The colonies which form upon gelatin are circular, have smooth borders, and look very much as if bored out with a tool. They have a brown color and are finely

FIG. 94. Spirillum aquatilis, from an agar-agar culture; x 1000 (Itzerott and Niemann).

granular. In gelatin puncture-cultures the growth occurs almost exclusively at the surface.

The agar-agar cultures are similar to those of cholera. Scarcely any development occurs in bouillon. By the growth of the organism sulphuretted hydrogen gas is produced.

The spirillum does not grow at all upon potato.

Günther did not find the organism to be pathogenic. Spirillum Terrigenus.-This species, also discovered by Günther, was secured from earth. It generally occurs in a slightly curved form, but sometimes is spiral. It is actively motile and has a terminal flagellum.

The colonies, which appear in twenty-four hours, are small, structureless, and transparent, and later take on a "fat-drop" appearance.

Upon agar-agar a thin white coating is formed. Milk is coagulated by the growth of the organism. No indol is produced.

The organism does not stain by Gram's method, and is said not to be pathogenic for guinea-pigs or for mice.

CHAPTER VIII.

PNEUMONIA.

THE term "pneumonia," while generally understood to refer to the lobar disease particularly designated as croupous pneumonia, is a vague one, really comprehending a variety of conditions quite dissimilar in character. This being true, no one will be surprised to find that a single organism cannot be described as "specific" for them all. Indeed, pneumonia must be considered as a group of diseases, and the various microbes found associated with it must be described successively in connection with the peculiar phase of the disease in which they occur.

1. Lobar or Croupous Pneumonia.-The bacterium, which can be demonstrated in at least 75 per cent. of the cases of lobar pneumonia, which is now almost universally accepted as the cause of the disease, and about whose specificity very few doubts can be raised, is the pneumococcus of Fränkel and Weichselbaum.

Priority of discovery in the case of the pneumococcus seems to be in favor of Sternberg, who as early as 1880 described an identical organism which he secured from his saliva. Curiously enough, Pasteur seems to have captured the same organism, also from saliva, in the same year. The researches of the observers whose names are attached to the organism were not completed until five years later. It is to Fränkel, Telamon, and particularly to Weichselbaum, however, that we are indebted for the discovery of the relation which the organism bears to pneumonia.

The pneumococcus should rather be called the pneumobacillus, for it habitually has an elongated form, and in its most typical form is so distinctly elongate as to be

described as lanceolate. However, popular parlance has now made it almost impossible to introduce Bacillus pneumoniæ instead of Diplococcus pneumonia (Weichselbaum), especially as there is already another organism bearing that name. (See Bacillus pneumonia of Friedländer.) The organism (Fig. 95) is variable in its morphology. When grown in bouillon it is oval, has a pronounced dis

FIG. 95.-Diplococcus pneumoniæ, from the heart's blood of a rabbit; × 1000 (Fränkel and Pfeiffer).

position to occur in pairs, and not infrequently forms chains of five or six members, so that some have been disposed to look upon it as a streptococcus (Gamaléia). In the fibrinous exudate from croupous pneumonia, in the rusty sputum, and in the blood of rabbits and mice containing them the organisms are arranged in pairs, exhibit a distinct lanceolate shape, the pointed ends generally approximated, and are usually surrounded by a distinct halo or capsule of clear, colorless, homogeneous material, thought by some to be a swollen cell-wall, by

others a mucus-like secretion given off by the cells. When grown ordinarily in culture-media, and especially upon solid media, the capsules are absent.

The organism is without motility, has no spores, and does not seem to be able to resist any unfavorable conditions when grown artificially. It stains well with the ordinary solutions of the anilin dyes, and gives most beautiful pictures in blood and tissues when stained by Gram's method. The capsule does not stain.

The bacillus is no stranger to us, but can frequently be found in the saliva of healthy individuals, and the inoculation of human saliva into rabbits generally causes a septicemia in which the bacillus is found abundantly in the blood and tissues. Because of its constant presence in the saliva it was described by Flügge as the Bacillus septicus sputigenus.

When desired for purposes of study, it can be obtained by inoculating rabbits with saliva and recovering the organisms from their blood, or it can be secured from the rusty sputum of pneumonia by the method employed by Kitasato for securing tubercle bacilli from sputum. A single mouthful of fresh sputum is secured, washed in several changes of sterile water to free it from bacteria of the mouth and pharynx, carefully separated, and a central portion transferred to an appropriate culture-medium.

The organism grows upon all the culture-media except potato, but only between the temperature extremes of 24° and 42° C.; the best development is at 37° C. The growth is always limited, probably because the formic acid produced serves to check it. The addition of an unusual amount of alkali to the culture-medium favors the growth.

The organisms readily lose their virulence in culturemedia, and cease to be pathogenic after a few days. Not only is this true, but they seem to be unable to accommodate themselves to a purely saprophytic life, and unless continually transplanted to new media die in a week or two, sometimes sooner.

The colonies which develop at 24° C. upon 15 per cent. gelatin plates are described as small, round, circumscribed, finely granular white points which grow slowly, never attain any considerable size, and do not liquefy the gelatin (Fig. 96).

If, instead of gelatin, agar-agar be used and the plates kept at the temperature of the body, the colonies which develop upon the plates appear as transparent, delicate, drop-like accumulations, scarcely visible to the naked eye, but under the microscope distinctly granular, the central darker portion being frequently surrounded by a paler marginal zone.

In gelatin puncture-cultures, made with 15 instead of the usual 10 per cent. of gelatin, the growth takes place along the entire path of the wire in the form of little whitish granules distinctly separated from each other. The growth in gelatin is always very limited.

Upon agar-agar and blood-serum the growth consists of minute, transparent, semi-confluent, colorless, dewdrop-like colonies, which die before attaining a size