Acidithiobacillus ferrooxidans is a chemolithoautotrophic (uses inorganic chemicals for energy and makes its own organic molecules from carbon dioxide), non-spore forming, Gram-negative organism that resides in extremely acidic environments. It is relatively short in size, measuring 0.4 μ by 0.8 μ, and can appear as single cells or in pairs. The bacterium gained attention for its unique ability to oxidize ferrous iron for energy and capacity to thrive in nutrient poor environments abundant in heavy metals, conditions that are typically aversive to most other microorganisms.

The genus name of the bacteria, Acidithiobacillus, contains Latin and Greek roots. It can be broken up into three words consisting of acidus (meaning “acidic”), thios (meaning “sulfur”), and bacillus (meaning “rod”), which highlight the rod-shaped organism’s affinity for acid and ability to oxidize sulfur. The species name of the bacteria, ferrooxidans, comes from the Latin word ferrum (meaning “iron”) and oxidare (meaning “to oxidize”), characterizing its distinct ability to oxidize ferrous iron.

Acidithiobacillus ferrooxidans is classified within the domain Bacteria, kingdom Pseudomonadati, phylum Pseudomonadota, class Acidithiobacilla, order Acidithiobacillales, family Acidithiobacillaceae, and genus Acidithiobacillus. There are currently 8 identified species within the Acidithiobacillus genus, with A. ferrooxidans serving as one of the most well-studied amongst the known species.

Acidithiobacillus ferrooxidans is most closely related to other iron and sulfur-oxidizing species within the Acidithiobacillus genus, including Acidithiobacillus caldus, Acidithiobacillus thiooxidans, and Acidithiobacillus ferrivorans. The species listed were organized into distinct clades to represent major evolutionary branches by comparing their 16S rRNA gene sequences — a gene sequence commonly used to study evolutionary relationships due to their presence in all bacteria and slow mutation rate. The genomes for the species were obtained from a publicly available database of nucleotide sequences.

The bacterium, Acidithiobacillus ferrooxidans, and its ability to oxidize ferrous iron was first reported by Colmer and Hinkle in 1947. Soon after, it was found repeatedly in various acid mine drainages across the United States, including the Pittsburgh, the Sewickley, and the Upper Freeport.

Isolation

To isolate the organism, Colmer and Hinkle incubated 50 mL of acidic mine drainage in sterile flasks at room temperature until turbidity developed, indicating microbial growth. They performed successive transfers by subculturing precipitate from the side of the flask into fresh acidic medium multiple times in an attempt to enrich or amplify the iron-oxidizing populations. Following the transfers, Colmer and Hinkle prepared streak plates on acid mine water agar, made from filtered acid mine drainage water and agar, to yield pure colonies. This process leveraged the bacterium’s natural environment to induce selective growth, a process that promotes the growth of a specific bacteria while limiting others.

Characterization