Softagar

Softagar, commonly written as soft agar, is a semi-solid gel used in microbiology to culture, observe, and analyze microorganisms. It refers to low-concentration agar or agarose media that remain more fluid than standard solid agar plates, enabling limited movement of cells and diffusion of substances through the gel.

Physical properties and composition

Softagar is made by incorporating agar or agarose into a nutrient medium at a much lower concentration than typical solid media. This yields a gel that is firm enough to hold colonies in place while still allowing motility, diffusion, and three-dimensional distribution of cells. The exact concentration varies by application, but it is generally in the range used for soft gel media, higher than liquids but well below the 1–2% levels used for solid plates.

Main uses

The most common applications of softagar include assessing bacterial motility, chemotaxis, and the diffusion of phages, antibiotics, or nutrients. Motility assays place bacteria in or on soft agar to distinguish motile strains, which spread through the gel, from non-motile ones. Soft agar overlays are also used in bacteriophage plaque assays to permit phage diffusion while keeping host cells relatively immobilized.

Preparation and handling

Softagar media are prepared like standard agar media but with reduced solid content. The mixture is sterilized, cooled, and poured at a temperature suitable for the organisms involved to avoid heat damage. The medium is then allowed to solidify into a semi-solid surface or overlay. Proper sterile technique and adherence to biosafety guidelines are essential.

See also: agar, agarose, motility test, phage plaque assay.