Which gram-negative species may swarm on a blood agar plate, forming a film over the surface?

Which gram-negative species may swarm on a blood agar plate, forming a film over the surface?

• A. Proteus sp.
• B. Pseudomonas sp.
• C. Staphylococcus sp.
• D. Escherichia coli

Answer: (A)

Swarming motility is a rapid, coordinated surface movement that many bacteria use to spread across an agar plate, often forming a continuous film rather than discrete colonies. Proteus species are classic for this on nutrient-rich media like blood agar. They differentiate into elongated swarmer cells with many flagella, allowing a large number of cells to move together and cover the surface quickly. On a blood agar plate, this produces a visible film that can obscure individual colonies. Other gram-negative bacteria may spread under some conditions, but the distinctive surface film produced by Proteus is the hallmark swarming behavior.

Swarming motility is a rapid, coordinated surface movement that many bacteria use to spread across an agar plate, often forming a continuous film rather than discrete colonies. Proteus species are classic for this on nutrient-rich media like blood agar. They differentiate into elongated swarmer cells with many flagella, allowing a large number of cells to move together and cover the surface quickly. On a blood agar plate, this produces a visible film that can obscure individual colonies. Other gram-negative bacteria may spread under some conditions, but the distinctive surface film produced by Proteus is the hallmark swarming behavior.