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Bacteria Beware!

The Life and Times of Bdellovibrio


by Laurel Crosby

A mild ocean breeze plays over the water surface, dispelling any notion that danger lurks in the murky depths. However, a gruesome event is about to occur as a silent attacker speeds forth toward an unsuspecting victim. In a furious collision, the savage meets its target and whittles its way into the body of the innocent prey. Once inside, the transformation begins - the predator ceases its frenzy and prepares to multiply. The host is reduced to a protective cocoon, supplying food and shelter for the growing parasite. Within hours, the nourishment is drained and the ghost-like shell of the host bursts open to release a new generation of deadly predators. And all the while, the waters remain still...

While the above description may sound like the season's next cinema thriller, it actually describes the life cycle of the bacterium, Bdellovibrio. The Bdellovibrio (which literally means "curved leech") make a living by attacking and devouring other bacteria, and are found in diverse environments such as marine and fresh waters, sewage, and soil. Bacteria of these type are characterized by two distinct stages in their life cycle, a predatory "attack" phase, and a parasitic "growth" phase.

The attack phase cell has a curved rod shape of approximately 1.4 micrometers (or microns) in length, and has a single hair-like projection called a flagellum. This bacterial flagellum rotates like a corkscrew to propel the bacterium at a rate of 100 microns per second. Considering the size of the cell, this corresponds to an incredible 70 body lengths per second! These highly motile attack phase cells have no sense of direction; instead, the flagellum propels the cell in whatever direction it happens to be pointing. Success in finding prey, therefore, is limited to "bumping" into a suitable prey cell that just happens to be in the right place at the wrong time. When a prey cell is encountered, the Bdellovibrio continues to rotate and bore its way into the prey. Once inside, the attacker loses its flagellum and prepares for the multiplication process.

The Bdellovibrio growth phase is the period when new cells are created and requires the parasitism of a suitable host cell. The attacker cells are not especially particular about the prey, except that it must be of the Gram negative type (i.e. having a thin cell wall and characteristic outer membrane.) When the Bdellovibrio enters, the host bacterium dies and bloats into a spherical shape called a bdelloplast. Essentially, the host cell loses its structural framework because it is being eaten from the inside out. The growing Bdellovibrio is now considered a parasite and continues to elongate into a filament. When the nutrients (proteins, lipids, structural polymers, RNA, and DNA, etc.) are exhausted from the host body, the filament partitions into the smaller attack phase cells and are released into the environment.

The unique lifestyle of the Bdellovibrio is of keen interest to scientists. We are curious as to why these organisms require a bacterial host in order to multiply. Could the host cell be supplying an essential nutrient? A vitamin? Perhaps a protein or two? Studies show that normal Bdellovibrio can multiply without living host cells under certain conditions. If the soluble components are extracted from host cells and fed into a beaker containing only Bdellovibrio, the Bdellovibrio are able to grow and multiply. This suggests that there is a factor which Bdellovibrio needs in order to survive - and only prey bacteria can provide! We are still searching for the factors that control the host-parasite relationship and we are continuing to make new discoveries. Meanwhile, Bdellovibrio remains a fierce predator in the natural environment.


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