Growth Factors

Prey abundance:

The question of the minimum prey abundance for Bdellovibrio's normal growth has been of some debate. It is clear that a large number of prey is required and without a suitable concentration they are susceptable to starvation and death, still this minimum threshold number is unknown. In the lab environment, it has been reported that survival is possible with concentrations of prey as low as 2-5 x 10ex4 cells per mL (Varon et al, 1984). Yet other studies have shown that numbers of 10ex6 and even upwards of 10ex7 cells per mL are required for normal population growth (Varon et al, 1978). The question arises, though, that if prey concentrations are driven down to 10ex2 to 10ex3 cells per mL as it has been found, then are Bdellovibrio also developing normally at these decreased levels (Rice et al, 1998)? Under conditions of starvation, the population of Bdellovibrio has been found to decrease by half every 5-6 hours (Pan et al, 1997).

The type of bacteria also plays a significant role, as Bdellovibrio is only able to prey on Gram-negative species. Of total species present in a given environment, it has been estimated that the range of susceptable bacteria present ranges from 40% to 76% (Taylor et al, 1974). More recent studies have even projected these values to be as high as 73% to 85% (Rice et al, 1998).

If necessary, Bdellovibrio can make due without viable prey to multiply within. Given certain circumstances, it is often able to prey on non-viable cells (Varon et al, 1969). Other evidence indicates that non-culturable cells also are adequate prey, which would mean the above estimates would be lower still than the actual percentage of susceptable bacteria.

Surface Association:

Many microbial species rely heavily upon submerged surfaces, such as oyster shells, for their habitat, and Bdellovibrio appears to be no different. In a recent study, Williams (et al, 1995) studied the association of Bdellovibrio with various surface types, and suggested that the majority of marine Bdellovibrio are associated with submerged surfaces.

By associating themselves with these submerged surfaces, they can successfully seek shelter from extreme conditions, such as the agitation incurred from currents, and harsh seasonal changes (see Life Cycle and Development). As such surfaces are shelters for so many species of bacteria, Bdellovibrio may also obtain essential nutrients here, regardless of the time of year.

Also, these associated bacteria make ideal prey as they as not motile and are easily caught. Oysters are found to be a particularly favorable surface as they naturally have a rough surface due to the presence of chitin, and this provides many contours in which Bdellovibrio may hide.

Temperature:

Bdellovibrio have been found to be mesophilic, normally found in the range of 25-35 degrees Celcius (Varon and Shilo, 1968). It appears that the optimal conditions for growth are more between 28 and 30 degrees, and at temperatures above 37 degrees and below 10 degrees we see poor growth (Burnham and Conte, 1988). With regards to surface associations, decreased temperatures caused decreased associations, and at the point of 6 degrees Celcius, the bacteria is not found attached at all (Williams and Kelley, 1995). Some psychotropic strains of Bdellovibrio have been found, which are active at temperatures below 10 degrees Celcius, however these strains are found to have increased activity when introduced to temperatures above 20 degrees (Fratamico and Whiting, 1994). Host composition may play a role in the temperatures that are tolerable for any given strain, as lipid membrane derivation is directly from the host.


Other considerations:

1. Bdellovibrio is obligately aerobic, though it seems to have a preference for low oxygen environments, which may reflect an adaptation to intracellular growth (Seidler et al, 1969).

2. Recent findings show that bacteria that have paracrystalline protein surface layes (S layers) may have increased resistance to attack by Bdellovibrio, which may require prey without this layer at all in order to survive (Koval and Hynes, 1991).

3. Evidence has shown that only certain serogroups of Escherichia and Salmonella are susceptable to attack, indicating a requirement for certain receptor sites on the prey wall (Fratamico and Whiting, 1994). Other studies show that increased attachment can occur on prey with no O-specific side chains and also have a complete R-core, indicating the lipopolysaccharides may be involved in the attachment receptor (Varon and Shilo, 1969).