56                                           THE   TECHNOGRAPH.
 

THE BACTERIOLOGICAL EXAMINATION OF SEWAGE.
 

»By R. J. DiCKmsoK, '94, Sanitary Enoineeeing Course.
 

The great obstacle in the study of bacteria is their extreme
smallness. Because of this, one can only see, even with the best
microscopes, the mere form, with often no distinguishing char¬
acteristics. For the same reason it is very difficult to seal the cul¬
ture to outside bacteria and still allow free access of air. Such being
the case too great care can not be taken to prevent contamination.

The method here recommended is merely an outline of the work,
every step being hedged in by precautions, and even then the culture
sometimes becoming contaminated.

In the examination of sewage two things are wanted: first,
the number of bacteria in a given volume; second, their species.

To determine the number: Melt a tube of gelatin (any gelatin
will liquefy at 40°C). Put into this a definite volume of the sewage
to be examined and mix thoroughing by shaking. Pour out on a plate
and cover with a bell-jar to prevent contamination. Place this culture
in a temperature slightly below the melting point of gelatin—about
30° to 33°C. The first indication of the growth of the bacteria will
be the appearance of small, unusually white dots. Each dot is a col¬
ony, reproduced from a single bacterium. When new-colonies cease
to appear (usually at the end of two or three days), place a counting
apparatus, a thin glass plate with the surface divided
into small equal t.areas,1 over the culture and count the colonies in
each of a number of these areas. From this counting estimate the
total number of colonies, which is the number of bacteria in the
volume examined.

The best ratio of the volumes of sewage and gelatin can be de¬
termined by experiment. It varies between very wide limits, the
object being to have as much sewage as possible without having the
colonies so numerous as to make counting difficult.

To determine the species it is necessary first to obtain a pure
culture. This may be done as follows: Transfer from the plate-
culture to a tube of distilled water, as small a quantity as possible
from the colony whose species is to be determined. Mix thoroughly
by shaking.     From this tube inoculate a tube of melted gelatin.