What type of structures do Listeria bacteria form?

Listeria bacteria are known for their unique motility and ability to form actin tails, which allows them to push against the cytoplasmic membrane of host cells. This movement often results in the formation of structures that resemble a spider web or tailing structures. As Listeria hijacks the host's actin polymerization machinery, it creates long filamentous projections that can be visualized in host tissues.

The characteristic "spider web-like structures" formed are indicative of the bacteria's capability to move from one cell to another by propelling themselves through the cytoplasm, thus enhancing their virulence and ability to spread. This mechanism is essential in understanding how Listeria can escape host immune responses and propagate within the host’s system.

In this context, the other options do not accurately reflect the nature of Listeria. Very small strings and small stick-like bodies do not capture the complexity of the structures formed by these bacteria. Large clusters of balls do not represent the filamentous nature of the actin tails produced, which is more accurately described as a web-like structure, facilitating the movement and infection process.