Soft rot control by the predators was concentration dependent and was higher when the predator was inoculated ahead of the prey. As residual prey remained, we investigated what determines their level and found that initial prey and predator population parameters affect prey response to predation.

Microbiome and Resistome Profiles along a Sewage-Effluent-Reservoir Trajectory Underline the Role of Natural Attenuation in Wastewater Stabilization Reservoirs

Antibiotic-resistant bacteria and antibiotic resistance gene (ARGs) loads dissipate through sewage treatment plants to receiving aquatic environments, but the mechanisms that mitigate the spread of these ARGs are not well understood due to the complexity of full-scale systems and the difficulty of source tracking in downstream environments. To overcome this problem, we targeted a controlled experimental system comprising a semicommercial membrane-aerated bioreactor (MABR), whose effluents fed a 4,500-L polypropylene basin that mimicked effluent stabilization reservoirs and receiving aquatic ecosystems. We analyzed a large set of physicochemical measurements, concomitant with the cultivation of total and cefotaxime-resistant Escherichia coli, microbial community analyses, and quantitative PCR (qPCR)/digital droplet PCR (ddPCR) quantification of selected ARGs and mobile genetic elements (MGEs). The MABR removed most of the sewage-derived organic carbon and nitrogen, and simultaneously, E. coli, ARG, and MGE levels dropped by approximately 1.5- and 1.0-log unit mL-1, respectively. Similar levels of E. coli, ARGs, and MGEs were removed in the reservoir, but interestingly, unlike in the MABR, the relative abundance (normalized to 16S rRNA gene-inferred total bacterial abundance) of these genes also decreased. Microbial community analyses revealed the substantial shifts in bacterial and eukaryotic community composition in the reservoir relative to the MABR. Collectively, our observations lead us to conclude that the removal of ARGs in the MABR is mainly a consequence of treatment-facilitated biomass removal, whereas in the stabilization reservoir, mitigation is linked to natural attenuation associated with ecosystem functioning, which includes abiotic parameters, and the development of native microbiomes that prevent the establishment of wastewater-derived bacteria and associated ARGs. IMPORTANCE Wastewater treatment plants are sources of antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs), which can contaminate receiving aquatic environments and contribute to antibiotic resistance. We focused on a controlled experimental system comprising a semicommercial membrane-aerated bioreactor (MABR) that treated raw sewage, whose effluents fed a 4,500-L polypropylene basin that mimicked effluent stabilization reservoirs. We evaluated ARB and ARG dynamics across the raw-sewage-MABR-effluent trajectory, concomitant with evaluation of microbial community composition and physicochemical parameters, in an attempt to identify mechanisms associated with ARB and ARG dissipation. We found that removal of ARB and ARGs in the MABR was primarily associated with bacterial death or sludge removal, whereas in the reservoir it was attributed to the inability of ARBs and associated ARGs to colonize the reservoir due to a dynamic and persistent microbial community. The study demonstrates the importance of ecosystem functioning in removing microbial contaminants from wastewater.