Bacterial movement at high matric potentials—II. In fungal colonies

Abstract

Bacillus subtilis spread readily along water films beside hyphae of killed colonies of Pythium ultimum on membrane filters at a matric potential of —50 cm of water, but movement was restricted at — 150 cm potential. Similar rates of spread occurred when the bacteria were inoculated at a point on the surface of a layer of artificial soil, placed over the killed fungal colony and equilibrated at either of the two matric potentials. This suggests that dead fungal mycelia do not aid bacterial spread to any extent at a matric potential of —150 cm of water. When live colonies of P. ultimum were grown in the artificial soil held at the two matric potentials, the rate of bacterial movement was comparable to that in killed colonies at −50 cm potential, but was erratic at the lower potential. The accumulation of inhibitory fungal metabolites in the thin water films beside the actively-respiring hyphae probably restricted bacterial spread at some points in the soil. It appears that dead fungal mycelia may aid in the local spread of bacteria at matric potentials higher than —150 cm of water, but live mycelia may not. The ability of bacteria to spread along dead fungal hyphae was explained by a consideration of the thickness of water films imposed by the prevailing matric potential.