Practical bacteriology for chemical students / by David Ellis.

  • David Ellis
Date:
1923
Catalogue details

Licence: In copyright

Credit: Practical bacteriology for chemical students / by David Ellis. Source: Wellcome Collection.

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    6. After two or three days (or longer if necessary) count the colonies. (See Exercises 68 and 69.) Suppose the whole plate contains 240 colonies. Therefore c c. Sewage contains 240 bacteria » 1 „ „ „ 24.000000 „ A cubic centimetre of water should be taken from No. 6 and plated in the same way. The number of colonies in this case must be multiplied by a million to find the number of bacteria per cub.cm. The average of the two readings must be taken as the number of bacteria per cub.cm. Note. 1. It is well to bear in mind that the exercise must be carried out with very strict attention to the sterility of the various apparatus that are employed. In our calculations above we have to multiply the number of colonies in the one case by 100.000, in the other by 1.000. 000. The result of the entrance of each 'intruder’ into the plates will mean therefore an error of either 100.000 or 1.000.000 in our final result. 2. When we blow our 1 c.c. from the pipette into the gelatine-tube, every microbe left adhering to the walls of the pipette results in an error of 100.000 or 1.000. 000 in the final result. Some allowance should be made for this, and a twentieth added to the final result. The observance of the following rules will help the student to obtain fairly consistent results. 1. See that all the apparatus is thoroughly sterile. 2. Make the dilutions with accuracy. 3. After each dilution mix thoroughly. 4. After blowing out the water from the pipette into the tube, which contains the 9 c.c. of water, suck up and blow out the mixture, several times so that the number of bacteria adherent to the walls of the pipette may be as few as possible. 5. A pipette must not be used more than once in the same experiment. 6. Count the colonies with great care. Ellis’ Practical Bacteriology. 7