Lysis of bacteria by the parasite Delovibrio bacteriovorus.

Normal speed. Slow motion two to eight times slower than normal. Delovibrio bacteriovorus is a predatory bacterium characterized by its high motility.

It inhabits soil and water and it obtains its nutrients by lysing other bacteria. If the movement of the bacterial parasite is slowed down by special treatment during the preparation,

its vibrio-like shape becomes apparent. This electron micrograph shows a typical cell of Delovibrio bacteriovorus. It is about one and a half microns long and 0.3 of a micron in width.

The strikingly thick flagellum enables the parasite to move rapidly. Here one can see the attack upon Pseudomonas bacteria. The organism attaches itself with the end opposite the flagellum. Immediately after contact with the host bacterium, the parasite rotates around its longitudinal axis.

Occasionally it detaches itself and attacks another host cell. Only gram-negative bacteria are infected. However, the various strains of Delovibrio can differ in their activity spectra. In the presence of non-susceptible bacteria, the parasites remain passive and no attachments occur.

With susceptible bacteria, such as spirillum serpens, attachments occur as soon as the host bacteria and the parasites are brought together in a preparation. Individual host bacteria may be attacked by one or more parasites.

In the presence of numerous parasites, the host bacteria are covered within a matter of seconds. Again, this slow-motion scene shows the attachment and the rotation of the parasites.

In the middle of the picture, a long parasite which has been inhibited in cell division is rotating about its longitudinal axis like a drill. Attachment and rotation of the parasites have been slowed down by a factor of eight.

Measurements and calculations reveal that as many as a hundred revolutions per second are possible. As far as the parasites' movement is concerned, the maximum speed has been estimated to be 100 body lengths per second.

Fast motion effect, 1 to 96 to 1 to 720. The processes taking place after parasite-host contact

are now shown in time-lapse cinematography of various host bacteria. First, observing Salmonella tifimurium. After attachment, the parasites penetrate the host cell. At the same time, the bacteria convert to spheroplasts.

The host cell in the upper right is simultaneously invaded by several parasites. The hosts are Escherichia coli, and they exhibit the transformation to spheroplasts very clearly.

Infected cells of Pseudomonas floresens respond accordingly. The short rods turn into spheroplasts already before and during the penetration of the parasites.

However, the long cell in the middle of the field is at first irregularly deformed.

The variable behaviour of the host bacteria depends upon their age and physiological condition.

The unusually large bacterium, Spirellum serpens, was found to be especially suitable for studying the host-parasite interaction. The short cell in the middle reacts fastest to the attack. After about 20 minutes, the first deformation takes place, and 10 minutes later, the host bacterium has become completely spherical.

The parasites can be clearly distinguished inside the now-transparent host cell.

This electron micrograph shows a thin section of a bacterial cell infected with a parasite. Part of the cytoplasm is already lysed.

In this sequence showing Spirellum serpens, the complete course of the host-parasite interaction is demonstrated. First, the formation of spheroplasts. The proteolytic activity of the invading parasites causes progressive lysis of the cell contents.

The insides of the bacteria therefore appear lighter in phase contrast, and the parasites can be clearly discerned. Their multiplication can be followed by the increasing number of dark bodies.

Finally, the parasites are released through the destruction of the surrounding cell wall. Out of each of these host bacteria emerge some 20 to 30 parasites. In this experiment, the whole process lasted about two and a half hours. Here, the process of intracellular multiplication

and the release of the parasites demonstrated once again.

Finally, the multiplication of delovibrio in Pseudomonas fluorescence. The parasites are free after the spheroplasts have disintegrated. Now they begin to swarm. Some of the released parasites move towards the growing bacterial colony at the bottom left.

They attack the young daughter cells, invade them, and so initiate a new life cycle.