The essentials of practical bacteriology : an elementary laboratory book for students and practitioners / by H. J. Curtis.

  • Curtis, Henry J.
Date:
1900
Catalogue details

Licence: In copyright

Credit: The essentials of practical bacteriology : an elementary laboratory book for students and practitioners / by H. J. Curtis. Source: Wellcome Collection.

Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.

    73/314 (page 55)
    of the individual bacteria such as we have abeady examined. Their diameter may be as much as 8 or 9-5 /x. The thin cell-wall encloses a granular protoplasm, containing one or more vacuoles, and some- times a nucleus (best seen in old cultures, and by staining with haematoxyhn, osmic acid, &c.). Multiplication, as already mentioned, usually occurs by a system of budding or gemmation, the buds, or daughter-cells, remaining attached for a variable time to the mother-cell. This process is well seen in an actively fermenting saccharine solution. In the true yeasts, Saccharomyces cerivisia, S. ellipsoideus, &c., under certain circumstances, three or four spores may be seen within the cell {endospores), whereas in the closely allied group, the true TondcB, no spore formation has ever been observed at any stage of develop- (gemmatiou)' ment, or under any circumstances by ^ 0^ Hansen, whose researches, together jiother-oeii—^^ with those of his pupils, have done so much to revolutionise the processes of Fig. 37.—yeast cells modern brewing. Thus, Tonola rosea, (sac™myces^ ceeivisi^). commonly called zjeast, is no ^-p^,^^ Schenk's 'Bacteriology'] longer considered to be a true yeast. Strictly speaking, it should be included in a separate order of its own, the Torulacece, though under the microscope, apart from the question of spores, it is not to be distinguished from a yeast. The conditions for spore formation in yeasts are that the cells must be young, and they should be freely supplied with air and moisture. The most favourable temperature, generally speaking, is 25° C. But the important observation was made by Hansen that at lower temperatures different yeasts require different periods for the development of spores. Thus S. cerivisice forms spores in 30 hours at temperatures ranging from 25° to 375° C, but it requires 10 days to do so at 11'5° G. At this lower temperatm'e, under similar conditions otherwise, another yeast (S. pastorianus II.) forms spores in 77 hours. This has been turned to practical account, and thus the presence of ' wild' yeasts, as they are called, may be ascertained if contaminating yeast about to be used for brewing pm'poses.' Appearances presented by the cultiLres A, B, C, D. A. Saccharomyces cerivisiae (or heer yeast) is a typical 'high' yeast plant (see p. 61). Cultivations.—Gelatine streak.—The growth consists of raised ' Sims Wooclhead. Bacteria and their Products.