Developed by Hans Christian Gram, this test is one of the most widely used microbiological techniques.

It differentiates bacteria by staining characteristics:

Gram-positive bacteria retain the crystal violet stain and appear purple/blue.

Gram-negative bacteria do not retain crystal violet but take up the counterstain (safranin) and appear pink/red.

Clinical uses include:

Rapid identification of bacterial type in infections.

Guiding initial antibiotic therapy.

Detecting presence of bacteria in blood, CSF, urine, sputum, and other body fluids.

Often performed as a first-line diagnostic test before culture and sensitivity.

Analyte: Bacterial cell wall characteristics (peptidoglycan thickness).

Normal Result: No bacteria seen.

Abnormal Findings:

Gram-positive bacteria: Thick peptidoglycan wall, purple/blue appearance (e.g., Staphylococcus, Streptococcus).

Gram-negative bacteria: Thin peptidoglycan wall, pink/red appearance (e.g., E. coli, Klebsiella, Pseudomonas).

Clinical Use:

Provides a rapid preliminary classification of bacteria.

Helps guide empirical antibiotic therapy.

Supports diagnosis of bacterial infections in various clinical specimens.