Question

Suspension of bacteria viewed by

Answer 1

When bacterial suspensions are used as vaccines or in serum
zeactions it is necessary or desirable to determine their concentration,
at least approximately. Many methods have been advocated. The
bacterial count per cc., the weight of the moist or dried bacterial
substance, the capacity of a platinum loop, the opacity of various
bacterial or chemical suspensions, the volume of the sedlmented
organisms, all have served as standards in various methods of
determining concentration already described. The more accurate
methods involve much time and labor; the simpler ones give only a
rough approximation of bacterial content.
If a wire loop is gradually pushed down into a suspension of
organisms in a test-tube (Text-fig. i) and viewed by looking down
into the tube through its mouth, the depth at which the loop dis-
appears will be determined by the opacity of the supervening column
of suspension. If two suspensions of the same organism are compared
in this manner, a longer column of the thinner suspension will be
required to effect the disappearance of the loop. The lengths of the
columns may be measured and compared, and the measurements
might be interpreted in terms of bacterial concentration if a suitable
standard were determined. Tests with such an instrument show that
with a little practice the length of the column of a bacterial sus-
pension that will just hide the loop (the depth of disappearance) can
be measured with considerable accuracy. For example, with sus-
pensions of such an opacity that the loop disappears between 1
and 4 cm. below the surface, the zone of most accurate measure-
ment, the depth of disappearance can be read within 1 mm. repeatedly,
an error of less than 10 per cent. Within certain limits, neither
the diameter of the test-tube nor the size of the wire loop affects
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