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The most common causes of blue pus are tied to a bacterium called pseudomonas aeruginosa. This infection usually appears in moderate to severe skin and soft tissue injuries such as second- to third-degree burns. It can also sometimes be responsible for urinary tract infections resulting from improper catheter use. The blue color of pus results from a by-product of the pseudomonas aeruginosa bacteria called pyocyanin. While this bacteria is found frequently in non-infectious colonizations on various surfaces, it can sometimes be serious when it invades open wounds in people with weakened immune systems.
Blue pus often appears in a closed abscess that can vary in size depending on the extent of the infection. These types of pus can range in color from dark blue to a lighter blue tint depending on the amount of pyocyanin present. This substance acts as a natural antibiotic agent that attempts to neutralize the pseudomonas aeruginosa when the rest of the body's immune defenses have been ineffective. A persistent wound with blue pus that does not heal usually indicates a compromised immune system and often requires further medical treatment with oral or topical medicines.
An advanced pseudomonas aeruginosa infection can sometimes result in green as well as blue pus, and this color sometimes indicates the presence of another antibacterial protein called myeloperoxidase. Both this protein and pyocyanin work to help the white blood cells clear up the infectious bacteria by a process of ingesting the invading microbes. Just as with other types of infections, the white blood cells eventually die off and accumulate with the neutralized infectious matter to form pus. Medical researchers sometimes report that lab test cultures of pus containing pseudomonas aeruginosa can give off an odd odor similar to that of fermenting grapes.
Most wounds with blue pus can be successfully healed by keeping the injured site clean and covered to prevent further infection. Physicians often find that topical medicines containing salicylic acid are particularly effective at fighting off pseudomonas aeruginosa bacteria. Some cases of pseudomonas aeruginosa invasions of internal organs need treatment with courses of stronger antibiotics. Since this species of bacteria can survive on non-biologic surfaces, it can sometimes be found in medical equipment, such as respirators, and cause infections including pneumonia in hospital patients. Pseudomonas aeruginosa-induced pneumonia is often characterized by expulsion of green or blue pus from the lungs, and doctors stress that these cases are completely preventable through proper equipment sterilization.