" " (avian).

" " (fish).

to contrast with Bacillus phlei (Timothy gra.s.s bacillus).

b.u.t.ter bacillus of Rabinowitch.

XV. _Plague group._

Bacillus pestis.

B. septicaemiae haemorrhagicae.

B. suipestifer.

XVI. _Influenzae group._

B. influenzae.

Bacillus aegypticus (Koch-Weeks).

Bacillus pertussis.

XVII. _Miscellaneous._

Bacillus leprae.

Bacillus mallei.

Micrococcus melitensis.

XVIII. _Streptothrix group._

Streptothrix actinomycotica.

Streptothrix madurae.

to contrast with Cladothrix nivea.

XIX. _Teta.n.u.s group._

Bacillus tetani.

Bacillus oedematis maligni.

Bacillus chauvei (symptomatic anthrax).

XX. _Enteritidis sporogenes group._

Bacillus enteritidis sporogenes.

B. botulinus.

B. butyricus.

B. cadaveris.

FOOTNOTES:

[15] See note on Vivisection License, page 334.

XXI. BACTERIOLOGICAL a.n.a.lYSES.

Each bacteriological or bacterioscopical a.n.a.lysis of air, earth, sewage, various food-stuffs, etc., includes, as a general rule, two distinct investigations yielding results of very unequal value:

1. Quant.i.tative.

2. Qualitative.

The first is purely quant.i.tative and as such is of minor importance as it aims simply at enumerating (approximately) the total number of bacteria present in any given unit of volume irrespective of the nature and character of individual organisms.

The second and more important is both qualitative and quant.i.tative in character since it seeks to accurately identify such pathogenic bacteria as may be present while, incidentally, the methods advocated are calculated to indicate, with a fair degree of accuracy, the numerical frequency of such bacteria, in the sample under examination.

The general principles underlying the bacteriological a.n.a.lyses of water, sewage, air and dust, soil, milk, ice cream, meat, and other tinned stuffs, as exemplified by the methods used by the author, are indicated in the following pages, together with the methods of testing filters and chemical germicides; and the technique there set out will be found to be capable of expansion and adaptation to any circ.u.mstance or set of circ.u.mstances which may confront the student.

~Controls.~--The necessity for the existence of adequate controls in all experimental work cannot be too urgently insisted upon. Every batch of plates that is poured should include at least one of the presumably "sterile" medium; plate or tube cultures should be made from the various diluting fluids; every tube of carbohydrate medium that is inoculated should go into the incubator in company with a similar but uninoculated tube, and so on.

BACTERIOLOGICAL EXAMINATION OF WATER.

The bacteria present in the water may comprise not only varieties which have their normal habitat in the water and will consequently develop at 20 C., but also if the water has been contaminated with excremental matter, varieties which have been derived from, or are pathogenic for, the animal body, and which will only develop well at a temperature of 37 C. In order to demonstrate the presence of each of these cla.s.ses it will be necessary to incubate the various cultivations at each of these temperatures.

Further, the sample of water may contain moulds, yeasts, or torulae, and the development of these will be best secured by plating in wort gelatine and incubating at 20 C.

~1. Quant.i.tative.~--

_Collection of the Sample._--The most suitable vessels for the reception of the water sample are small gla.s.s bottles, 60 c.c. capacity, with narrow necks and overhanging gla.s.s stoppers (to prevent contamination of the bottle necks by falling dust). These must be carefully sterilised in the hot-air steriliser (_vide_ page 31).

(a) If the sample is obtained from a ~tap~ or ~pipe~, turn on the water and allow it to run for a few minutes. Remove the stopper from the bottle and retain it in the hand whilst the water is allowed to run into the bottle and three parts fill it. Replace the stopper and tie it down, but _do not seal it_.

(b) If the sample is obtained from a ~stream~, ~tank~, or ~reservoir~, fasten a piece of stout wire around the neck of the bottle, remove the stopper, and retain it in the hand. Then, using the wire as a handle, plunge the bottle into the water, mouth downward, until it is well beneath the surface; then reverse it, allow it to fill, and withdraw it from the water. Pour out a few cubic centimetres of water from the bottle, replace the stopper, and tie it down.

[Ill.u.s.tration: FIG. 203.--Esmarch's collecting bottle for water samples.]

(c) If the sample is obtained from a ~lake~, ~river~ or the ~sea~; or when it is desired to compare samples taken at varying depths, the apparatus designed by v. Esmarch (Fig. 203) is employed. In this the sterilised bottle is enclosed in a weighted metal cage which can be lowered, by means of a graduated line, until the required depth is reached. At this point the bottle is opened by a thin wire cord attached to the stopper; when the bottle is full (as judged by the air bubbles ceasing to rise) the pull on the cord is released and the tension of the spiral spring above the stopper again forces it into the neck of the bottle. When the apparatus is taken out of the water, the small bottles are filled from it, and packed in the ice-box mentioned below.

An inexpensive subst.i.tute for Esmarch's bottle can be made in the laboratory thus:

Select a wide-mouthed gla.s.s stoppered bottle of about 500 c.c. capacity (about 20 cm. high and 8 cm. in diameter).

Remove the gla.s.s stopper and insert a rubber cork with two perforations in its place.

Through one perforation pa.s.s a piece of gla.s.s tubing about 5 cm. long and through the other a piece 22 cm. long, reaching to near the bottom of the bottle, each tube projecting about 2.5 cm. above the rubber stopper. Plug the open ends of the tubes with cotton wool. Secure the stopper in place with thin copper wire.

[Ill.u.s.tration: FIG. 204.--Thresh's deep water sampling bottle.]

Sterilise the fitted bottle in the autoclave. Remove the cotton wool plugs and connect the projecting tubes by a piece of loosely fitting stout rubber pressure tubing about 5 cm. long, previously sterilised by boiling.

Take a piece of stout rubber cord about 33 cm. long, and of 10 mm.

diameter (such as is used for door springs) thread a steel split ring upon it and secure the free ends tightly to the neck of the bottle by cord or catgut.

Attach the cord used for lowering the bottle into the water to the split ring on the rubber suspender. The best material for this purpose is cotton insulated electric wire knotted at every metre.