Bacterial motility concept: In microbiology, what does the term “chemotaxis” describe with respect to bacterial movement in the presence of a chemical compound or gradient?

Introduction / Context:
Chemotaxis is a classic topic in microbiology and cell behavior. It explains how bacteria modulate their motility to navigate chemical landscapes, moving toward favorable conditions (attractants) or away from harmful ones (repellents). Understanding chemotaxis helps explain colonization, infection patterns, biofilm formation, and laboratory motility assays.

Given Data / Assumptions:

• Bacteria can swim using rotating flagella.
• The environment contains chemical cues such as nutrients or toxins.
• The question asks for the most accurate definition of chemotaxis.

Concept / Approach:

Bacterial chemotaxis involves biased random walks controlled by runs and tumbles. In attractant gradients, the frequency of tumbles decreases when movement improves conditions, extending runs up-gradient. With repellents, the signaling inverts the bias, increasing the chance to leave the harmful zone. The molecular basis involves chemoreceptors (MCPs), the Che signaling proteins, and flagellar motor switching between counterclockwise and clockwise rotation to alternate runs and tumbles.

Step-by-Step Solution:

Verification / Alternative check:

In soft-agar motility assays, rings expand more rapidly in the presence of attractants and contract or disperse differently with repellents, confirming directed motility rather than random diffusion. Mutants in Che proteins lose directional control, demonstrating the signaling dependence of chemotaxis.

Why Other Options Are Wrong:

Swimming away only: incomplete because attraction is equally important.

Swimming toward bacteria: chemotaxis is about chemicals, not necessarily other bacteria.

None of the above: incorrect because a precise definition exists.

Common Pitfalls:

Confusing chemotaxis with general motility; forgetting that both attractants and repellents exist; assuming movement is deterministic rather than probabilistic via runs and tumbles.

Final Answer:

Swimming toward or away by bacteria in response to a chemical compound