R2A

R2A stands for Reasoner's 2A agar, a low-nutrient microbiological medium developed by Reasoner and Geldreich for isolating and enumerating heterotrophic bacteria from potable water and other aquatic environments. It is designed to recover slow-growing and stressed organisms that are often not captured on richer media, providing a broader view of the microbial contribution to water quality.

The formulation uses modest nutrient levels, including carbon and nitrogen sources such as peptone and yeast extract, along with buffering minerals and agar to solidify the medium. The intentionally limited nutrients slow growth, reduce dominance by fast-growing bacteria, and encourage formation of visible colonies by a wider range of organisms present in environmental and drinking-water samples.

R2A is widely used for heterotrophic plate counts (HPC) in water quality assessments. Plates are incubated at room temperature or roughly 20–28°C for several days, after which colonies are counted and reported as colony-forming units per milliliter (CFU/mL). Results on R2A can differ substantially from those obtained on richer media, often yielding more diverse and slower-growing taxa.

There are standardized variants and commercially available formulations of R2A produced by multiple suppliers. The medium is referenced in international and national standards for water testing, including ISO 6222, which describes methods for enumerating microflora in water using agar plates. R2A thus remains a common tool in aquatic microbiology and drinking-water quality assessment.