Pneumonia is caused by virulent pathogenic bacteria, but the same microbe is not always the infective agent. In some cases of secondary pneumonia, the ordinary pyogenic micrococci have been found, but in proper primary cases one or other of two distinct forms can in the great majority of cases be detected. These are the Diplococcus of Fraenkel, and the Pneumococcus of Friedliinder. Weichselbaum found, in 83 cases, the former 54, and the latter 6 times.

(A) Diplococcus Pneumoniae. Capsule-Coccus

This microbe, whose pathogenic characters were first distinguished by Fraenkel, has, apart from pneumonia, pathogenic relations extending widely. It is not only the prevalent microbe in pneumonia, but it has been frequently observed as the infective agent in acute leptomeningitis, whether the epidemic, sporadic, or traumatic form, also occasionally in ulcerative endocarditis, in acute nephritis, and in acute synovitis affecting several joints.

The form is a slightly elongated coccus, generally two united together (Diplococcus), and sometimes in longer chains. When found in the animal body it is surrounded by a capsule (see Fig. 142), but in cultures it loses this. It is readily stained by ordinary aniline dyes, and when so treated the capsule appears faintly coloured around the microbe. The microbe stains by Gram's method, but the capsule is rendered invisible. It is readily cultivated on nutrient media, but requires a temperature over 22° C, and grows best at 35°. The cultures lose their virulence and readily die.

Capsule coccus from exudation in case of lepto meningitis.

Fig. 142. - Capsule-coccus from exudation in case of lepto-meningitis. x about 1000.

The microbe is highly virulent in rabbits, and less so in mice and guinea-pigs. If cultures be used for injection under the skin of a rabbit, an acute febrile disease is produced, of which the animal dies in one or two days. The capsule coccus is found in enormous numbers in the blood.

Immunity has been produced in rabbits in various ways, as by injection of the toxines, and the blood serum is stated to confer immunity on other animals. The blood serum of patients who have recovered from pneumonia is also said to confer immunity on rabbits.

An exceedingly interesting fact is that the Sputum of healthy persons frequently contains the capsule-coccus. The secretion of the mouth in such persons when inoculated subcutaneously in rabbits produces the regular form of disease (Sputum-septicaemia).

(B) Bacillus Pneumoniae. Friedlander's Pneumococcus

This somewhat resembles the former, but is longer, and hence is a bacillus. It also possesses a capsule, but only when obtained in the living body. It is non-motile. It grows on the ordinary culture media, and does not require a temperature above that of the ordinary room. In this respect it differs from the diplococcus, as well as in the fact that it does not stain by Gram's method.

It is not pathogenic in rabbits, forming thus a striking contrast to the capsule-coccus. It is pathogenic in mice and dogs, less so in guinea-pigs.