Suppuration, or the formation of pus, is one of the results of the action of bacteria on the tissues. The invading organism is usually one of the staphylococci, less frequently a streptococcus, and still less frequently one of the other bacteria capable of producing pus, such as the bacillus coli communis, the gonococcus, the pneumococcus, or the typhoid bacillus.

So long as the tissues are in a healthy condition they are able to withstand the attacks of moderate numbers of pyogenic bacteria of ordinary virulence, but when devitalised by disease, by injury, or by inflammation due to the action of other pathogenic organisms, suppuration ensues.

It would appear, for example, that pyogenic organisms can pass through the healthy urinary tract without doing any damage, but if the pelvis of the kidney, the ureter, or the bladder is the seat of stone, they give rise to suppuration. Similarly, a calculus in one of the salivary ducts frequently results in an abscess forming in the floor of the mouth. When the lumen of a tubular organ, such as the appendix or the Fallopian tube is blocked also, the action of pyogenic organisms is favoured and suppuration ensues.

Pus.—The fluid resulting from the process of suppuration is known as pus. In its typical form it is a yellowish creamy substance, of alkaline reaction, with a specific gravity of about 1030, and it has a peculiar mawkish odour. If allowed to stand in a test-tube it does not coagulate, but separates into two layers: the upper, transparent, straw-coloured fluid, the liquor puris or pus serum, closely resembling blood serum in its composition, but containing less protein and more cholestrol; it also contains leucin, tyrosin, and certain albumoses which prevent coagulation.

The layer at the bottom of the tube consists for the most part of polymorph leucocytes, and proliferated connective tissue and endothelial cells (pus corpuscles). Other forms of leucocytes may be present, especially in long-standing suppurations; and there are usually some red corpuscles, dead bacteria, fat cells and shreds of tissue, cholestrol crystals, and other detritus in the deposit.

If a film of fresh pus is examined under the microscope, the pus cells are seen to have a well-defined rounded outline, and to contain a finely granular protoplasm and a multi-partite nucleus; if still warm, the cells may exhibit amœboid movement. In stained films the nuclei take the stain well. In older pus cells the outline is irregular, the protoplasm coarsely granular, and the nuclei disintegrated, no longer taking the stain.

Variations from Typical Pus.—Pus from old-standing sinuses is often watery in consistence (ichorous), with few cells. Where the granulations are vascular and bleed easily, it becomes sanious from admixture with red corpuscles; while, if a blood-clot be broken down and the debris mixed with the pus, it contains granules of blood pigment and is said to be “grumous.” The odour of pus varies with the different bacteria producing it. Pus due to ordinary pyogenic cocci has a mawkish odour; when putrefactive organisms are present it has a putrid odour; when it forms in the vicinity of the intestinal canal it usually contains the bacillus coli communis and has a fścal odour.

The colour of pus also varies: when due to one or other of the varieties of the bacillus pyocyaneus, it is usually of a blue or green colour; when mixed with bile derivatives or altered blood pigment, it may be of a bright orange colour. In wounds inflicted with rough iron implements from which rust is deposited, the pus often presents the same colour.

The pus may form and collect within a circumscribed area, constituting a localised abscess; or it may infiltrate the tissues over a wide area—diffuse suppuration.