Ashdown’s medium is a selective culture medium for the isolation and characterization of Burkholderia pseudomallei (the bacterium that causes melioidosis).
Ashdown’s medium was first described by LR Ashdown in 1979.
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Composition of Ashdown’s Agar
Tryptone soya broth
Crystal violet 0.1%
Neutral Red 1%
Principle of Ashdown’s Agar
The medium contains crystal violet and gentamicin as selective agents to suppress the growth of other bacteria.
Colonies of B. pseudomallei also take up neutral red which is present in the medium, and this further helps to distinguish it from other bacteria.
Gentamicin slightly inhibits the growth of B. pseudomallei and so specimens inoculated onto Ashdown’s agar needs to be incubated for a minimum of 96 hours instead of 48 hours.
The medium is also enriched with 4% glycerol, which is required by some strains of B. pseudomallei to grow.
At 24 hours of incubation of the medium colonies are often pinpoint, clear and pale pink, changing to become pinkish-purple, flat, and slightly dry in the next 2 days. The most characteristic feature of B. pseudomallei is its metallic sheen, and the usual progression to dry and wrinkled colonies at 96 hours.
Further tests must be carried out for identification of B. pseudomallei. Any organism that has a colonial appearance typical of B. pseudomallei on Ashdown agar and is resistant to colistin and susceptible to co-amoxiclav can be presumptively identified as B. pseudomallei if it is positive for latex agglutination.
It has been noted in local clinical laboratory use that ASA is inhibitory to some strains of B. pseudomallei.
Isolation from sputum is particularly difficult when low numbers of B. pseudomallei are present and there is an abundant commensal upper respiratory tract flora.
Howard, K., & Inglis, T. J. (2003). Novel selective medium for isolation of Burkholderia pseudomallei.Journal of clinical microbiology, 41(7), 3312-6.