rapidly fatal; for the mesophilic, 1 about 60° (Forster); for the thermophilic, 75°. A temperature of 100° is not withstood by any bacterium free of spores for more than a few minutes.

7. MECHANICAL AND ELECTRICAL
INFLUENCES.

In the first edition, at this point I reported the astonishing statements of Meltzer, according to whom short, feeble shaking would operate favorably upon the growth of fluid cultures of bacteria, while more prolonged and more vigorous shaking or long-continued very feeble shaking would operate very unfavorably (Meltzer, Zeit. f. Biol. xxx, p. 464).

Otto Appel, who, at my request, restudied the whole question, arrived at entirely different results. No shaking of longer or shorter duration injured the bacterial growth, except where very severe agitation and the addition of glass pearls caused direct mechanical lesions of the bacteria. Slight shaking, such as cultures experience when placed upon the foundations of strongly working steam-engines, was without effect. (Further communications thereon are found in the Archiv für Hygiene.)

Most of the previously observed effects of the electric current are easily explained as due to the action of heat and electrolysis. Thiele and Wolf demonstrated by means of new investigations, which are not open to objection, that neither the passage of a constant or interrupted current through a culture of bacteria, if electrolysis be avoided, nor the placing of a culture in an interrupted current induction coil, injures the bacteria (C. B. XXV, 650); l. c., also the previous literature, which in part contains remarkable assertions.

8. INFLUENCE OF LIGHT AND RÖNTGEN

RAYS.

The cultures of all bacteria are restricted in their growth by direct sunlight. If the action is more prolonged, they are subsequently less able to grow luxuriantly in the dark, and there results a generation of weakened organisms, shown, for example, by incomplete liquefaction, slight production of pigment, lessened virulence, etc., which only regain their original properties after repeated transplanta

1According to Sternberg the following die at 56°: Streptococcus pyogenes, Bac. anthracis, Bact. mallei, and Vibrio cholera (Amer. Jour. Med. Sciences, July, 1887, 146).

tion on fresh medium in the dark. With still longer action of the direct sunlight the micro-organisms die.

Bact. putidum and Bact. prodigiosum were materially disturbed in their ability to produce pigment and trimethylamin by direct sunlight, in July and August in one-half hour and in November in one and a half hours. They grew slowly and prodigiosum liquefied slowly (Dieudonné). Death was produced in these organisms in one and a half and two and a half hours.

Dieudonné (A. G. A. IX, 405 and 537, also an extensive review of literature) has found ultra-violet, violet, and blue light to be very injurious, green but slightly, and red and yellow not at all. On the contrary, Beck and Schultz (Z. H. XXIII, 490), who employed better light-filters, generally observed no injury from the colored light obtained from sunlight. This is to be explained by the slight intensity of the light employed. Also, Beck and Schultz deny that diffuse daylight works injury to bacteria (the chromogenic function is lost in many varieties even when grown in the dark), while Dieudonné asserts :

In diffuse daylight, in spring and summer in three and a half hours and in winter in four and a half hours, there occurs interference with growth, while in from five to six hours death is produced. Electric arc light of 900 candle-power checks growth in five hours and kills in eight hours. Incandescent light injures growth in from seven to eight hours, and kills in eleven hours. Similar results occur with Bact. coli, typhi, and B. anthracis. Naturally, Dieudonné's positive results are not disproved by Beck and Schultz's negative ones.

1

For testing the sensitiveness to light, thickly sown gelatin or agar plates are exposed to diffuse or direct sunlight, after the method of H. Buchner, a dark paper cross being placed upon the illuminated side. To exclude the action of heat, one may first carry the light through a layer of water or alum a few centimeters in thickness. After the light has acted for one-half, one, one and a half, two, etc., hours, the plates are placed in a dark room, and it is observed whether the bacteria develop only on the part covered by the cross; in complete death of the exposed bacteria there occurs a sharply outlined cross consisting of the colonies in a clear field.

1 The action of the heat is entirely without interest.

The action of the light seems to occur under the cooperation of the oxygen of the air; obligate anaerobes (tetanus) and facultative aerobes (B. coli), when oxygen is completely excluded, withstand the sunlight very well; for example, B. coli withstood direct intense sunlight for four hours. (Compare also Wesbrook, Journal of Pathology and Bacteriology, IV, 352.)

Regarding the mechanism of the action of light, the observations of Richardson and later of Dieudonné appear important, if not furnishing a complete explanation. They assert that in illuminated agar plates, and indeed only in blue to ultra-violet light, in a short time (even after ten minutes in direct sunlight) peroxid of hydrogen1 appears. For its demonstration one exposes to the light an agar plate, half covered with dark paper, then pours over the same a weak iodid of potassium paste, and upon this a weak solution of ferrous sulphate; the illuminated side becomes dark blue. (With gases that do not contain oxygen there is no formation of H2O2, nor injurious action from light:) This also explains what has been often observed, that one may obtain a slight attenuation of bacilli if they are inoculated upon agar plates that have previously stood in the sunlight.2 Bacteria previously exposed to light develop especially badly upon media that have been illuminated, much more so than upon good media.

According to Rieder, strong Röntgen rays injure bacterial growth in a way similar to light (Münch. med. Woch., 1898, No. 4, 101).

9. THE EFFECTS ON BACTERIAL GROWTH OF OTHER BACTERIA.

Although it is the endeavor of every bacteriologist to always obtain bacteria in pure culture, we must not forget that in nature bacteria often occur in combination. If

1 With gelatin it is hours before H2O2 can be recognized.

2 Also other decompositions of the nutrient medium by sunlight may occasionally render difficult a subsequent growth of fungi; for example, the origin of formic acid from tartaric acid (Duclaux).

Sy

we examine water, milk, or the intestinal contents in health or disease, we always find many varieties simultaneously present. This mixture certainly appears to us as a pure accident, but upon closer study it is found that also in the domain of bacteriology there are synergists (mutual or one-sided aid) and antagonists (mutually injurious, or one to the other). Nencki speaks of symbiosis and enantobiosis.

Experimentally, Garrè has demonstrated the antagonism by inoculating various bacteria simultaneously in streaks upon gelatin plates as parallel or intersecting lines. It then appears that many varieties thrive but slightly or not at all if another variety grows in the immediate neighborhood. The antagonism is very often only on one side; for example, the Bact. putidum grows very well if inoculated between closely placed, well-developed streaks of staphylococci; on the contrary, the Micr. pyogenes does not grow if inoculated between luxuriantly developed cultures of Bact. putidum, and if the two are simultaneously inoculated in alternating streaks, the former grows very slightly (Garrè, "Correspondenzbl. f. Schweizer Aerzte," 1887, 387).

Another way of showing antagonism is by preparing plates of gelatin or agar (for liquefying varieties), which, while liquid, are inoculated with equal quantities of two different varieties of bacteria; often only one variety will develop (Lewek, C. B. VII, 107).

A third way of carrying out the investigation is to inoculate simultaneously the same fluid nutrient medium with two varieties, and, later, determine microscopically or by plates which is triumphant in the battle. This is what is commonly observed when a cause of fermentation is abundantly introduced into a suitable nutrient medium; the contaminating bacteria are overgrown and sometimes perish.

From these observations the practical conclusion is reached that for determining the number of bacteria in a material the colonies in the plates must not be very thick, and also that for the isolation of definite varieties, thin plates are necessary; for example, if one wishes to isolate the Bact. Pflügeri from abundant Bact. putidum. In an area of several millimeters about each putidum colony no Bact. Pflügeri will grow (K. B. Lehmann).

Finally, in the animal body bacteria may counteract each other as antagonists; as Emmerich has pointed out, animals infected with anthrax may be saved by subsequent infection with Streptococcus pyogenes. Literature by Mühlmann (C. B. xv, 895).

Symbiosis of bacteria appears to be of more practical

importance, and of this the following examples may be cited:

1. Some bacteria thrive much better together with others than when alone. Some anaerobes thrive even with the admission of oxygen, if only certain aerobic varieties are present. (Compare B. tetani.)

2. Certain chemical transformations-for example, the breaking up of nitrite with liberation of free nitrogen-are not accomplished by many bacteria alone, while two varieties jointly may do so. This fact is well worth considering when searching for the cause of certain decompositions. Always when the isolated varieties, acting singly, operate partially or not at all, the effects of combinations must be investigated.

3. In a similar manner it is observed that, for example, the single individuals of a series of soil bacteria are not pathogenic, while certain combinations, inoculated into animals, make them sick. 1 (Liermann, C. B. vIII, 364.) This experience also demands that especial care be taken in searching for the cause of a new or puzzling disease-picture. Many authors believe that cholera has its origin in two germs, "diblastic theory" (Nägeli, Buchner).

4. Attenuated pathogenic varieties-for example, attenuated tetanus bacilli-may increase in virulence when cultivated together with other bacteria; for example, Bact. vulgare.

E. The Conditions of Spore-formation and Spore-germination.

The extent of the formation of endogenous spores appears hitherto to have been very insufficiently understood. Except in a large group of important varieties of bacilli, related to the B. anthracis and the B. tetani, undoubted

1Not quite appropriate here is the experience that the metabolic products of one variety of bacterium, under some circumstances, may enhance the action of another variety; for example, the metabolic products of the Bact. vulgare the action of the tetanus bacillus.