assume an orange-yellow colour—­hence the name aureus.  It is of high vitality and resists more prolonged exposure to high temperatures than most non-sporing bacteria.  It is capable of lying latent in the tissues for long periods, for example, in the marrow of long bones, and of again becoming active and causing a fresh outbreak of suppuration.  This organism is widely distributed:  it is found on the skin, in the mouth, and in other situations in the body, and as it is present in the dust of the air and on all objects upon which dust has settled, it is a continual source of infection unless means are taken to exclude it from wounds.

The staphylococcus albus is much less common than the aureus, but has the same properties and characters, save that its growth on artificial media assumes a white colour.  It is the common cause of stitch abscesses, the skin being its normal habitat.

[Illustration:  FIG. 3.—­Streptococci in Pus from an acute abscess in subcutaneous tissue. x 1000 diam.  Gram’s stain.]

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 pyaemia.

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. x 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 foetid, or even a faecal odour, and often contains gases resulting from putrefaction.