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close this bookBasic Laboratory Procedures in Clinical Bacteriology (WHO; 1991; 128 pages)
View the documentPreface
View the documentIntroduction
open this folder and view contentsQuality assurance in microbiology
close this folderPart I. Bacteriological investigations
open this folder and view contentsBlood
open this folder and view contentsCerebrospinal fluid
open this folder and view contentsUrine
open this folder and view contentsStool
open this folder and view contentsLower respiratory tract infections
open this folder and view contentsUpper respiratory tract infections
open this folder and view contentsSexually transmitted diseases
close this folderPurulent exudates, wounds, and abscesses
View the documentIntroduction
View the documentCommonly encountered clinical conditions and the most frequent etiological agents
View the documentCollection and transportation of specimens
View the documentMacroscopic evaluation
View the documentMicroscopic examination
View the documentCulture
View the documentIdentification
View the documentSusceptibility testing
open this folder and view contentsAnaerobic bacteriology
open this folder and view contentsAntimicrobial susceptibility testing
open this folder and view contentsPart II. Essential media and reagents for isolation and identification of clinical pathogens
View the documentSelected further reading
View the documentSelected WHO publications of related interest
View the documentBack Cover

Macroscopic evaluation

Specimens of pus or wound discharge collected on swabs are difficult to evaluate macroscopically, particularly when the swab is immersed in a transport medium. Specimens of pus, received in a syringe or in a sterile container, should be evaluated carefully by an experienced technician for colour, consistency, and odour.


The colour of pus varies from green-yellow to brown-red. A red colour is generally due to admixture with blood or haemoglobin. The aspirate from a primary amoebic liver abscess has a gelatinous consistency and a dark brown to yellowish brown colour. Pus from postoperative or traumatic wounds (bums) may be stained blue-green by the pyocyanin pigment produced by Pseudomonas aeruginosa.


The consistency of pus may vary from a turbid liquid to one that is very thick and sticky. Exudates, aspirated from a joint, the pleural cavity, the pericardial sac, or the peritoneal cavity, are generally liquid, with all possible gradations between a serous exudate and frank pus.

Pus originating from a draining sinus tract in the neck should be inspected for small yellow “sulfur” granules, which are colonies of the filamentous Actinomyces israelii. The presence of sulfur granules suggests a diagnosis of cervicofacial actinomycosis. Small granules of different colours (white, black, red, or brown) are typical of mycetoma, a granulomatous tumour, generally involving the lower extremities (e.g., madura foot), and characterized by multiple abscesses and draining sinuses. The coloured granules correspond to either filamentous bacteria or fungal mycelium.

Pus from tuberculous “cold abscesses” (with few signs of inflammation) is sometimes compared with soft cheese and called “caseum” or “caseous pus”.


A foul feculent odour is one of the most characteristic features of an anaerobic or a mixed aerobic - anaerobic infection, although it may be lacking in some instances. The odour, together with the result of the Gram-stained smear, should be reported at once to the clinician as it may be helpful in the empirical selection of an appropriate antibiotic. It will also help in determining whether anaerobic cultures are needed.

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