Complex media are culture media that contain ingredients of unknown exact chemical composition, such as peptones, meat extract, yeast extract and other digests. They provide a rich mixture of amino acids, vitamins, minerals and carbohydrates to support the growth of a wide variety of microorganisms.
Explanation
Peptones and extracts are produced by enzymatic or acid digestion of proteins and tissues. The precise composition of each batch varies, so complex media are undefined. Their nutrient‑rich nature makes them suitable for cultivating heterotrophic bacteria and fungi that have diverse and unknown nutritional requirements. These media are often used for routine cultivation, isolation and enumeration in clinical, environmental and industrial microbiology.
Examples of complex media include nutrient broth and nutrient agar prepared from beef extract and peptone, tryptic soy broth and tryptic soy agar containing enzymatic digests of casein and soybean meal, and brain heart infusion made from animal tissues. Lysogeny broth (LB) used to grow Escherichia coli is a widely used complex medium. The undefined composition makes them unsuitable for studies that need known nutrient concentrations, but they are valuable when the goal is to obtain robust growth or to recover stressed cells.
Examples and uses
Laboratories use complex broth or agar to cultivate clinical isolates, test antibiotic susceptibility and enumerate bacteria in food and water samples. Plate count agar, a variant of nutrient agar, is used to determine the total viable count in food microbiology. Sabouraud dextrose agar, containing peptone and glucose, supports the growth of yeasts and moulds. Adding blood or serum to a complex base produces enriched media such as blood agar or chocolate agar that can grow fastidious organisms like Haemophilus and Neisseria. Selective agents, dyes and antibiotics can be incorporated into complex bases to create selective media for specific groups of microbes.
Complex media are versatile and easy to prepare but they lack the consistency of defined media. Variation between batches can affect experimental reproducibility.
Related Terms: defined media, nutrient broth, LB broth, heterotroph, enrichment medium