Imágenes de páginas
PDF
EPUB

forms conidia (Fig. 130 e). Fig. 131 shows two derelor mental series of such endogenous conidia, and Fig. 132 thgermination of the same. As with Oidium, the phenomeno appears when young, vigorous mycelium is seeded in a litt. water; when placed in wort or on moist gypsum blocks, the mycelium can also display inter-growth, but it does not take place nearly so frequently.

This fungus forms a very strong layer on nutrient liquids. v. Skerst observed the following limits of temperature for its growth in wort: Maximum 31° to 32° C., optimum 16° C., and minimum 0:59 to 2° C. In a related form Hoffmann had found that the gelatinised cell walls were stained blue by iodine. The author observed that this also occurs sometimes in the typical Dematium pullulans.

The fungus is extremely common in nature, especially on fruit. It is found in moist places in breweries.

It decolorises wort and makes it ropy. According to Lindner it clouds white beer wort; according to Wortmann it also transforms grape must into a thready gelatinous substance, which is derived from a product of the outer parts of the cell membrane. This appearance is especially marked when cane sugar is present in the liquid. In wine, however, the fungus is quickly suppressed, though not killed, by the evolution of carbonic acid. In the presence of about 8 vol. per cent. of alcohol it does not grow, but is also not killed. The so-called cork or stopper flavour of wine arises from the cork of the bottle being overgrown with various fungi, Dematium and others.

According to Wortmann, Dematium pullulans is the cause of a disease of wine grapes. The attacked grapes look like those affected with "schwarzen Brenner," but the black spots are soft, not brittle, and depressed; further they have a white, mealy spot in the middle. At this place the mycelium of the fungus breaks through the epidermis of the grapes, and the yeast buds appear. The spots often reach half round the grapes.

Dematium pullulans is one of the mould fungi which

has been repeatedly regarded as the original form of the saccharomycetes. Communications relative to this have, however, suffered the same fate as those in which the same is maintained for Penicillium, Aspergillus, Mucor, etc. ; as soon as exact experiments were made, these ideas were found to be quite incorrect.

Finally, it may also be mentioned that various Ascomycetes are found, e.g., Sphærulina intermixta, Berk. and Br., and Dothidea ribesia, Pers., which can produce Dematium-like growths. If Brefeld says that the

OOOI

Fig. 132.- Dematium pullulans, De Bary. Germination of endogenous conidia

in wort in a cover glass preparation. The conidia (a) swelled up after twentyfour hours and put forth germ threads (ag) ; the same has also taken place with the uppermost two cells in the mycelium thread. 50%. (After Klöcker and Schiönning.)

species in question produce Dematium pullulans, he is in error, because these growths do not entirely correspond with the species named. Moreover, Brefeld has also been unable to produce Ascomycetes from a typical Dematium pullulans. Under certain conditions it is also found that some of the forms of Cladosporium herbarum produce Dematium-like growths.

Cladosporium herbarum, Link, is, as already mentioned, a collective name. In addition to the conidial form of Sphærella Tulasnei, referred to on p. 283, a number of other fungi are found, which have been classed under the name Cladosporium herbarum ; none of these, however, is known

to possess ascus fructification. Some are similar to Dematium pullulans, and some investigators (e.g., Laurent) have, for this reason, classed Cladosp. herbarum and Demat. pullulans in the same development series. Lopriore has studied a parasitical Cladosporium herbarum form occurring on wheat, which forms sclerotia. He states that it produces Hormodendron cladosporioides as well as Demat. pullulans. It is therewith stated that several species occur that produce conidial forms, which can be identified with the species referred to above.

II.–Fission FUNGI (SCHIZOMYCETES).

General.
1.-STRUCTURE AND FORM OF BACTERIAL CELLS.

The Cell Contents.—Like yeast cells, the bacterial cell consists of a mass of protoplasm surrounded by a membrane. It has been contended as to whether it contains a cell nucleus or not. Later investigations, however, show that in this respect also bacterial cells resemble other cells.

As with yeast cells, vacuoles and granules occur in the protoplasm. In some bacteria a substance is found which is probably granulose or an allied carbohydrate, as it is stained blue by iodine. Sulphur granules or oxide of iron are found in other forms; occasionally fat globules are observed, especially in the cells of old cultures. Colouring matter is found in many species, often in such quantity that the colonies are highly coloured, e.g., red, yellow, blue, violet, brown, etc. The colouring matter is found partly in the interior of the cells, and partly secreted as granules lying between the cells.

The Cell Wall and its Gelatinous Formation. If the cell is placed in a solution of common salt, the plasma separates from the membrane so that the latter becomes plainly visible. This phenomenon is called plasmolysis. The membrane does not consist of cellulose but of albuminoids, probably modifications of those forming the protoplasm. The membrane often possesses the property of forming gelatine and swells up. The growths are then enveloped in mucilage and form gelatinous films or masses, called Zooglææ (Fig. 133). In some species the mucilage is stained blue by iodine, in others it gives the cellulose reaction (blue stain by iodine in zinc chloride or iodine and sulphuric acid).

Fig. 133. -- Bacterium Pasteurianum, Hansen. Gelatinous formation in an old

growth on beer. The three cells to the left have fallen out. The cells are prepared and stained by Löffler's method. 199. (After Hansen.)

Flagella. — Many bacteria are observed under the microscope to be in motion; this motion is often due to physical causes and is then the so-called Brownian molecular movement. Another kind of movement is effected by special organs of motion, flagella or cilia. The rapidity which this motion attains, has, with some species, an average value of about į mm. per second. The flagella or cilia were discovered in 1836 by Ehrenberg, and, as a rule, can only be observed after a special preparation (see p. 91). In some forms they are found at the poles of the cells, in others at the sides : occasionally several are found to

gether. Alfr. Fischer distinguishes between bacteria (1) with one flagellum at the end (Fig. 135), (2) with a cluster of Hagella at the end, and (3) with flagella over the whole surface (Fig. 134). Flagella furnish important characteristics for the determination of species, and for classifi

Fig. 134. --Clstridium butyricum, Prazmowski. Butyric acid bacterium with

flagella. a, Vegetative motile cell ; b, sporulating motile cell. me. (After Alfr. Fischer.)

cation it is of importance to determine if the species concerned occurs with flagella or without.

Bacteria with flagella movement are called motile. This motion may be temporarily stopped by certain means, e.g., by an increase in the acid content of the nutrient medium or by a deficiency of oxygen; the condition of the organism

FIG. 135. -- Termobacterium aceti, Zeidler. Acetic acid bacterium with one

flagellum. 1200. (After Zeidler.)

is then known as flagella-stiffness (Geisselstarre). By neutralisation or by aëration the stiffness can again be removed. When, therefore, bacteria in a culture do not move just at the instant, one cannot be always certain that the power of motion has altogether left them. Various sub

« AnteriorContinuar »