_Streptococcus pyogenes_ has been isolated from the membrane of diphtheria, and from small-pox, scarlet fever, vaccinia, and other diseases. In such cases it is not the causal agent, but merely associated with the complications of these diseases. Suppuration and erysipelas are the only pathological conditions in which the causal agency of streptococcus has been sufficiently established.
3. The _Bacillus pyocyaneus_ occurs in green pus, and is the cause of that colouration. Gessard was the first to prove its significance, and he describes two varieties.
It is a minute, actively motile, non-sporulating bacillus, which occasionally complicates suppuration and produces green pus. Oxygen is necessary for pigmentation, which is due to two substances: _pyocyanin_, a greenish-blue product extracted with chloroform, and _pyoxanthose_, a brown substance derived from the oxidation of the former pigment. Both these colours are produced in cultivation outside the body. On gelatine the colour is green, passing on to olive. There is liquefaction. On potato we generally obtain a brown growth (compare _Bacillus coli_, _B. mallei_, and others). The organism grows rapidly on all the ordinary media, which it has a tendency to colour throughout.
It will be remembered that when speaking of the antagonism of organisms, we referred to the inimical action of _Bacillus pyocyaneus_ upon anthrax.
4. _Micrococcus Tetragonus._ This species occurs in phthisical cavities and in certain suppurations in the region of the face. It is a micrococcus usually in the form of small tetrads. A capsule is always present and sometimes discernible.
5. _Bacillus coli communis_ and many putrefactive germs commonly occur in suppurative conditions, but they are not restricted to such disorders (see p. 64).
6. _Micrococcus gonorrhœæ_ (Neisser, 1879). This organism is more frequently spoken of as a diplococcus. It occurs at the acute stage of the disease, but is not readily differentiated from other similar diplococci except by technical laboratory methods. Each element presents a straight or concave surface to its fellow. A very marked concavity indicates commencing fission. The position which these diplococci take up in pus is _intracellular_, and arranged more or less definitely around the nucleus. Difficulty has often been found in cultivating this organism in artificial media outside the body. Wertheim and others have suggested special formulæ for the preparation of suitable media, but it is a very simple matter to secure cultures on agar plates smeared with human blood from a pricked finger. The plate is incubated at 37° C. At the end of twenty-four hours small raised grey colonies appear, which at the end of about four days show adult growth. The margin is uneven, and the centre more opaque than the rest of the colony. This diplococcus is readily killed, and sub-cultures must be frequently made to retain vitality and virulence. Light, desiccation, and a temperature of 55° C. all act germicidally. The organism stains readily in Löffler's blue, but is decolourised by Gram's method. It is more or less strictly parasitic to man. Its shape, size, character of growth, and staining properties assist in differentiating it from various similar diplococci.[98]