Pus

For other uses, see Pus (disambiguation).

Pus
An abscess is an enclosed collection of pus.
Classification and external resources
Specialty	Infectious disease
ICD-9-CM	xxx
Patient UK	Pus

Eye with conjunctivitis exuding pus

Duodenoscopy image of hepatopancreatic ampulla with pus exuding from it, indicative of cholangitis

Pus is an exudate, typically white-yellow, yellow, or yellow-brown, formed at the site of inflammation during infection. 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.

Despite normally being of a whitish-yellow hue, changes in the color of pus 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, there evacuate it").

Some disease processes caused by pyogenic infections are impetigo, osteomyelitis, septic arthritis, and necrotizing fasciitis.

Pyogenic bacteria

A great many species of bacteria may be involved in the production of pus. The most commonly found include:

• Staphylococcus aureus
• Staphylococcus epidermidis
• Streptococcus pyogenes
• Escherichia coli (Bacillus coli communis)
• Streptococcus pneumoniae (Fraenkel's pneumococcus)
• Klebsiella pneumoniae (Friedländer's bacillus)
• Salmonella typhi (Bacillus typhosus)
• Pseudomonas aeruginosa
• Neisseria gonorrhoeae
• Actinomyces
• Burkholderia mallei (Glanders bacillus)
• Mycobacterium tuberculosis (tubercle bacillus)

Staphylococcus aureus bacteria is the most common cause of boils.

Historical terminology

In the pre-asepsis era, wrote surgeon Frederick Treves (1853–1923), "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 staphylococci, while "ill-conditioned" pus usually meant the more dangerous streptococcus was present.

See also

• Pyoderma
• Serous fluid