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. |
