The Genus Bdellovibrio
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The Genus Bdellovibrio EDOUARD JURKEVITCH
Introduction Bdellovibrio spp. are Gram negative, motile, and uniflagellated bacteria. What characterizes this bacterial genus as unique is its predatory behavior: Bdellovibrios attack other Gram negative cells, penetrate their periplasm, multiply in their cytoplasm, and finally burst their cell envelopes to start anew (Figure 1). Although many predator bacteria may lyse other bacteria, and many “micropredators” have been described, these Bdellovibrio-like organisms can grow without bacterial prey and do not divide inside the periplasm (Varon and Shilo, 1978). Bdellovibrio research began with their serendipitous discovery by Stolp and Petzold (1962). The physiology, ecology, taxonomy, interactions with prey, and cell cycle of Bdellovibrio were established in the 1960’s and 70’s mainly by the groups of Shilo and Varon, Stolp and Starr, Rittenberg, Hespell, Diedrich, Ruby, Tudor, Thomashow, and more recently Williams. Bdellovibrios can be described as predators, or parasites, or symbionts (Starr, 1975). In this review, the terms predator-prey and parasitehost will be used interchangeably. Bdellovibrios are found in wet, aerobic environments. As described in other sections, their importance in shaping or influencing bacterial community dynamics and structure is still unknown. Similarly, the way these predators interact with potential prey, their survival mechanisms, and their preferred habitats in natural settings or man-made biotops are poorly understood. Particularly fascinating is the two-phased cell cycle of bdellovibrios, which are dimorphic. During the “attack phase,” small cells (0.5 to 1.5 mm long, 0.5 mm in diameter, Figure 2) swim at speeds up to one hundred body lengths◊sec-1 (Stolp, 1967) to find prey. Prey-encounter is apparently by random collision, as evidence of chemotaxis has not been detected. After encountering and attaching to a substrate cell, a predatory cell penetrates the periplasm, shedding its long-sheathed flagellum in the process. The “growth phase” can then be initiated.
The invaded bacterium usually rounds into the ensemble predator-prey, called a bdelloplast (Figure 3). Within this confined space Bdellovibrio engages in restricted development, turning the short cell into an unseptated filament at the expense of the host’s cytoplasm (Figure 4). After a lag phase of about 45 min, DNA begins to replicate within the bdelloplast and proceeds for a couple of hours along with cell growth, and then the filament divides by multiple fission into progeny attack cells, the number determined by the size of the host. Progeny bursts from the ghost cell, ready for another attack cycle. Within the bdelloplast, the bdellovibrio cell is protected from photooxidation (Friedberg, 1977) and phage attack (Varon and Seijffers, 1975) and shows increased resistance to pollutants (Varon and Shilo, 1981). Hostindependent mutants growing axenically can be isolated. The cell cycle can be extended to include the development of a cyst-like
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