One of the most commonly observed infectious disease processes is the production of a purulent (sometimes seropurulent) exudate as the result of bacterial invasion of a cavity, tissue, or organ of the body. Such infections may be relatively simple, innocuous “pimples” or a series of multiple pockets of pus found in abscesses in one or more anatomical sites. The exudate consists of white blood cells, predominantly polymorphonuclear leukocytes, the invading organisms, and a mixture of body fluid and fibrin. In some instances, the exudate may be found as a coating on the surface of an organ, e.g., the surface of the brain in acute bacterial meningitis. In other cases, the exudate may be walled off by layers of fibrin and a network of tissue cells, e.g., a carbuncle or subcutaneous “boil”, while in other cases the exudate may be associated with an open wound, which therefore drains thick fluid or pus.

Just as the anatomical site of exudate production can vary considerably, so too can the organisms involved in the underlying infection. Virtually all bacteria that are part of the normal flora, or that gain access to the body, may be involved in the production of an exudate. Some fungi, particularly those that are able to multiply in body tissues, can also be involved in the production of an exudate. In contrast, a purulent exudate is rarely produced in a viral infection.

The bench microbiologist should be aware of the diversity of anatomical sites and microorganisms involved, and be prepared to initiate the appropriate macroscopic and microscopic examinations and the proper primary media inoculations to recover the major organism(s) involved. Once the organisms have been isolated in pure culture, the identification process and antimicrobial susceptibility tests should be set up as soon as possible.

Communication between the clinician and the microbiologist is particularly important in the diagnosis and management of patients with suppurative infectious diseases. The microbiologist must collaborate with the physician to ensure proper specimen collection and the expeditious delivery of the specimen to the laboratory for prompt and proper processing.