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Most known acidophiles can be grown on a range of solid media. Following sufficient dilution of bioreactor samples, individual cells are spread onto the surface of media gelled with agarose, and these cells grow into colonies that are made of many cells arising from the original cell deposited on that location of the solid medium. In principle, the lowest limit of detection of acidophiles by enumeration on a solid medium is 10 per ml of sample. Microbes attached to solids can also be enumerated in this manner when detached from the solids (e.g. by shaking in the presence of a non-ionic detergent).


  • High sensitivity, as few as 10 cells per ml sample;
  • Microbial identities can be preliminarily established based on colony morphology, and easily confirmed by 16S rRNA gene amplification and sequencing;
  • Yields pure cultures of microbes found in a bioreactor.


  • Protracted time required for colony development, on the order of 3 to 20 days;
  • The production of the solid media is time-consuming, and requires a great deal of effort to process many samples at one time;
  • Currently used solid media are not suitable for the study of acidophiles that grow at high temperatures (>50°C);
  • While most known acidophiles can grow on solid media, acidophiles have been detected by molecular means but not by growth on solid media.

Current use in bioleaching studies

Solid media have been used to elucidate and characterize microbial populations in many bioleaching environments, including bioreactors (Okibe et al., 2003), wetlands constructed for remediation of iron-rich acid mine drainage (Hallberg and Johnson, 2005), naturally acidic environments (Johnson et al., 2005), and mine tailings (Bryan et al., 2006).

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