Halophilic Bacteriovorax (Bx) were formerly known as marine Bdellovibrio. They are obligate Gram-negative predatory bacteria which can be found in various environments. Alike Bdellovibrio, Bx have a very unique biphasic life cycle which is reminiscent of viruses. They swim at very high speeds (160 um s-1) and can detect prey using a chemosensory system which is not very well understood. They then attach to their prey and enter the periplasmic space. Once inside they can lay dormant however usually they consume the cytoplasm of the prey, elongate and then differentiate into several motile cells which lyse the prey cell and start a new cycle. Its therefore quite possible they have an important role in contributing to bacterial mortality and nutrient cycling.
They differ from Bdellovibrio and freshwater Bx species mainly due to their salt tolerance, %G+C ratio, fatty acid profiles, antigenic variations and habitat. Phylogenetic trees have shown that Bx have two major branches of which one contains two freshwater species and the other two marine species. However previous studies of Bdellovibrio and Bx have more or less completely ignored the marine strains. The aim of this study is to gain some perspective of the diversity and geographical distribution of halophilic Bx by analysing the ssu-rRNA sequences from isolates collected from a very wide range of geographical and environmental sites.
The method I'm about to give is an extremely shortened, general version of what they actually did in the study. Samples were collected from marine, salt lakes and estuarine environments from literally all over the world (27 sites). They were mainly pelagic water samples however some from biofilms from oyster shells and a few from gills of aquarium fish. There don't seem to be any sediment samples taken which I thought would probably lower the diversity a bit but I'm sure they had their reasons. The water samples were filtered to leave the small Bx which were then enriched in a prey-seawater broth. The prey organism was Vibrio parahaemolyticus which had been shown through previous studies to be the best prey. They were then plated out using a double agar overlay method and the plaques derived where confirmed to contain Bx using fluorescent microscopy. Several rounds of sub culturing were performed to produce pure cultures. Culture suspensions were then filtered to get rid of any prey cells and centrifudged leaving Bx cells in the pellet. The supernatant was removed and a DNeasy Tissue Kit was used to extract DNA from the pellet. The ssu-rRNA gene sequences were then amplified using PCR with universal rRNA gene primers. They then used various programs to produce a phylogenetic tree.
One hundred and eleven novel marine strains of Bacteriovoracaceae were isolated from the marine environments. A phylogenetic tree based on the ssu-rRNA gene sequence data and using Geobacter as an outgroup revealed two main branches. One branch included one hundred and ten of the isolates and other branch included the other isolate, several freshwater isolates and two soil isolates. Analysis showed eleven distinct clusters (isolates with >96.5% similarity) however only eight contained two or more isolates. Almost half of the isolates fell into two clusters (3 and 9) which were also the most widespread geographically. Although most clusters were found in many different locations some were specific to certain areas. Cluster 5 was only found in estuarine waters and cluster 12 was only found in warmer tropical waters. Isolates from many different clusters could also be found in the same site.
The paper very convincingly suggests that Bx are incredibly more diverse and geographically dispersed in marine environments that previously thought. The experiment itself is going to be a huge underestimation of the true diversity of these bacteria. For instance Bx have been shown to be quite particular about choosing their prey and therefore it is quite possible that many are prey-specific to bacteria which cannot be cultured or are unknown. It is also suggested that this prey-specificity may be what drives their distribution. The divergent boundaries for clusters (>3.5%) were intentionally quite high so its possible that there could have been many new species isolated. I quite liked the paper as it looked at the phylogenetic tree of these marine bacteria which is incredibly important for understanding them and future studies. There doesn't seem to be many papers at all like this one as most studies seem to neglect the marine strains. I strongly agree that these bacteria could have a much more important role in nature than first thought. They are also being extensively studied for obvious reasons in the medical field as a possible solution to antibiotic-resistant Gram-negative pathogens.
A review of Pineiro, S. A. et al.(2007) Global survey of diversity among environmental saltwater Bacteriovoracaceae, Environmental Microbiology, 9(10), 2441-2450
2 comments:
Fascinating! We usually disregard these predators in consideration of carbon cycling etc. It would be great to have some idea of their actual abundance as well as their diversity. If they are really abundant, we would expect to see quite a lot of genes represented in metagenomes. Do the authors say anything about the genes appearing in GOS data?
They dont seem to mention whether they were present in the GOS data or not. However others similar papers have used many genes sequences which they have found from other databases like genbank. There was also a paper (Davidov et al (2006)) who suggests that they may have had a role in the mitochondrial endosymbiotic theory however I havent fully read the paper and dont know how far fetched the idea is.
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