Gram staining of preparations

Synonyms: microscopy of Gram-negative and Gram-positive microorganisms.
All body sites and specimens, except blood.

In specimens stained by the Gram method, the material under study can be evaluated quickly.

Gram-negative bacteria stain red, Gram-positive bacteria stain blue. These slides also determine the presence of leukocytes (indicator of infection) and epithelial cells (indicator of mucosal contamination). If sputum is being examined, finding more than 25 epithelial cells in one field of view indicates a questionable result of the culture, as this sign indicates saliva contamination. Usually, the results of slides stained by the Gram method and the culture match in over 90% of cases. If microorganisms are found in many fields of view in a portion of the smear of centrifuged urine sediment, it is one of the best signs of bacteriuria.

This sign should be evaluated together with the NIT indicator in the urine analyzer and the culture results from the midstream urine portion. Often, in asymptomatic bacteriuria, bacteria are found when examining urine under a microscope. In the case of bacterial meningitis, staining the cerebrospinal fluid by the Gram method is also important because its results can be known quickly without waiting for culture growth results.

Staining part of the material under study by the Gram method allows to start preliminary treatment. Some pathogens do not stain well with this method and may not be visible, although they are actually present in the material under study (Legionella, Mycobacterium, Treponema, Rickettsia). The absolute majority of pathogens, classified by their morphological appearance and staining by the Gram method, are listed below.

Source | Handbook of Basic Laboratory Tests | Doctor of Medical Sciences Gintaras Zaleskis