Thiobacillus is a genus of Gram-negative Betaproteobacteria. Thiobacillus thioparus is the type species of the genus, and the type strain thereof is the Starkey<sup>T</Sup> strain, isolated by Robert Starkey in the 1930s from a field at Rutgers University in the United States of America. While over 30 "species" have been named in this genus since it was defined by Martinus Beijerinck in 1904, (the first strain was observed by the biological oceanographer Alexander Nathansohn in 1902 - likely what we would now call Halothiobacillus neapolitanus), most names were never validly or effectively published. The remainder were either reclassified into Paracoccus, Starkeya (both in the Alphaproteobacteria); Sulfuriferula, Annwoodia, Thiomonas (in the Betaproteobacteria); Halothiobacillus, Guyparkeria (in the Gammaproteobacteria), or Thermithiobacillus or Acidithiobacillus (in the Acidithiobacillia). The very loosely defined "species" Thiobacillus trautweinii was where sulfur oxidising heterotrophs and chemolithoheterotrophs were assigned in the 1910-1960s era, most of which were probably Pseudomonas species.
- Thiobacillus neapolitanus to Halothiobacillus
- Thiobacillus thyasiris to Thiomicrospira thyasirae
- Thiobacillus hydrothermalis and Thiobacillus halophilus firstly to Halothiobacillus and later to Guyparkeria
- Thiobacillus tepidarius to Thermithiobacillus