Bdellovibrio and like organisms: current understanding and knowledge gaps of the smallest cellular hunters of the microbial world.

Almost sixty years ago, Bdellovibrio and like organisms (BALOs) were discovered as the first obligate bacterial predators of other bacteria known to science. Since then, they were shown to be diverse and ubiquitous in the environment, and to bear astonishing ecological, physiological, and metabolic capabilities. The last decade has seen important strides made in understanding the mechanistic basis of their life cycle, the dynamics of their interactions with prey, along with significant developments towards their use in medicine, agriculture, and industry. This review details these achievements, identify current understanding and knowledge gaps to encourage and guide future BALO research.

Diversity, Dynamics, and Distribution of Bdellovibrio and Like Organisms in Perialpine Lakes.

Microbes drive a variety of ecosystem processes and services, but many of them remain largely unexplored because of a lack of knowledge on both the diversity and functionality of some potentially crucial microbiological compartments. This is the case with and within the group of bacterial predators collectively known as Bdellovibrio and like organisms (BALOs). Here, we report the abundance, distribution, and diversity of three families of these obligate predatory Gram-negative bacteria in three perialpine lakes (Lakes Annecy, Bourget, and Geneva). The study was conducted at different depths (near-surface versus 45 or 50 m) from August 2015 to January 2016. Using PCR-denaturing gradient gel electrophoresis (PCR-DGGE) and cloning-sequencing approaches, we show that the diversity of BALOs is relatively low and very specific to freshwaters or even the lakes themselves. While the Peredibacteraceae family was represented mainly by a single species (Peredibacter starrii), it could represent up to 7% of the total bacterial cell abundances. Comparatively, the abundances of the two other families (Bdellovibrionaceae and Bacteriovoracaceae) were significantly lower. In addition, the distributions in the water column were very different between the three groups, suggesting various life strategies/niches, as follows: Peredibacteraceae dominated near the surface, while Bdellovibrionaceae and Bacteriovoracaceae were more abundant at greater depths. Statistical analyses revealed that BALOs seem mainly to be driven by depth and temperature. Finally, this original study was also the opportunity to design new quantitative PCR (qPCR) primers for Peredibacteraceae quantification.IMPORTANCE This study highlights the abundance, distribution, and diversity of a poorly known microbial compartment in natural aquatic ecosystems, the Bdellovibrio and like organisms (BALOs). These obligate bacterial predators of other bacteria may have an important functional role. This study shows the relative quantitative importance of the three main families of this group, with the design of a new primer pair, and their diversity. While both the diversity and the abundances of these BALOs were globally low, it is noteworthy that the abundance of the Peredibacteraceae could reach important values.

Adaptive traits of cysts of the snow alga Sanguina nivaloides unveiled by 3D subcellular imaging.

Sanguina nivaloides is the main alga forming red snowfields in high mountains and Polar Regions. It is non-cultivable. Analysis of environmental samples by X-ray tomography, focused-ion-beam scanning-electron-microscopy, physicochemical and physiological characterization reveal adaptive traits accounting for algal capacity to reside in snow. Cysts populate liquid water at the periphery of ice, are photosynthetically active, can survive for months, and are sensitive to freezing. They harbor a wrinkled plasma membrane expanding the interface with environment. Ionomic analysis supports a cell efflux of K+, and assimilation of phosphorus. Glycerolipidomic analysis confirms a phosphate limitation. The chloroplast contains thylakoids oriented in all directions, fixes carbon in a central pyrenoid and produces starch in peripheral protuberances. Analysis of cells kept in the dark shows that starch is a short-term carbon storage. The biogenesis of cytosolic droplets shows that they are loaded with triacylglycerol and carotenoids for long-term carbon storage and protection against oxidative stress.

A Comparative Study of the Dynamics and Diversity of Bdellovibrio and Like Organisms in Lakes Annecy and Geneva.

Bdellovibrio and like organisms (BALOs) are obligate bacterial predators of other Gram-negative bacteria. Here, we used quantitative PCR (qPCR) and recently developed specific primers which target the 16S rRNA gene to explore the abundance and distribution of three families of BALO belonging to the Oligoflexia class (i.e., Bdellovibrionaceae, Peredibacteraceae and Bacteriovoracaceae) over one year in the epilimnion and hypolimnion of Lakes Annecy and Geneva. Peredibacteraceae was the dominant group at all sampling points except at the bottom of Lake Geneva, where Bdellovibrionaceae was found in higher number. In addition, the abundance of BALOs increased significantly during the warmer months. Using high-throughput sequencing (Illumina Miseq), hundreds of OTUs were identified for Bdellovibrionaceae and Peredibacteraceae. Phylogenetic analysis suggests that Bdellovibrionaceae are more diverse than Peredibacteraceae and that some OTUs belong to new species of Bdellovibrionaceae. We also found that dominant OTUs were present simultaneously in the two lakes, while some others were specific to each lake, suggesting an adaptive pattern. Finally, both abundance and diversity of BALOs were poorly associated with abiotic factors except temperature, suggesting the importance of studying biotic relationships, assumed to play a greater role than physico-chemical variables in BALOs' dynamics and distribution.

New 16S rRNA primers to uncover Bdellovibrio and like organisms diversity and abundance.

Appropriate use and specific primers are important in assessing the diversity and abundance of microbial groups of interest. Bdellovibrio and like organisms (BALOs), that refer to obligate Gram-negative bacterial predators of other Gram-negative bacteria, evolved in terms of taxonomy and classification over the past two decades. Hence, some former primers have become inadequate while others are yet to be designed, for both PCR (especially with the advent of NGS) and qPCR approaches. Thus, to study BALOs' abundance and diversity in a variety of aquatic ecosystems, we designed in silico specific primer sets for each BALO genera and tested them in vitro on a variety of cultures and environmental samples. Also, we performed Sanger and Nano Miseq sequencing to reveal the exact degree of specificity of the most promising primers set. Here we report our success in designing specific primers for some BALOs genera, i.e. Bdellovibrio (PCR), Bacteriovorax (qPCR), Peredibacter (PCR).

Bdellovibrio and Like Organisms in Lake Geneva: An Unseen Elephant in the Room?

When considering microbial biotic interactions, viruses as well as eukaryotic grazers are known to be important components of aquatic microbial food webs. It might be the same for bacterivorous bacteria but these groups have been comparatively less studied. This is typically the case of the Bdellovibrio and like organisms (BALOs), which are obligate bacterial predators of other bacteria. Recently, the abundance and distribution of three families of this functional group were investigated in perialpine lakes, revealing their presence and quantitative importance. Here, a more in-depth analysis is provided for Lake Geneva regarding the diversity of these bacterial predators at different seasons, sites and depths. We reveal a seasonal and spatial (vertical) pattern for BALOs. They were also found to be relatively diverse (especially Bdellovibrionaceae) and assigned to both known and unknown phylogenetic clusters. At last we found that most BALOs were positively correlated to other bacterial groups, mainly Gram-negative, in particular Myxococcales (among which many are predators of other microbes). This study is the first shedding light on this potentially important bacterial killing group in a large and deep lake.