Bdellovibrio bacteriovorus is a very curious bacterium with a very unusual life-cycle! It is a parasite or predator of other bacteria. It
is a member of the Spirillum Group or spirilla, vibrioid (comma-shaped) or helical aquatic and Gram-negative bacteria. The group
includes the fresh-water Spirillum and Aquaspirillum, the marine Oceanospirillum and the soil-dwelling Azospirillum. These bacteria
are aerobic (require oxygen for respiration) chemoheterotrophs (obtain energy and carbon from organic sources made by other
organisms). The group also includes Campylobacter, a microaerophile (prefers small amounts of oxygen, or microaerobic
environments) and a parasite of mammals (an intestinal parasite that causes diarrhoea), Vampirovibrio and Bdellovibrio.
Bdellovibrio are very small bacteria (0.3 to 0.4 micrometres wide, and 0.8-1.2 micrometres long) with a single sheathed polar
flagellum. (Sheathed flagella are covered by an extension of the cell-surface membrane and so appear thicker, more often
bacterial flagella are non-sheathed, as in Escherichia coli). Bdellovibrio is a parasite of other Gram-negative bacteria (bacteria in
which the 'cell wall' consists of an outer membrane and a thin peptidoglycan layer in the perisplasm between the outer membrane
and cell membrane (inner membrane or cytoplasmic membrane) - see bacterial envelopes). The life-cycle is illustrated below:
The stages are as follows:
- The Bdellovibrio rams the target cell at high speed (Bdellovibrio is a very fast swimmer, reaching up to 100 cell-lengths
per second, or about 100 micrometres per s; in contrast, a typical prey like an Escherichia coli cell, though larger moves
at about 10 cell lengths per s).
- It attaches immediately to the host cell envelope by its non-flagellated end and rotates about its long axis at over 100 rps,
presumably drilling into the host cell wall. About 5-10 minutes lapse between attachment and the initiation of penetration.
Enzymes (proteases, lipases and muramidase) help punch a hole in the host cell wall. Penetration takes several seconds.
- The host cell rounds up (to form a so-called bdelloplast) as the penetration enzymes damage components of the
peptidoglycan layer of the cell wall, a hole appears in the host's cell wall at the site of attachment and the parasite enters
the host periplasm. The flagellum is lost during penetration, and remains outside the host. The cytoplasmic (inner)
membrane is not penetrated, but it becomes porous and leaks cytoplasmic contents into the perisplasm where they are
digested and absorbed by the growing Bdellovibrio.
- The Bdellovibrio grows without cell division, and so elongates to form a helical filament with multiple copies of the genome.
- As the food supply becomes exhausted, the filament divides by multiple fission, splitting into several new Bdellovibrio,
each of which has one genome (a single molecule of DNA) and grows a flagellum.
- The host cell swells and eventually the new bdellovibrios are liberated from the degraded host cell as its cell wall ruptures.
The whole life-cycle takes about 4 hours.
Bdellovibrio occurs in soil (at about 100 to 100 000 cells per gram of soil), sewage, pond water and sea water. It can not use
carbohydrate as a fuel, but is proteolytic, digesting proteins and absorbing the products (short peptides and amino acids) and
using them as carbon and energy sources. They derive all their nutrients from the host and can only respire aerobically (via the
TCA cycle). The yield or synthesis of dry cell mass per mole of ATP is about 10 in most bacteria, but is 20-30 for Bdellovibrio,
since they do not synthesise all their components, but assimilate nucleotides directly from the host and incorporate host lipids,
which are modified or incorporated directly) - a very efficient system!
Other Predatory Bacteria
Vampirococcus attaches to the surface of its prey/host, where it draws out the cytoplasm contents and reproduces. Daptobacter
penetrates the cytoplasm of its host, punching through the cytoplasmic membrane, and devours the cytoplasm. Myxobacteria
release enzymes that can degrade dead and living bacteria, dissolving them into nutrients that can be absorbed. Whole groups
or rafts of myxobacteria glide along the surface, surrounding and digesting other bacteria that stand in their way!