Attachment of acidophilic bacteria to solid surfaces: The significance of species and strain variations

Sixteen strains of acidophilic bacteria were screened for their abilities to adhere to pyrite ore, glass beads and ferric hydroxysulfates. These were three culture collection and two isolated strains of the iron- and sulfur-oxidizer, Acidithiobacillus ferrooxidans, two each of the sulfur-oxidizer Acidithiobacillus thiooxidans and the iron-oxidizer Leptospirillum ferrooxidans (the type strain and a mine isolate in either case), five heterotrophic acidophiles (four Acidiphilium and one Acidocella sp.) and two moderately thermophilic iron/sulfur-oxidizers (Sulfobacillus thermosulfidooxidans and Sulfobacillus acidophilus). Considerable variations were found between different species of acidophiles, and also between different strains of the same species, in how they attached to the three solid materials tested. Attachment to the solid substrata generally increased with time (over 100 min) though > 99% of one At. ferrooxidans isolate (strain OP14) were attached to pyrite after just 10 min exposure. Most acidophiles attached more readily to pyrite than to glass beads, and attachment to ferric hydroxysulfates was highly variable, though one At. ferrooxidans isolate (strain SJ2) and one heterotrophic acidophile (Acidocella sp. het-4) both attached strongly to ferric iron precipitates (jarosites and schwertmannite) that formed in cultures of At. ferrooxidans grown at pH > 2. The results of these experiments showed that even closely related strains of acidophilic bacteria can display very different propensities to attach to solid materials, an observation that may explain the somewhat disparate results reported on occasions by research groups that have examined single, or limited numbers of strains, of acidophiles (mostly At. ferrooxidans). The significance of differential attachment of mineral-oxidizing and other acidophiles to pyrite and other solids is discussed in the context of biohydrometallurgy.