Helium ion microscopy reveals mysteries of spiky bacterial filaments

Some iron-oxidizing bacteria make organic-mineral filaments that extend from their surfaces. But how they are formed remains a puzzle. Now, using helium ion microscopy (HIM), James M. Byrne of the University of Tübingen and his colleagues have captured these stalks forming in unprecedented detail (Environ. Sci. Technol. Lett. 2018, DOI: 10.1021/acs.estlett.8b00077). Previous imaging efforts used scanning electron microscopy, but this technique has inferior resolution and in this context requires coatings that distort the structures being studied. The group cultured bacteria isolated from low-oxygen sediments in a Denmark bay and analyzed the samples with HIM over a month to document how the structures changed. First, spiral-shaped stalks formed. Eventually, mineral crystals coated the spirals so heavily that their shapes were no longer discernible. “It really blew my mind when we started to see these objects forming,” Byrne says. The spirals were mostly organic, whereas the mineral deposits were lepidocrocite, an iron oxide-hydroxide mineral, elemental analysis showed. Some researchers think the structures sequester toxic, dissolved iron(II) in the environment as insoluble iron(III), protecting the microbes. More work may solve that mystery, too.