SELECTIVE SOLID MEDIA FOR ISOLATING AND ENUMERATING ACIDOPHILIC BACTERIA

Published solid media formulations designed for T. ferrooxidans and other acidophilic microorganisms are reviewed. Many designated media have been found to be unsatisfactory both for enumerating and isolating acidophiles from environmental samples (acid mine drainage etc.) and for some species and strains of chemolithotrophs grown in the laboratory. However, the 'overlay' technique, in which an acidophilic heterotrophic bacterium is incorporated into the underlayer of a two-layer gel, has been used successfully to culture all known iron-oxidising mesophilic and moderately thermophilic bacteria. A variant (FeSo) of the original medium, which includes tetrathionate as well as ferrous iron and tryptone soya broth, is described. This medium supports the growth of Thiobacillus thiooxidans as well as iron-oxidising mesophiles (T. ferrooxidans and Leptospirillum ferrooxidans) and many heterotrophic isolates, and facilitates differentiation and identification of isolates based on colony characteristics. Medium FeSo has also been used to culture a range of moderately thermophilic acidophilic microorganisms. A second novel solid medium is described (WAYE) for isolating and enumerating oligotrophic, heterotrophic acidophiles from the environment. This non-overlay medium contains yeast extract (at 0.02% w/v) and is gelled by water- or acid-washed agarose. Data recording results from using these novel media for plating mine waters and laboratory cultures are presented, and a summary is made of which media are most appropriate for targeting specific groups of acidophilic microorganisms.