Effects in hyperoxia upon microorganisms. I. Membrane culture technique for exposing cells directly to test atmospheres.

A membrane culture technique was developed for directly exposing microorganisms to test atmospheres. Inhibition and killing were calculated from comparisons with air-grown cultures. Direct colony counts were used with low inocula. With mass inocula, plate colony counts and optical-density measurements were made on resuspended filter populations. Bacteria, including Escherichia coli, were more sensitive to oxygen than previously reported. With inocula of a few hundred cells per membrane, five of seven species failed to produce colonies while exposed to oxygen at one atmosphere. Upon reincubation in air, the survival of five species ranged from near 0 to 12% of the cells. Aerobacter aerogenes was neither inhibited nor killed. With this technique, bacteria are in direct contact with the test atmosphere and cells which survive are detected but do not obscure the response of other cells in the population.