Microbial Predators And Parasites Biology Essay

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The life-cycle of Bdellovibrio is composed of two distinct stages, the Extraperiplasmic attack phase and the Intrapreiplasmic growth phase. During the first phase, the predator locates and collides with bacterial prey, attaches and penetrates the membrane. Subsequently, the Bdelloplast is formed, in which the predator assimilates host macromolecules for development of the filament inside the host cells, followed by the lysis of the host and release of progeny cells. Stolp and Starr () were the first to describe the interaction between of the of Bdellovibrio life cycle as observed with a phase-contrast microscope. Following an extracellular attack phase, where the parasite penetrates the host membrane and enters the cell, Most early information regarding this subject was observational, 11 but much research has been carried out to elucidate the stages of the of the Bdellovibrio life cycle, shown in Figure () below. In the following section, the exact structure of the flagellum and basal apparatus is not discussed in detail, but this has been studied in detail by L. Thomashow and Rittenburg.

Attackphase

bdellovibrios are capable of carrying out a wide range

of cellular functions: they are motile, they can elongate, they

respire a number of substrates, and they synthesize macromolecules,

including RNA, protein, and peptidoglycan 21, 30). They do not, however, initiate rounds of DNA

replication. Indeed, all attempts to culture wild-type Bdellovibrio

isolates axenically on commercial media have failed

(5, 8, 14, 21). One possible explanation for this host dependency

is that wild-type bdellovibrios might be auxotrophs

that require a nutritional factor that is absent from standard

complex media. Alternatively, attack-phase cells might locked into a search mode, requiring a signal from the host initiate IP growth. Such a negative regulatory model was

initially suggested by Shilo (26). This model takes account the fact that mutations are generally more likely result in a loss of function than in a gain of function.13

Flagellum

Bdellovibrio bacteriovorus exhibits a single, polar flagellum with an unusual thickness and dampened waveform morphology (fig-) that is essential to predation in liquid environments(T2006). The flagellum is sheathedAs early as 1963, Stolp and Starr recognised that it was very similar to that of other bacteria, and more recent studies (Thomashow and Evans 2006), following the gene sequencing of strain HD100, have further revealed its characteristics. The flagellar filament of Bdellovibrio bacteriovorus HD100 and 109J is composed of six different flagellins, encoded by a cluster of six flagellin genes, fliC 1-6. (Lambert and Evans, Rendulic 2004) By systematically inactivating each of the flagellin genes, Lambert et al. (2006) determined that loss of certain genes, fliC4 and fliC5, reduce motile speed and predatory efficiency by altering the shape of the filament shape or the length of the flagellar filament. Inactivation of fliC3 proved impossible in host-dependant cultures, but produced non-flagelated and consequently non-motile mutants when using a host-independent 109J strain. A change to any of the other genes, however, confers no observable alteration in the structure of the flagellum, although a slight reduction in speed was observed in some. 1 Lambert 2006 characterising/rendulic. (Figure?) Lambert and Evans (2006) showed that a non-flagelated mutant applied to a lawn of prey bacteria was still able to attack and invade, however flagella are essential for predation in liquid media.

Chemotaxis

When Host-dependant strains of Bdellovibrio are lysed from host cells, they must find and penetrate new prey or risk starvation. A very high concentration of prey bacteria (approximately 1.5 x 105 cells/ml) is required for Bdellovibrio to encounter cells by random collision alone, and even under these conditions; the cells have only a 50% chance of survival. (Hespell) Since Bdellovibrio lacks genes for known quorum-sensing compounds, it seems unlikey that it responds to high densities of prey cells, despite evidence of chemoattraction toward high concentrations (108)(Rendulic) (Straley, LMare pure compounds) It still remains unclear whether Bdellovibrio employ a chemotaxis system to locate and respond to high concentrations of potential prey bacteria.7 Studies have demonstrated that Bdellovbrio is attracted (and in some cases repelled by) to specific amino acids and other compounds such as NH4+, Mg2+ and K+, and concluded, perhaps surprisingly, that B. bacteriovorus is not attracted to amino acids that it is able to readily oxidise. However, LaMare et al. also showed that Bdellovibrio was attracted to amino acids that susceptible bacteria such as E. coli are also attracted to, rather than amino acids that are commonly excreted by bacteria or to prey cells themselves which may present a disadvantage. It is also possible that attraction to these compounds allows the predatory cells to maintain themselves while in starvation conditions, rather than acting directly as chemo attractants. In addition, Straley et al. have suggested that chemoattraction to certain inorganic compounds may prevent predators from being washed from soil into open water, where random collision with prey would be dramatically reduced. The same authors have also suggested aerotaxis as a tactic for locating potential prey and allowing aerobic Bdellovibrio to locate regions of optimal oxygen concentration in order to maintain motility.

To evaluate the data shown in the Tables

and in Fig. 5 to 7 in terms of the usefulness of

chemotaxis for the location of prey by B. bacteriovorus,

we need to translate them into distances

through which prey can be detected by

bdellovibrios. As pointed out by Hespell et al.

(20), the probability that a randomly swimming

bdellovibrio will collide with a prey cell increases

in proportion to the square of the radius

of the prey cell. If the prey is chemotactically

attractive to bdellovibrios, its "radius" includes

the radius of the sphere through which the

attraction is effective. Thus, a chemotactic attraction

that could substantially increase the

effective radius of the prey cell would greatly

reduce the concentration of susceptible bacteria

that must be present in an envrionment for

bdellovibrios to be able to survive (20).

Varon and Zeigler Bacterial Predator-Prey Interaction at Low Prey Density, Applied and Environmental Microbiology, 36 (1): 11. (1978)

Why Microbial Predators and Parasites do not Eliminate their Prey and Hosts

Annual Review of Microbiology

Vol. 35: 113-133 (Volume publication date October 1981)

Hespell, R. B., M. F. Thomashow, and S. C. Rittenberg.

1974. Changes in cell composition and viability

of Bdellovibrio bacteriouorus during starvation. Arch.

Microbiol. 97:313-327.

Isolation and Composition of Sheathed Flagella from

Bdellovibrio bacteriovorus 109J

LINDA S. THOMASHOW AND SYDNEY C. RITTENBERG JOURNAL OF BACTERIOLOGY 164(3) 1985, p. 1047-1054

Katy J. Evans, Carey Lambert, and R. Elizabeth Sockett Predation by Bdellovibrio bacteriovorus HD100 Requires Type IV Pili Journal of Bacteriology, July 2007, p. 4850-4859, Vol. 189, No. 13

SUSAN C. STRALEY, ARTHUR G. LAMARRE, LOWELL J. LAWRENCE, AND S. F. CONTI

Chemotaxis of Bdellovibrio bacteriovorus Toward Pure Compounds;JOURNAL OF BACTERIOLOGY, Nov. 1979, p. 634-642

Thomashow, M. F., and S. C. Rittenberg. 1979. The intraperiplasmic

growth cycle-the life style of the bdellovibrios, p.

115-138. In J. H. Parish (ed.), Developmental biology of prokaryotes.

Blackwell Scientific Publications, Ltd., Oxford.

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