The Elements of Bacteriological Technique Part 118

EXAMINATION OF CREAM AND b.u.t.tER.

~Collection of the Sample.~--Collect, store, and transmit samples to the laboratory, precisely as is done in the case of ice cream.

~Quant.i.tative.~--

_Apparatus Required_:

Sterile test-tube.

Sterilised spatula.

Water-bath regulated at 42 C.

Case of sterile plates.

Case of sterile graduated pipettes, 1 c.c. (in hundredths).

Tubes of gelatine-agar (+10 reaction).

Plate-levelling stand, with its water chamber filled with water at 42 C.

METHOD.--

1. Transfer a few grammes of the sample to a sterile test-tube by means of the sterilised spatula.

2. Place the tube in the water-bath at 42 C. until the contents are liquid.

3. Liquefy eight tubes of gelatine-agar and place them in the water-bath at 42 C, and cool down to that temperature.

4. Inoculate the gelatine-agar tubes with the following quant.i.ties of the sample by the help of a sterile pipette graduated to hundredths of a cubic centimetre--viz.,

To tube No. 1 add 1 c.c. liquefied b.u.t.ter.

2 add 0.5 c.c. liquefied b.u.t.ter.

3 add 0.3 c.c. liquefied b.u.t.ter.

4 add 0.2 c.c. liquefied b.u.t.ter.

5 add 0.1 c.c. liquefied b.u.t.ter.

6 add 0.05 c.c. liquefied b.u.t.ter.

7 add 0.03 c.c. liquefied b.u.t.ter.

8 add 0.02 c.c. liquefied b.u.t.ter.

9 add 0.01 c.c. liquefied b.u.t.ter.

5. Pour a plate cultivation from each of the gelatine-agar tubes and incubate at 28 C.

6. "Count" the plates after three days' incubation, and from the figures thus obtained estimate the number of organisms present per cubic centimetre of the sample.

~Qualitative.~--

_Apparatus Required_:

Sterile beaker, its mouth plugged with sterile cotton-wool.

Counterpoise for beaker.

Scales and weights.

Sterilised spatula.

Water-bath regulated at 42 C.

Separatory funnel, 250 c.c. capacity, its delivery tube protected against contamination by pa.s.sing it through a cotton-wool plug into the interior of a small Erlenmeyer flask which serves to support the funnel. This piece of apparatus is sterilised _en ma.s.se_ in the hot-air oven.

Large centrifugal machine.

Sterile tubes (for the centrifuge) closed with solid rubber stoppers.

Case of sterile pipettes, 10 c.c.

Case of sterile graduated pipettes, 1 c.c. (in tenths of a cubic centimetre).

METHOD.--

1. Weigh out 100 grammes of the sample in a sterile beaker.

2. Plug the mouth of the beaker with sterile cotton-wool and immerse the beaker in a water-bath at 42 C. until the contents are completely liquefied.

3. Fill the liquefied b.u.t.ter into the sterile separatory funnel.

4. Transfer the funnel to the incubator at 37 C. and allow it to remain there for four days.

At the end of this time the contents of the funnel will have separated into two distinct strata.

(a) A superficial oily layer, practically free from bacteria.

(b) A deep watery layer, turbid and cloudy from the growth of bacteria.

5. Draw off the subnatant turbid layer into sterile centrifugal tubes, previously warned to about 42 C., and centrifugalise at once.

6. Pipette off the supernatant fluid and fill the tubes with sterile 1 per cent. sodium carbonate solution previously warmed slightly; stopper the tubes and shake vigourously for a few minutes.

7. Centrifugalise again.

8. Pipette off the supernatant fluid; filling the tubes with warm sterile bouillon, shake well, and again centrifugalise, to wash the deposit.

9. Pipette off the supernatant fluid.

10. Prepare cover-slip preparations, fix and clear as for milk preparations, stain carbolic methylene-blue, Gram's method, Ziehl-Neelsen's method, and examine microscopically with a 1/12 inch oil-immersion lens.

11. Proceed with the examination of the deposit as in the case of milk deposit (see pages 450 _et seq._).

EXAMINATION OF UNSOUND MEATS.

(INCLUDING TINNED OR POTTED MEATS, FISH, ETC.)

The bacterioscopic examination of unsound food is chiefly directed to the detection of those members of the Coli-typhoid group--B. enteritidis of Gaertner and its allies--which are usually a.s.sociated with epidemic outbreaks of food poisoning, and such anaerobic bacteria as initiate putrefactive changes in the food which result in the formation of poisonous ptomaines, consequently the quant.i.tative examination pure and simple is frequently omitted.