Proteus vulgaris, on a blood agar plate incubated 24h at 37°C. The colonies are spreading on the surface of the agar due to P. vulgaris are svarming.
Credit: Ingrid Hansson (BVF, SLU) & Lise-Lotte Fernström (BVF, SLU)
A swarming bacterium does not form discrete colonies on an agar plate, but flows out over the entire agar surface at a rate of 2–10 μm/s. If a bacterium swarms, one cannot determine its macromorphology, nor could colonies be counted. Swarming can be prevented for some bacteria by growing them on a medium with low electrolyte content (= low salt concentration), such as e.g. CLED agar. A variant of MacConkey-agar without NaCl can also be used to avoid swarming. Bacteria that can swarm are commonly found in the genera: Aeromonas, Bacillus, Brachyspira, Campylobacter, Clostridium, Escherichia, Paeniclostridium, Proteus, Pseudomonas, Salmonella, Serratia, Vibrio and Yersinia. It is not known why some bacteria swarm, but it has been suggested that it is to move quickly to areas with less competition for nutrients. Swarming bacteria usually have more flagella per unit area than planktonic (freely floating) bacteria. It has been observed that actively swarming bacteria of Salmonella Typhimurium exhibit an increased resistance to certain antibiotics as compared to non-swarming cells.