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Unfortunately the fermentation of cellulose by bacteria is insufficiently studied. So much seems certain, that at least one anaerobic variety converts cellulose into marsh-gas and carbonic acid. Yet Van Senus maintains that the anaerobic "Bacillus amylobacter," isolated by him, attacks cellulose only in symbiosis with another small bacillus. (Compare the résumé by Herfeld, C. B. L. 1, 74, 114, and also the special part.)

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Fig. 11.-Bacterium coli upon sugar-agar, after twelve, twentyfour, and forty-eight hours.

14. Gas-production from Carbohydrates and Other Fermentable Bodies of the Fatty Series.

The only gas eventually arising in visible quantity1 in nutrient media which contain no sugar is nitrogen (compare page 82).

1 Sulphuretted hydrogen and ammonia can scarcely occur in visible quantity.

If sugar is broken up energetically by bacteria, gasformation may be absent, only lactic or acetic acid being produced (for example, Bac. typhi on grapesugar), but very often an enormous production of gas occurs, especially if air is excluded. About one-third of the vigorous acid-forming varieties produce abundant gas, which consists always of carbonic acid, with a constant admixture, according to Smith (C. B. xviii, 1), of hydrogen. Marsh-gas appears to be rarely formed (aside from the bacteria causing fermentation of cellulose). Compare in special part: Bact. brassicæ acidæ.

To determine whether gas is formed, the shake-culture in 1% grape-sugar agar is very useful (Fig. 11). After

Fig. 12.-Fermentation tube.

If it

twenty-four hours (if incubator temperature is available, often after six to twelve hours) the agar is beset by gas bubbles or cleft by numerous deep holes and cracks. is desirable to collect and measure the gas, to investigate the curve of the intensity of gas production, or to analyze it, the gas is best collected after the method of Th. Smith in a fermentation tube, such as has been long employed in physiologic and pathologic chemistry (Fig. 12).

The tubes, which preferably have the same form, are filled with 1% grape-sugar peptone-bouillon (without airbubbles) and sterilized in the steam sterilizer.

After inoculating with a platinum loop, they are kept

in the incubator and the following observations made (Th. Smith):

1. If cloudiness occurs only in the open limb, then one is dealing with an aerobic variety; if only the fluid in the closed limb becomes cloudy, while that in the bulb remains clear, then an anaerobic variety is present.

2. One notes the daily gas-formation by an ink mark, and, if the tubes are graduated, in four to six days, after the gas production has ceased, the percentage of gas formed on each day can be determined.

With this

3. A rough analysis of the gas is made. object in view, after indicating the quantity of gas produced by means of a mark, the open bulb is completely filled with 10% solution of caustic soda and closed tightly with the thumb. The fluid is shaken thoroughly with the gas and allowed to flow back and forth from bulb to closed branch and the reverse several times. Finally, the gas is allowed to again rise in the closed limb, and after removing the thumb, the new volume of gas is read. The part removed consists of CO2; that remaining consists of nitrogen, hydrogen, and marsh-gas. For quantitative analysis of these gases it is best to employ Hempel's gas pipet. (Compare Cl. Winkler, Lehrbuch der techn. Gasanalyse, Freiberg, 1892.) The principle of the method is that hydrogen mixed with oxygen carried over red-hot palladium asbestos is converted into water, thus disappearing; marsh-gas in a red-hot platinum capillary is burned up to carbonic acid, and as such is determined; what is left is nitrogen. With some practice the investigation is easy and accurate.

15. Production of Acids from Alcohols and from Other Organic Acids.

The transformation, in weak solutions, of ethyl alcohol, under energetic consumption of oxygen, into acetic acid by the Bact. aceti or its nearest relatives has long been known (compare p. 66 and special part):

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Also higher alcohols: Glycerin, dulcite, and mannite are changed into acids; glycerin as constantly as sugar is (v. Sommaruga, Z. H. xv, 291).

Finally, numerous results have been obtained regarding the transformation of acids of the fatty series (or their salts) into other fatty acids. by bacteria. Unfortunately, the earlier observations were made without the employment of such pure cultures as satisfy the present demands. Usually lactate, malate, tartrate, citrate, and glycerate of calcium were employed, while almost always mixtures of acids were obtained as a result of the activity of the bacteria. Among these, butyric, propionic, valerianic, and acetic acid play the principal rôle; often also there occur succinic acid, ethyl alcohol, and more rarely formic acid. Of gases, carbonic acid and hydrogen occur especially.

Such investigations were formerly carried out particularly by Fitz, and more recently they have been performed on an extensive scale with undoubtedly pure cultures and with interesting results by P. Frankland.

Here only two illustrations are given: Pasteur found that anaerobic bacteria converted lactate of calcium into the butyrate:

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According to P. Frankland, the Bacillus æthaceticus Fitz forms from the glycerate of calcium, (CH2OH-CHOH-COO), Ca, ethyl alcohol, acetic acid, carbonic acid, and hydrogen.

5. THE PATHOGENIC ACTION OF BACTERIA (PATHOGENESIS, PREDISPOSITION, RESISTANCE, IMMUNITY).

I. How Do Bacteria Act Pathogenically? If micro-organisms enter the tissues or blood of an animal, there occurs an infection if, at the same time1. The micro-organisms can remain alive and increase within the host.

2. If the micro-organisms produce substances which are injurious to the host.

Theoretically, the blood and organs of healthy animals contain no germs, yet we must accept the fact that often solitary streptococci, tubercle bacilli, etc., are present in the healthy body, circulating in the lymph- and bloodstream and becoming fixed at loci minoris resistentiæ, from which places they extend further. Perez found, in a systematic examination of healthy bodies, that only the lymph-glands contain bacteria, but here the bacterial flora was very rich. In dead animals after sixteen to twenty hours at room temperature or after five to six hours in the incubator, bacteria are found in the blood and organs (Trombetta), having for the greater part wandered from the intestine. In the most frequent artificial method of infection, i. e., subcutaneous injection, the bacteria are absorbed through the lymph-stream, and in part are held back in the lymph-glands, weakened in their virulence. and even killed; but if the organisms are strongly "pathogenic," they resist total destruction, and, on the contrary, begin to increase a few hours after entrance. Regarding the method of destruction of bacteria, compare page 97.

Wherever we inspect the character of the pathogenic action of bacteria, they are found to operate through chemical substances which they produce or which are produced from them in the animal body. Thus far we are able to understand the action of only those bacteria which in cultures form poisonous substances, by means of which we can also reproduce the characteristic picture of the disease more or less accurately. Bacteria of this class are: Bac. tetani and Corynebact. diphtheria, Streptococcus pyogenes, Micrococc. pyogenes, Vibrio cholera, etc. There has already (p. 73) been given a sketch of what we know chemically about these poisonous substances.

On the contrary, the means for an explanation upon a chemical basis are almost entirely absent as yet in a series of important infectious diseases, among which are, for example, anthrax (Conradi, Z. H. xxxi, 287) and swine

'It may be mentioned here that often a considerable number of micro-organisms are also rapidly secreted through the bile and urine without any perceptible injury to the organ being demonstrable (Biedl and Krans, Z. H. XXVI, 353), yet many authors assume, in these cases, at least microscopic rupture of vessels (Opitz, Z. H. XXIX, 505).

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