What Is Pus?
The pus, typically white-yellow, yellow, or yellow-brown, we see oozing out from a septic cut is made up of the dead bodies of white blood cells which have fought against the bacterial or fungal infection. These white cells, or corpuscles, live in the blood and float around the body ready to fight infection.
There are at least five kinds of white cells. The ones which fight the kind of infection that happens when a spot or a cut goes bad are called leucocytes. A speck of blood, the size of a pinhead, normally contains about 5,000 leucocytes. But it may have 30,000 in a really bad septic wound. This is because the leucocytes gather at a point of infection and multiply.
Antibodies in the blood help to fight bacteria. When these have killed the bacteria, the white cells move into destroy them. If a doctor suspects that the blood may not contain enough antibodies to counteract the infection, he prescribes extra antibiotics, like penicillin, which do the same job.
Often the infection is defeated even without help from injections and pills, but not before many of the white cells has been killed, and discarded as pus. An accumulation of pus in an enclosed tissue space is known as an abscess, whereas a visible collection of pus within or beneath the epidermis is known as a pustule, pimple, or spot.
Pus consists of a thin, protein-rich fluid, known as liquor puris, and dead leukocytes from the body’s immune response (mostly neutrophils) During infection, macrophages release cytokines which trigger neutrophils to seek the site of infection by chemotaxis. There, the neutrophils release granules which destroy the bacteria. The bacteria resist the immune response by releasing toxins called leukocidins. As the neutrophils die off from toxins and old age, they are destroyed by macrophages, forming the viscous pus. Bacteria that cause pus are called pyogenic.
Although pus is normally of a whitish-yellow hue, changes in the color can be observed under certain circumstances. Pus is sometimes green because of the presence of myeloperoxidase, an intensely green antibacterial protein produced by some types of white blood cells. Green, foul-smelling pus is found in certain infections of Pseudomonas aeruginosa. The greenish color is a result of the bacterial pigment pyocyanin that it produces. Amoebic abscesses of the liver produce brownish pus, which is described as looking like “anchovy paste”. Pus can also have a foul odor, particularly pus from anaerobic infections.
In almost all cases when there is a collection of pus in the body, the clinician will try to create an opening to drain it. This principle has been distilled into the famous Latin aphorism “Ubi pus, ibi evacua” (“Where there is pus, evacuate it”). Some disease processes caused by pyogenic infections are impetigo, osteomyelitis, septic arthritis, and necrotizing fasciitis.
In the pre-asepsis era, surgeon Frederick Treves (1853–1923) wrote, “Practically all major wounds suppurated. Pus was the most common subject of converse, because it was the most prominent feature in the surgeon’s work. It was classified according to degrees of vileness.” But pus of the right kind was considered desirable. “If a patient was lucky … a thick cream-colored odorless fluid would appear within five or six days”; such “laudable” pus was considered “a sure sign that the wound would heal” because it meant “Nature has put up a bold fight against the invader”.
“On the other hand, if the pus gradually became watery, blood tinged and foul smelling, it was designated ‘sanious’ [or ‘ill-conditioned’ ] and the wound condition was considered unfavorable”. It later came to be understood that “laudable” pus generally implied an invasion of relatively benign staphylococcus, while “ill-conditioned” pus usually meant the more dangerous streptococcus was present.