A text-book of general bacteriology / by Edwin O. Jordan.

  • Edwin O. Jordan
Date:
1931
Catalogue details

Licence: In copyright

Credit: A text-book of general bacteriology / by Edwin O. Jordan. Source: Wellcome Collection.

    39/830 page 37
    SPECIAL MEDIA AND BIOCHEMICAL TESTS At the time of plating there is added to every 100 cc. of the above medium while hot 0.5 cc. of a 1 per cent alcoholic basic fuchsin solution and 0.5 cc. of a freshly prepared 5 per cent sodium sulfite solution. On this medium members of the colon-aerogenes group give medium-sized red colonies, usually surrounded by a deep red ring. Other organisms that develop give pink or uncolored colonies. Special Media and Biochemical Tests.—A great variety of special media are used in connection with the study of different organisms, either because such media are particularly favorable to the growth of those organisms or because they reveal certain char¬ acteristic features and biochemical reactions. The addition of glycerol to nutrient agar, for example, favors the development of the tubercle bacillus. Certain organisms, of which the so-called “ influ¬ enza bacillus” is the type, are favored by the presence of hemoglobin in the culture medium, and are often designated on that account as the hemophilic bacilli.1 A few media have a decisive differential value: the typhoid bacillus does not produce either gas or acid in lactose broth, whereas a closely allied bacillus, Bact. coli, found in the normal human intestine, actively ferments lactose. In general, the ability to ferment carbohydrates or substances like mannitol and glycerol which are added to the ordinary sugar-free culture media, constitutes one of the most important differential characters of bacteria. The reducing power of bacteria may be measured by the loss of color of litmus (as in litmus milk) or of methylene-blue, or by the reduction of nitrates to nitrites. The reduction of nitrates may be determined in the following way: After four days' incubation at 37 C. in nitrate broth (0.1 per cent peptone, 0.02 per cent nitrite- free potassium nitrate) add to 3 cc. of the culture in a clean test- tube 2 cc. each of the following solutions: (1) Sulphanilic acid solution made by dissolving 8 grams of the purest sulphanilic acid in 1000 cc. of 5N acetic acid (sp. gr. 1.041); (2) a-amidonaph- thalene acetate solution prepared by dissolving 5.0 grams solid a-naphthylamine in 1000 cc. of 5N acetic acid and filtering the solu¬ tion through absorbent cotton. The development of a rose color indicates the presence of nitrites. An uninoculated tube of the medium should always be treated in the same way for a control. 1 Davis: Jour. Infect. Dis., 1907, 4, p. 73.
    digitised image 1
    digitised image 2
    digitised image 3