_Streptococcus Pyogenes._--This organism also varies greatly in its virulence; in some instances--for example in erysipelas--it causes a sharp attack of acute spreading inflammation, which soon subsides without showing any tendency to end in suppuration; under other conditions it gives rise to a generalised infection which rapidly proves fatal. The streptococcus has less capacity of liquefying the tissues than the staphylococcus, so that pus formation takes place more slowly. At the same time its products are very potent in destroying the tissues in their vicinity, and so interfering with the exudation of leucocytes which would otherwise exercise their protective influence. Streptococci invade the lymph spaces, and are associated with acute spreading conditions such as phlegmonous or erysipelatous inflammations and suppurations, lymphangitis and suppuration in lymph glands, and inflammation of serous and synovial membranes, also with a form of pneumonia which is prone to follow on severe operations in the mouth and throat. Streptococci are also concerned in the production of spreading gangrene and pyæmia. Division takes place in one axis, so that chains of varying length are formed (Fig. 3). It is less easily cultivated by artificial media than the staphylococcus; it forms a whitish growth. [Illustration: FIG. 4.--Bacillus coli communis in Urine, from a case of Cystitis. × 1000 diam. Leishman's stain.] _Bacillus Coli Communis._--This organism, which is a normal inhabitant of the intestinal tract, shows a great tendency to invade any organ or tissue whose vitality is lowered. It is causatively associated with such conditions as peritonitis and peritoneal suppuration resulting from strangulated hernia, appendicitis, or perforation in any part of the alimentary canal. In cystitis, pyelitis, abscess of the kidney, suppuration in the bile-ducts or liver, and in many other abdominal conditions, it plays a most important part. The discharge from wounds infected by this organism has usually a fœtid, or even a fæcal odour, and often contains gases resulting from putrefaction. It is a small rod-shaped organism with short flagellæ, which render it motile (Fig. 4). It closely resembles the typhoid bacillus, but is distinguished from it by its behaviour in artificial culture media. [Illustration: FIG. 5.--Fraenkel's Pneumococci in Pus from Empyema following Pneumonia. × 100 diam. Stained with Muir's capsule stain.] _Pneumo-bacteria._--Two forms of organism associated with pneumonia--_Fraenkel's pneumococcus_ (one of the diplococci) (Fig. 5) and _Friedländer's pneumo-bacillus_ (a short rod-shaped form)--are frequently met with in inflammations of the serous and synovial membranes, in suppuration in the liver, and in various other inflammatory and suppurative conditions.