Microbial utilisation of two proteins adsorbed to a vertisol clay fraction: toxin from Bacillus thuringiensis subsp. tenebrionis and bovine serum albumin

Clay minerals have been shown to reduce the extent and rate of biodegradation of several compounds. Here, we investigated the ability of soil clays to protect proteins from biodegradation: the insecticidal protein from Bacillus thuringiensis subsp. tenebrionis (Btt toxin) and Bovine Serum Albumin (BSA). The two proteins adsorbed in large amounts (up to 0.24 g BSA g⁻¹ clay and 0.74 g Btt toxin g⁻¹ clay) and irreversibly to smectite clay particles from a vertisol. We measured the growth of a soil inoculum in the presence of each of proteins as the sole source of carbon. When clay was present in the medium, microbial growth was directly proportional to the amount of free protein (i.e. nonadsorbed). Hence, the two proteins were unavailable when adsorbed to clay. The clay had little influence on the ability of microorganisms to hydrolyse a soluble substrate. The inhibitory effect of clays on utilisation of BSA and Btt toxin was interpreted as being the result of the adsorption of the proteins to clay, which rendered the proteins unavailable for microbial utilisation.