Preservation of Strictly Acidophilic Autotrophic Bacteria Acidithiobacillus ferrooxidans by Lyophilization Using Steel

Abstract Acidithiobacillus ferrooxidans are strictly acidophilic bacteria, which are used in metal recovery practices worldwide. Guaranteed preservation of the viability and genetic potential of A. ferrooxidans strains remains important due to the need for long-term maintenance of industrial cultures without constant subcultures, which increase the likelihood of spontaneous mutations. Storage of strictly acidophilic microorganisms by cryopreservation or lyophilization is difficult due to the acidic culture medium: sulfuric acid freezes and evaporates slower than water, so its residual concentration increases and damages the cells. Various protectants for preserving A. ferrooxidans cells during cryopreservation and lyophilization have been described in the literature. This study presents the results of lyophilization of two A. ferrooxidans strains in the presence of protectors: sucrose + gelatin + agar (SGA) and SGA supplemented with steel wire (SGA + steel). Fluorescence microscopy showed that the proportion of viable A. ferrooxidans cells after rehydration was higher with the wire than without it (63–68 and 32–33%, respectively). The article discusses the possible functions of the introduced steel, which ensures higher numbers of surviving A. ferrooxidans cells after their lyophilization, storage, and rehydration. It is assumed that the presence of steel in the “SGA + steel” protector ensures not only the neutralization of residual acid in the lyophilized material, but also additional substrates and optimization of conditions during rehydration. Microscopic examination of A. ferrooxidans cells after rehydration revealed their abundant growth and the formation of biofilms on the steel wire.