The cryoprotective additives (CPAs) used in the frozen storage of microorganisms (viruses,
bacteria, fungi, algae, and protozoa) include a variety of simple and more complex
chemical compounds, but only a few of them have been used widely and with satisfactory
results: these include dimethylsulfoxide (Me2SO), glycerol, blood serum or serum albumin,
skimmed milk, peptone, yeast extract, saccharose, glucose, methanol, polyvinylpyrrolidone
(PVP), sorbitol, and malt extract. Pairwise comparisons of the cryoprotective activity
of the more common CPAs used in cryomicrobiology, based on published experimental
reports, indicate that the most successful CPAs have been Me2SO, methanol, ethylene
glycol, propylene glycol, and serum or serum albumin, while glycerol, polyethylene
glycol, PVP, and sucrose are less successful, and other sugars, dextran, hydroxyethyl
starch, sorbitol, and milk are the least effective. However, diols (as well as some
other CPAs) are toxic for many microbes. Me2SO might be regarded as the most universally
useful CPA, although certain other CPAs can sometimes yield better recoveries with
particular organisms. The best CPA, or combination of CPAs, and the optimum concentration
for a particular cryosensitive microorganism has to be determined empirically. This
review aims to provide a summary of the main experimental findings with a wide range
of additives and organisms. A brief discussion of mechanisms of CPA action is also
included.