Influence of clay minerals on microorganisms. II. Effect of various clay species, homoionic clays, and other particles on bacteria.

Respiration of Agrabacterium radwbacter was stimulated most by certain clay minerals of the 2:1 type (montmorillonite, vermiculite, illite, prochlorite, talc). Some 2:1 minerals (pyrophyllite, verrniculite–mica mixtures), 1:1 minerals (kaolinite, halloysite), and a variety of particles which possess some of the characteristics of clay minerals (ion exchangers, polystyrene latex, glass beads, etc.) had relatively little influence on respiration. One mechanism by which clay minerals and other particles stimulated respiration was by maintaining the pH of the environment suitable for sustained growth. This mechanism was dependent on both the initial pH of the systems and the buffering ability of the particles. Studies with homoionic clays showed that stimulation of respiration and maintenance of favorable pH levels was related to the relative basicity of the exchangeable cations on the clays.