The predator keeps its prey closed until the meal is over
Bdellovibrio bacteriovorus is a bacterium that grows inside the outer layers of another bacterium. It makes a controlled entrance, remodels the victim’s envelope into a rounded container, and uses the prey’s contents to produce new predators. Only when those offspring are ready does the envelope break open.
The timing is central to the strategy. Destroying the victim immediately would release its contents and remove the protected space in which the predator grows. Keeping the envelope intact does not keep the prey alive. It preserves a shelter around a meal.
This is bacterial predation, rather than infection by a virus or hunting by a tiny animal. The attacker is itself a single bacterial cell. Its unusual life cycle depends on a structure that many of its victims possess: two membranes with a narrow compartment between them.
“Inside” means between the membranes
A typical Gram-negative prey cell has an outer membrane, a supporting cell wall, and an inner membrane enclosing its cytoplasm. The space between the membranes is the periplasm. Bdellovibrio enters this region instead of moving straight into the central cytoplasm.
That placement makes the attack easier to follow. The predator occupies part of the victim’s envelope while dismantling and consuming material from the victim’s interior. It is enclosed by the prey without simply floating among all the prey’s original internal contents.
The double-membraned envelope also helps explain the limits of its menu. Many Gram-negative bacteria, including E. coli, can be prey. Gram-positive bacteria have a different outer structure and are not its usual targets. Human cells are not the cells this invasion mechanism is built to exploit.
Contact must become a workable grip
During its swimming phase, Bdellovibrio is roughly a micrometer long and uses a flagellum for movement. It encounters other cells and responds to their surfaces. An encounter may end with the predator leaving; contact is not automatically a successful invasion.
That is a difficult recognition problem because bacterial surfaces carry different combinations of sugars and proteins. Research published in 2024 identified 21 members of a family of adhesive fibre proteins in the studied strain. Their varying tips provide multiple possible interactions with prey surfaces.
One protein tip bound sugars from Proteus mirabilis in a laboratory test. The finding supports a specific molecular interaction, but does not explain every susceptible species or every failed encounter. The predator also has pili involved in attachment. Together, these structures help establish contact without acting as eyes, teeth, or a conscious choice of victim.
Entry is controlled, and its driving force remains uncertain
Once attached, the predator loosens the prey’s mesh-like cell wall with enzymes and passes into the periplasm. High-resolution imaging has revealed a portal fitting closely around the entering cell. After entry, the opening seals sufficiently for the prey envelope to remain a container.
An intuitive explanation would be that the flagellum pushes the predator through, or a pilus pulls it in. But cryo-electron tomography has shown the flagellum retracting into the predator’s own envelope before entry, while pili appear to disassemble. The precise force moving the whole cell through the opening remains unresolved.
Read an invasion diagram in two layers: the observed portal and cell arrangement, then the proposed force moving the predator through it. The first is supported by imaging; the second remains unresolved in the account described here. Drawing a drill or winch would supply an explanation the observations do not yet establish.
The rounded victim is a bdelloplast
The invaded prey often changes from a rod into a rounded structure called a bdelloplast. Enzymes cut some of the cross-links that maintain the original cell-wall shape. The wall becomes less rigid while the surrounding envelope remains intact enough to contain the growing predator.
Experiments removing two wall-remodeling enzymes showed why this change is useful. Altered predators could still enter E. coli, but invasion took longer and many victims failed to round up. A second predator was also more likely to enter prey already occupied by the first.
Wall modification therefore contributes to both entry and control of a limited food supply. One prey cell cannot provide unlimited resources to multiple predators. Making further invasion harder can leave more of that supply for the first occupant and its eventual offspring.
Researchers have also observed a small vesicle left near the entry site, associated with components of the predator’s envelope and adhesive machinery. It helps locate parts of the invasion apparatus. Its exact function and every step of sealing are still being investigated.
One hunter grows into several
Within the bdelloplast, the prey’s proteins and genetic material are broken down into smaller building blocks. The predator takes up resources and lengthens into a filament, rather than immediately dividing into two equal cells.
When growth is complete, that filament partitions into new predators. The number depends on the resources available from the victim; it is not a fixed brood size. Some E. coli experiments produced about four predators per prey cell on average, but a schematic with four attackers is not a universal rule.
Finally, the remodeled envelope breaks down and the new cells escape. The cycle unfolds over hours rather than the few seconds a compressed animation might suggest. Swimming cells are the dispersal stage; the enclosed filament is the growth stage.
What a microscope image does and does not establish
The bright blue and orange cover is an illustration, not a natural-color photograph of an attack. Fluorescent images used in experiments likewise often show labels introduced to distinguish predator, prey, and particular proteins.
A rounded bacterium alone is not proof of a bdelloplast. Identifying one requires evidence that predation occurred. Conversely, the absence of a conventional animal mouth does not make the process merely “living together”: the predator consumes the victim and releases its own descendants.
Bdellovibrio has attracted interest because it can attack some antibiotic-resistant bacteria. That interest does not establish a ready treatment for people. Its prey range, delivery, interaction with an immune system, and effects on beneficial bacteria all matter. A laboratory attack is one part of a much larger biological question.
Follow the sequence as attachment → controlled entry → enclosed growth → offspring release. Preserving the prey’s envelope serves the middle stages; breaking it open completes the last. The fungal ring-trap explanation describes another microbial predator whose capture and growth stages operate on very different timescales.
