Impact of bdellovibrio-and-like organisms (BALOs) on mixed species biofilm communities
Date of Issue2016-02-01
School of Biological Sciences
Predation has been found to play essential roles in ecosystems, where predatory activity may play an important role in helping to maintain the diversity of population and communities by preventing a single species from becoming dominant. Therefore, predators are considered as integral components of the food webs within the whole ecosystem. Predation is important not only at macro-scale, but also for single celled microorganisms. Microorganisms, especially bacteria, are generally preyed upon by viruses, protozoa and a groups of specific bacterial predators defined as Bdellovibrio- and-like organisms. Microbial predators are found in various environments where bacteria are inhabited. Activated sludge in wastewater treatment plants is comprised of highly diverse bacterial species, which carry out essential functions for wastewater treatment. Given that the microbial predators may prey upon diverse bacteria, it is of great interest to investigate the impact of predation on microbial community composition and function. In this study, one member of the Genus Bdellovibrio was isolated from activated sludge collected at the Ulu Pandan Wastewater Reclamation Plant (Singapore). Based on the 16S rRNA gene sequencing, this isolate was 99% identical to ‘Bdellovibrio bacteriovorus strain Tiberius’ and was designated here as ‘Bdellovibrio bacteriovorus UP’. The growth pattern of this Bdellovibrio isolate on specific prey species was shown to be similar with the well-known predator-prey interaction described by the Lotka-Volterra equation. The prey range and potential specificity of B. bacteriovorus UP was tested by quantifying growth in the presence of several model species, including Escherichia coli, Klebsiella pneumoniae and Pseudomonas protegens Pf-5 and the results showed that all of the tested model species were vulnerable to B. bacteriovorus UP predation. To further explore the potential impact of B. bacteriovorus UP on microbial communities in wastewater treatment plant, 78 isolates from activated sludge samples, were grown as single-species biofilms in the presence of this predator. As expected, none of the Gram-positive (14 isolates) or fungi (4 isolates) supported the growth of the predator. In contrast, with the exception of one species, O. anthropi, all of the Gram-negative species tested, represented by 18 Alpha-Proteobacteria isolates and 22 Gamma-Proteobacteria isolates were sensitive to predation by B. bacteriovorus UP irrespective of whether they were present as biofilms or as planktonic cells. Biofilms and planktonic cells were reduced by 20% – 80% and 10 – 1000 fold, respectively. It has been suggested that growth in a mixed species community can protect sensitive species from stressors, including predation. To test this possibility, two-membered mixed species communities were developed that included one species, O. anthropi, which was shown to be resistant to predation and the second species, which was shown to be sensitive to predation. The results indicated that the predator was able to significantly reduce the biomass of dual-species biofilms and planktonic cultures, suggesting that there was no cross-protection against predation. These results also suggest that the mechanism by which O. anthropi is protected is not a diffusible molecule, but is more likely cell associated. To further explore the role of biofilm based predation protection and feeding preference, mixed species communities with high density (> 3500 OTUs) represented by floccular or granular sludge were exposed to the predator and alterations in biomass, viability and community composition were quantified. The results showed significant reductions in the viability and total biomass of both floccular and granular sludge, although the floccular sludge was more sensitive to predation. Due to predation, the microbial community compositions of both floccular and granular sludge were also significantly affected. For example, the microbial diversities of floccular and granular sludge were reduced by 11.5% and 23.1% in their contribution to the total community as a consequence of predation. The results presented in this study suggest that B. bacteriovorus UP isolated from a wastewater treatment plant has a broad spectrum of prey species and whilst it does not exhibit a specific feeding preference amongst the Gram-negative bacteria, some species are more sensitive to predation, especially when grown as biofilms. This suggests that whilst there is no explicit feeding preference for B. bacteriovorus UP, it may nonetheless impact specific communities members and hence reduce their contribution to overall system performance more than others. Both floccular and granular sludges were significantly impacted by the predator, although the granular sludge showed slightly less sensitivity to predation pressure. In aggregate, the data demonstrate that the B. bacteriovorus UP is a generalist predator and has the potential to strongly impact the performance of activated sludge communities.
DRNTU::Science::Biological sciences::Microbiology::Microbial ecology