Life Cycle and Development

Intraperiplasmic growth phase:

Bdellovibrio enters its hosts for reproduction, penetrating the Gram-negative cell wall of its prey so it can multiply within the periplasm. In this, the intraperiplasmic growth phase, the cell finds itself not only physically protected, but within a nutrient favorable environment.

Prey are located by mere chance collision, and then become attached by an unknown receptor (Fratamico and Whiting, 1994). As a result of this, large prey are generally the targets of attack (Burnham and Conte, 1983). Penetration occurs quite rapidly from this point. The process of attachment and penetration involves many enzymes, including: glycanase, peptidase, LPS degrading activity, an N-deactylase, and an acylase (though more are probably involved as well; Burnham and Conte, 1983).

Once inside, the prey's own cytoplasmic contents can be used as the lone nutrient source, making growth highly efficient; monomeric units for DNA, RNA, lipids and proteins all can be derived from the host, which results in a lipid composition that reflects that of the invaded cell (Burnham and Conte, 1983). For the majority of the growth phase the invading cell elongates as a coiled, aseptate filamentous body and becomes many cell units long. The size this single-cell filament grows to, and thus the number of progeny, is proportional to the volume of the host cell, as Bdellovibrio is optimized to utilize all the nutrients it can obtain (filamentous cells up to 90 cell units in length have even been reported; Gray et al, 1989). Replication follows the typical exponential rate that is typically seen in bacteria, which suggests that there are multiple elongation sites present, as this type of growth is not possible from merely two points (Gray et al, 1989). At the end of the growth phase, the elongated filament fragments to motile, single-unit attack-phase cells, destroying the host cell from within and releasing itself into the surrounding environment.

Free-living attack phase:

Once it has burst from host cell, Bdellovibrio begins what is called the attack-phase of its life cycle, where it is not free-living in the water column. This stage of development is now exposed to the elements, and also subject to starvation and death, unlike its host-residing counterpart.

Unbalanced growth:

An unusual characteristic of the Bdellovibrio growth cycle is its unbalanced growth. This occurs during the intraperiplasmic replication and is in reference to the proportions of DNA and cell volume.

Gray (et al, 1989) found that the rate of DNA replication and protein synthesis were both exponential in the filamentous cell, but DNA was synthesized at a greater rate. They also found that the number of chromosomal origins per unit cell mass was not constant, but actually increased with each round of replication. Likewise, it was concluded that there must also be multiple elongation sites along the filamentous wall for exponential increase of cell mass. The original ratio of DNA:protein was only restored when protein synthesis was given time to recover after the septation period, when DNA replication has been terminated.

A few ideas have been suggested as to why the DNA and protein growth rates are not coordinated. Gray suggested that it is perhaps an adaptation to produce the maximum progeny possible from the determinant amount of substrate available, and so the minimum sustainable cell mass is maintained. Alternately, it was suggested that the attack phase is already larger in mass than the necessary "initiation volume" for replication, and so DNA synthesis is uncoupled from cell volume.

Overwintering:

Bdellovibrio is unable to tolerate extremes of temperature, and during winter months, its numbers decrease to levels that are nearly undetectable, as shown by Williams et al (1995). To survive these cold months it is now thought that Bdellovibrio associate themselves with the inner surfaces of shellfish such as oysters, where they may seek shelter (see Growth Factors). It is these shell surfaces that act as a reservoir for the spring population once the waters warm up and prey abundance increases.