Bacteriological techniques for dairy purposes.

  • Great Britain. Ministry of Agriculture, Fisheries and Food.
Date:
1968
Catalogue details

Licence: Public Domain Mark

Credit: Bacteriological techniques for dairy purposes. Source: Wellcome Collection.

    37/192 (page 29)
    6. Incubation of Bottles Place the bottles in divided baskets with the seals upwards. Proceed as detailed previously in para. I4. 7. Counting of Colonies Proceed as detailed previously in para. I5 with the following modifications: a colony illuminator designed for this method is desirable; only bottles with counts between 15 and 250 should be used for assessing the results; and to record the count per ml, multiply the count per bottle by two and multiply the answer by the reciprocal of the dilution from which the bottle was inoculated. Ill Drop Plate Method 1. Introduction This method consists of dropping from a capillary pipette a known volume of bacterial suspension on to the surface of an agar plate, allowing the liquid to be absorbed, incubating the plate and counting the resulting colonies. It is based upon a technique originally described by ais and Misra (see Ref. 7). When compared with the conventional pour-plate technique it erates a saving of media and glassware to be made, it is quicker and more flexible in its application, it enables the colony form of the bacteria to be studied and it does not expose bacteria to the temperature of molten agar. The same degree of accuracy can be achieved as with the pour-plate method. 2. The Preparation of Capillary Pipettes A suitable method is described by the Medical Research Council (see Ref. 6). Pipettes are made from an 8 in. length of clean, soda glass tubing of 8 mm external diameter. By heating in the centre, the glass can be drawn out to make two pipettes with relatively parallel sides. ‘The size of the drop expelled from such a pipette is governed by the external diameter of the tip, the speed at which the drop is formed and the nature of the medium in which the ‘bacteria are suspended. For performing colony counts a drop size of 0:2 ml is convenient and this may be achieved by the use of a pipette with a capillary tip diameter of 0-043 in. and by dropping at the rate of 20-30 drops per min. This will be correct for suspensions in water, physiological saline or quarter- strength Ringer’s solution, but if other suspending media are used, e.g., nutrient broth, correction factors should be determined by comparing the weight of a known number of drops of the particular medium with the weight of the same number of drops of water. In order to ensure that the capillary tip is of the correct size it is in- troduced into the No. 57 hole (0-043 in. diam) of a Morse Twist Drill or Starrett Gauge and pressed down until it engages. The pipette is then cut off squarely with a sharp glass cutter at the point of impaction. The pipette should be rotated with the tip at eye level and discarded if seen to be cracked or not cut at right angles to the axis. To ensure that the tip of the pipette is free from grease it is advisable to dip it in acetone. The pipette is plugged at the wide end with cotton wool, placed in a suitable container, and sterilized in the hot-air oven.