172 PATHOGENIC BACTERIA.

them before it was frozen. In Hudson-River ice Prud-
den found an average of 398 colonies in a cubic centi-
meter.

A sample of water when collected for examination
should be placed in a clean sterile bottle or in a her-
metically-sealed pre-sterilizecl glass bulb, and must be
examined as soon as possible, as the bacteria multiply
rapidly in water which is allowed to stand for a short
time. In determining the species of bacteria found in
the water reference must be made to the numerous mono-
graphs upon the subject, and to tables such as those
compiled by Eisenberg.

The discovery of certain important pathogenic bacteria,
as those of cholera and typhoid, will be considered under
the specific headings.

Unfortunately, the bacteriologic examination of waters
does not throw satisfactory light upon their exact hygi-
enic usefulness. Of course, if cholera or typhoid-fever
bacteria are present, the water is harmful, but the quality
of the water cannot be gauged by the number of bacteria
it contains.

The drinking-water furnished large cities is not infre-
quently contaminated with sewage, and contains intes-
tinal bacteria—Bacillus coli communis. For the ready
determination of this organism, which is an important one
as an indicator that the water is polluted, Smith1 has
made use of the fermentation-tube in addition to the
plate. His method is to add to each of the fermentation-
tubes containing i per cent, dextrose-bouillon a certain
quantity of water. The evolution of 50-60 per cent, of
gas by the third day is a strong indication that the colon
bacillus is present. Plates may be used to confirm the
presence of the bacillus, but are hardly necessary, as
there is scarcely another bacterium met with in water
that is capable of producing so much gas.

Filtration with sand, etc. diminishes the number of
bacteria for a time, but, as the organisms multiply in

1 American Journal of the Medical Sciences, 1895, IIO> P- 3O1-