Differential media are culture media formulated to reveal differences among microorganisms by producing visible changes in colony morphology or the appearance of the medium. They contain substrates and indicators that organisms metabolise differently, allowing identification based on colour changes or other reactions.
Explanation
In clinical and environmental microbiology, differential media help distinguish between species or groups based on their biochemical properties. These media include specific carbohydrates, amino acids, red blood cells or other substrates along with pH indicators or dyes that change colour when metabolic byproducts alter the medium. For example, MacConkey agar contains lactose and neutral red; lactose-fermenting bacteria such as Escherichia coli produce acid and turn colonies pink, whereas non‑fermenters remain colourless. Mannitol salt agar contains high salt and mannitol; most staphylococci grow, but only Staphylococcus aureus ferments mannitol to produce yellow zones. Blood agar includes defibrinated sheep blood; bacterial toxins produce patterns of haemolysis: β‑haemolysis results in clear zones, α‑haemolysis produces greenish discolouration and γ‑haemolysis shows no change. Eosin methylene blue agar uses eosin and methylene blue dyes to differentiate strong lactose fermenters, which show metallic sheen, from weak or non‑fermenters. Differential media are often also selective, including inhibitors that suppress unwanted organisms while revealing traits of those that do grow. Interpretation requires careful observation and confirmation with additional tests because some organisms may show atypical reactions depending on incubation conditions or inoculum size. By highlighting metabolic differences, differential media streamline the process of identifying pathogens and assessing microbial diversity.
Common media and uses
Beyond MacConkey and mannitol salt agar, there are many differential media. Hektoen enteric agar differentiates Salmonella and Shigella based on hydrogen sulfide production, giving black-centred colonies for Salmonella. Xylose lysine deoxycholate (XLD) agar changes colour when organisms ferment xylose or produce lysine decarboxylase. Chromogenic agars incorporate artificial substrates that release coloured compounds when specific enzymes are present, enabling rapid detection of Listeria, MRSA or Candida species. Thiosulfate–citrate–bile salts–sucrose (TCBS) agar differentiates Vibrio cholerae, which ferments sucrose and forms yellow colonies, from other vibrios. In teaching laboratories, these media provide students with visual demonstrations of microbial physiology. Laboratories must prepare and store media properly and interpret results within recommended time frames to avoid false readings.
Differential media are valuable tools for isolating and presumptively identifying microorganisms based on observable biochemical traits.
Related Terms: Selective media, Culture medium, Indicator, Agar, Hemolysis