Investigations into the life cycle of the bacterial predator Bdellovibrio bacteriovorus 109J at an interface by atomic force microscopy

Abstract

Atomic force microscopy was used to image Bdellovibrio bacteriovorus 109J, a gram-negative bacterial predator that consumes a variety of other gram-negative bacteria. In predator-prey communities grown on filters at hydrated air-solid interfaces, repeated cycles of hunting, invasion, growth, and lysis occurred readily even though the cells were limited to near two-dimensional movement. This system allowed us to image the bacteria directly without extensive preparation or modification, and many of the cells remained alive during imaging. Presented are images of the life cycle in two species of prey organisms, both Escherichia coli (a small prey bacterium that grows two-dimensionally on a surface) and Aquaspirillum serpens (a large prey bacterium that grows three-dimensionally on a surface), including high-resolution images of invaded prey cells called bdelloplasts. We obtained evidence for multiple invasions per prey cell, as well as significant heterogeneity in morphology of bdellovibrios. Mutant host-independent bdellovibrios were observed to have flagella and to excrete a coating that causes the predators to clump together on a surface. Most interestingly, changes in the texture of the cell surface membranes were measured during the course of the invasion cycle. Thus, coupled with our preparation method, atomic force microscopy allowed new observations to be made about Bdellovibrio at an interface. These studies raise important questions about the ways in which bacterial predation at interfaces (air-solid or liquid-solid) may be similar to or different from predation in solution.

FIGURE 1

Diagram of the two-stage life cycle of the bacterial predator Bdellovibrio bacteriovorus, which consists of a free-swimming stage spent living in water or soil, and a growth stage spent inside its prey bacterium. A Bdellovibrio predator and the killed prey cell in which it is growing are together termed a bdelloplast.

FIGURE 2

Bdellovibrio bacteriovorus and invaded and uninvaded prey E. coli were imaged by atomic force microscopy in tapping mode (amplitude image). Bacteria were rinsed and dried on a smooth mica surface before imaging. (A) The prey E. coli, (B–D) the predator Bdellovibrio, and (E) invaded prey cells (bdelloplasts) were all observed. The arrow in E indicates a bdelloplast inside of which the predator was visible; a second predator was seen attached to the external surface of the cell. Another bdelloplast was visible at the top left, identified by its roundness, bumpy shape, and smooth surface texture.

FIGURE 3

The life cycle of Bdellovibrio in a population of E. coli was imaged by atomic force microscopy in contact mode (height and deflection images). Solutions of predator and prey were mixed and deposited on smooth, small-pore filters over DNB plates. At various times over several days, filters were removed to the AFM and imaged. (A) Prey E. coli grew quickly on the filters to form a continuous layer of cells. The total depth range of the height image was only ≈200 nm, compared to a cell height of ∼400 nm, which indicated that the cells were all roughly the same height and were therefore growing next to but not on top of each other. (B) A few invaded cells, or bdelloplasts, were initially present in the population of healthy cells. (C) Given a few days to hunt in the monolayer of prey, Bdellovibrio invaded every cell, and only bdelloplasts and predators were seen. (D) Eventually only the predator bacteria and some cell debris remained. The total depth range of the height image was ≈150 nm. The exact time elapsed between different stages depended upon the concentration and vigor of the two initial bacterial cultures, as well as the location on the filter that was imaged.

FIGURE 4

High-resolution AFM images of a mixed E. coli-Bdellovibrio system, taken in contact mode (deflection). Several bdelloplasts were visibly mixed in with healthy prey cells. The bdelloplasts were round or irregular in shape and had a smooth surface texture, whereas the uninvaded cells generally had a more oblong shape and a rough surface texture. Furthermore, the growing Bdellovibrio were visible inside of many of the bdelloplasts as smaller, oblong, or coiled shapes. Arrows indicate bdellovibrios that were in the process of attacking prey cells.

FIGURE 5

An advancing “tide” of bdellovibrios and bdelloplasts overtaking a flat expanse of uninvaded prey cells. In the deflection picture at right (B), it is clear that the uninvaded E. coli (top left) were slightly wrinkled in texture, whereas the bdelloplasts (bottom right) were smoother and more rounded. Notably, in the height image at left (A), the interface between the invaded and uninvaded cells, where the attacking predators and newly invaded bdelloplasts are located, was raised compared to the cells on either side by 150–250 nm. The prey bacteria, which have a life cycle with a much shorter period, multiplied at the edges of the circular community away from the predator bacteria at the center, and thus a gradient of predation stages was present on the filter.

FIGURE 6

The life cycle of Bdellovibrio in a population of Aquaspirillum serpens imaged by atomic force microscopy in contact mode (height and deflection). Again, solutions of predator and prey were mixed and deposited on smooth, small-pore filters over DNB plates, and at various times over several days, filters were removed to the AFM and imaged. (A) Healthy prey Aquaspirillum were very long and lumpy, and unlike E. coli many grew in multiple layers on top of the filter. The total depth range of this height image was ≈400 nm, and in other fields was >600 nm, which indicated that the cells grew on top of each other. The presence of “lumps” in the uninvaded cells made bdelloplasts difficult to identify. (B) Two Bdellovibrio invaded the same Aquaspirillum. (C) A long coil of growing Bdellovibrio was visible inside of the Aquaspirillum bdelloplast, as proposed earlier from electron microscope images. (D) As with E. coli mixed monolayers, given a few days to “graze” on the prey, Bdellovibrio invaded and consumed every cell. One remaining spirillum was being attacked and consumed at the left side of the figure. Note that because the Aquaspirillum prey cells grew three-dimensionally on top of the filter, the predator cells were also piled on top of each other and the cell debris. The total depth range of this height image was ≈1.2 μm, and in other fields was at least 800 nm.

FIGURE 7

Images of wild-type and host-independent mutant Bdellovibrio taken in contact mode (deflection). The morphology varied widely. (A) Cocultured with E. coli on filters over DNB plates, the wild-type Bdellovibrio consumed all of the prey. (B) These wild-type Bdellovibrio was grown in solution with E. coli, and then after clearing, the solution of prey were spotted onto a filter and imaged. (C and D) The host-independent mutant Bdellovibrio stuck together on filters, even when pelleted, rinsed, and deposited on plates with minimal nutrients. (E and F) These mutants had flagella and, when left to grow on nutrient media plates overnight, were very long.