Nondestructive methods for removal of bacteria from silicate surfaces

In order to study biosignatures documenting the effects of microbiota on mineral surfaces, microbial matter must be removed without chemically or physically changing the surface. In previous work for analysis by scanning electron microscopy (SEM) we removed soil bacteria from silicate surfaces with lysozyme; however, when Iysozyme-treated surfaces were analyzed at nanoscale by atomic force microscopy (AFM), residual matter was observed. Therefore, we tested removal of bacteria by each of five methods. including four different detergents and CO₂ snow cleaning. AFM observations revealed residue on glass surfaces that underwent CO₂ snow cleaning after 13 days of growth of Bacillus sp. (a soil microbe). In contrast, treatment with each of four detergents followed by acetone completely removed Bacillus sp. without altering the topography of the surfaces even after 77 days of growth. However, X-ray photoelectron spectroscopic (XPS) analyses of the upper 25 Å of the glass surfaces revealed chemical alterations by some detergents, but not by the detergent sodium dodecyl sulfate (SDS). Therefore, for strictly topographic analyses such as AFM, any of the four detergents tested can be considered a viable option for bacterial removal, but for analyses of surface chemistry, the pH-neutral, anionic detergent SDS is recommended. Additionally, AFM imaging after detergent treatments revealed etch pits on glass surfaces exposed to bacteria that were similar in size and distribution to small colonies of Bacillus sp. These surfaces were also depleted in Fe and Al relative to Si as measured by XPS, yet similar controls incubated in media without bacteria showed no depletion in Fe and Al.