Fig. 147.—A villous stab or puncture culture. × ½.
In broth cultures the presence or absence of growth on the surface and the amount of the same. Whether the broth is rendered cloudy or remains clear, and whether there is a deposit at the bottom or not ([Fig. 144]). An abundant surface growth with little or nothing below indicates a strict aërobe, while a growth or deposit at bottom and a clear or nearly clear medium above, an anaërobe. These appearances are for the first few days only of growth. If the broth is disturbed, or after the culture stands for several days many surface growths tend to sink to the bottom. So an actively motile organism causes in general a cloudiness, especially if the organism is a facultative anaërobe, which tends to clear up by precipitation after several days when the organisms lose their motility. Non-motile facultative anaërobes usually cloud the broth also, but settle out more rapidly than the motile ones.
In gelatin and agar punctures the oxygen relationship is shown by surface growth for aërobes, growth near the bottom of the puncture for anaërobes, and a fairly uniform growth all along the line of inoculation for facultative anaërobes
. In the case of these last organisms, a preference for more or less oxygen is indicated by the approach to the aërobic or anaërobic type of growth.
Fig. 148
Fig. 149
