A Microbial Biorealm page on the genus Bdellovibrio bacteriovorus
Higher order taxa
Bacteria; Proteobacteria; Deltaproteobacteria; Bdellovibrionales; Bdellovibrionaceae; Bdellovibrio
Bdellovibrio bacteriovorus: HD100, 109J
Description and significance
Bdellovibrios were discovered by Stolp and Petzhold in 1962, in an attempt to isolate bacteriophage from soil samples. Stolp and Petzhold observed unique plaques in their samples that took several days to develop and continued to grow for over a week, instead of plaques caused by bacteriophages that would appear within hours. A closer inspection of the plaques under a light microscope revealed cells that were small, highly motile, and vibrio-shaped. These cells were Bdellovibrios.
After the discovery of Bdellovibrios further observations revealed many interesting and unique properties. One property that makes Bdellovibrios interesting is that it is a parasite to other Gram-negative bacterias. Bdellovibrios have biphasic life-cycles that include an attack phase, and a free living and mobile phase.(2) The attack phase is when it attaches to another Gram-negative bacteria and imbeds itself into its periplasm, it then procedes to grow and replicate itself by degrading the host bacterium from the inside out.(1) In the free living and mobile phase, Bdellovibrios move about in search of host or prey bacteria so it can intiate attack phase again.(2) Each of these phases are of interest to researchers because it reveals unique cell-cell interactions and unusual cell metabolism.(1)
Bdellovibrio bacteriovorus is a small, curved, and highly motile Gram-negative bacteria approximately 0.2 to 0.5μm wide and 0.5 to 2.5μm long.(3) It has been found in many environments that include soil, sewage, and other terrestial and aquatic habitats. B. bacteriovorus has been observed to only attack Gram-negative bacteria which includes many plant, animal and human pathogens making it an execellent candidate as a biocontrol agent.(2) The study of its degradative enzymes and host targeting system has shed some insight in possible designs for new antimicrobial agents.(3)
The sequencing of the B. bacteriovorus HD100 genome was completed on 01/31/2004. The complete genome consists of a single circular chromosome that is 3,782,950 nucleotides long. The entire genome has a 50% GC content and contains 3629 genes which codes for 3587 proteins and 42 structural RNAs.(4)
Of the large number of proteins that the B. bacteriovorus genome encodes, many of them are degradative and lytic enzymes(2). These enzymes are necessary for B. bacteriovorus to breakdown its host or prey cell. Some of these enzymes needs to be secreted out of B. bacteriovorus and into its prey cell, while degradative products needs to be transported into the B. bacteriovorus from the prey cell cytoplasm. Therefore, the genome of B. bacteriovorus also includes a large amount of transport proteins. In fact, B. bacteriovorus has the potential protein secretory capabilities of at least five types of outer membrane secretion systems and four types of inner membrane secretion systems.(5)
Other than encoding for degradative enzymes and transport systems, the chromosome also contains multiple biosynthetic gene clusters that are necessary for the formation of the flagella and pilus. These gene clusters are essential for the motility B. bacteriovorus, which is important to its life cycle because it directly relates to its ability to find and approach the prey cell. However, the organism sheds its flagellum once it makes irreversible contact with its prey cell suggesting that the flagellum is not required for it to grow.(2)
Cell structure and metabolism
The general cell structure of B. bacteriovorus is small, curved, and highly motile Gram-negative bacteria approximately 0.2 to 0.5 μm wide and 0.5 to 2.5 μm long.(3) It has a flagellum that is membrane bound, allowing it to swim up to 100 body length per second while it looks for its prey.(9)
The unique biphasic life-cycle of B. bacteriovorus is directly related to its metabolism. In the free-living or mobile phase, B. bacteriovorus actively seeks other Gram-negative bacteria by chemotaxic means. During the mobile phase, most of the metabolic activity is involve in motility and prey detection.(7) In the attack phase, B. bacteriovorus burrows itself into the periplasm of the prey cell and begins to degrade the prey cell from the inside out. A large variety of metabolic activities occur during this phase.(2)
In the attack phase, B. bacteriovorus begins by attaching itself to the outer membrane of the prey cell via hook-like structures.(8) It then proceeds to secrete enzymes which breaks down a large enough hole in the outer membrane for B. bacteriovorus to enter. After entry into the prey cell, B. bacteriovorus sheds its flagellum and seals the hole in the outer membrane of the prey cell.(1) B. bacteriovorus then secretes degradative enzymes that act on the inner membrane of the prey cell allowing it to access the prey's cytoplasm. The degradation of the inner membrane causes the prey cell to become a bdelloplast, host cell of Bdellovibrios with a stable spherical structure, and also the destruction of the host cell's ability to generate energy.(1) At this point, B. bacteriovorus degrades and metabolizes the macromolecules and nutrients from the host's cytoplamic material allowing it to grow and begin DNA replication. B. bacteriovorus subsequently becomes a long filament and then fragments into individual mobile progenies which will lyse the remaining bdelloplast.(1)
The metabolism of B. bacteriovorus has many interesting areas. One of these areas is that B. bacteriovorus can survive while lacking numerous amino acid biosynthetic pathways. The reason for this is that B. bacteriovorus utilizes the metabolites of its prey for its own protein synthesis.(2) B. bacteriovorus is very efficient at utilizing the DNA, RNA, and other pre-existing precursors from the host cell. It can break down or rearrange these precursors for the biosynthesis of homologous polymers. The uptake of nucleoside monophosphates and glycerol phoshphates greatly reduces the amount of energy that B. bacteriovorus has to produce during nucleic acid synthesis and lipid synthesis. There have also been evidence that B. bacteriovorus incorporates intact fatty acid molecules from its host. Experiments have shown that the energy efficiency of B. bacteriovorus can be 0.8 to 1.5 times more efficient than bacteria growing in rich media.(6)
B. bacteriovorus seems to be pretty ubiquitous in nature and manmade habitats. They have been found in soil samples, rhizosphere of plant roots, rivers, oceans, sewage, intestines and feces of birds and mammals, and even in oyster shells and the gills of crabs.(9) B. bacteriovorus are able to thrive in almost any habitat, the general requirements are that there needs to be oxygen and some other Gram-negative bacteria present in its environment. Its optimal temperature is between 28-30C, making B. bacteriovorus a mesophile.(2)
B. bacteriovorus are somewhat tolerant to its environment. It can live in a range of salinity and are resistant to pollution of its environment.(2) During its attack phase or as bdelloplasts, B. bacteriovorus can tolerate dry spells and short periods of anaerobic environment. This makes B. bacteriovorus more tolerant to environmental changes than other permanantly aerobic bacterias.(9)
B. bacteriovorus has also been associated with biofilms. It seems that the rich environment of biofilms supply B. bacteriovorus with many possible host cells. Research has shown that B. bacteriovorus can significantly decrease E. Coli biofilm that is grown on stainless steel.(9)
B. bacteriovorus is not a known pathogen to humans. It is a known pathogen to Gram-negative bacteria, making it a possible biocontrol agent to many human pathogens.(2)
Application to Biotechnology
The major applications of B. bacteriovorus to biotechnology seems to be its potential as a biocontrol agent. B. bacteriovorus have been used in certain water treatment plants to reduce the number of Gram-negative organisms in the water. Agriculturally, B. bacteriovorus has been use to limit or prevent the start and spread of plant pathogens with certain crops. The use of B. bacteriovorus in agriculture requires extensive knowledge on the crop and its ecology; B. bacteriovorus may attack growth promoting Gram-negative organisms needed for healthy crops such as rhizobacterias.
Currently, the main field of research with B. bacteriovorus is its high potential as a biocontrol agent. Many experiments are performed to test how it specifically recognize its prey.(10) The goal is to provide possible elements to create more specific antimicrobial agents. It is also highly possible that a slight change in its genetic code can allow B. bacteriovorus to truly become a living antibiotic.
(1) Thomashow Michael F., Cotter Todd W., ‘‘Bdellovibrio Host Dependence: the Search for Signal Molecules and Genes That Regulate the Intraperiplasmic Growth Cycle’’, Journal of Bacteriology Sept.1992, p. 5767-5771
(5)Barabote RD, Rendulic S, Schuster SC, Saier MH Jr. “Comprehensive analysis of transport proteins encoded within the genome of Bdellovibrio bacteriovorus’’ Pubmed, 2007 Aug 14
(6)Rittenberg Sydney C., Hespell Robert B. “Energy Efficiency of Intraperiplasmic Growth of ‘‘Bdellovibrio bacteriovorus’’, Journal of Bacteriology Mar. 1975, p.1158-1165
(7)Lambert, C., Smith, M. C. M., & Sockett, R. E., “ A novel assay to monitor predator-prey interaction for Bdellovibrio bacteriovorus 109 J reveals a role for methylaccepting chemotaxis proteins in predation.” Environmental Biology, 5(2), 127.(2003, February).
(8)Fox, J. F. . Bdellovibrio make fast food of gram-negative bacteria . In Microbe Magazine. (2006) http://www.asm.org/
(9)YAIR Shemesh, YAACOV Davidov, SUSAN Koval, JURKEVITCH Edouard "Small eats big: ecology and diversity of Bdellovibrio and like organisms, and their dynamics in predator-prey interactions", Agronomie 23, (2003), p.433–439, http://www.agronomy-journal.org/index.php?option=article&access=standard&Itemid=129&url=/articles/agro/pdf/2003/05/A3505.pdf