The Gram stain test is the most extensively performed procedure in diagnostic microbiology. It is used to classify bacteria as either gram-positive or gram-negative based upon their ability to retain the crystal violet stain following decolorization. In addition, the Gram stain provides vital diagnostic information, aids in the selection of culture media, and dictates initial selection of antibiotics for treatment and antimicrobial susceptibility testing.
The Gram stain is used to detect the presence of bacteria, yeast, and other cells in direct smears prepared from swabs, aspirates, secretions, etc. from any part of the body where infection is suspected. Direct smears are often made of throat swabs, sputum, genital swabs, wounds, abscesses, cerebrospinal fluid (CSF), serous fluids, joint fluid, urine, and stool. Gram stain is also performed to help identify colonies isolated from cultures. In addition to gram-negative or gram-positive, organisms are evaluated for size, shape, arrangement, number, and any special characteristics such as bipolar staining and the presence of spores. These characteristics often point the way to the most efficient selection of biochemical tests needed to identify the organism. The finding of organisms on direct examination of some specimens is sufficient to establish a preliminary diagnosis and justify immediate antibiotic treatment pending confirmation by culture or other means. The Gram stain is very useful in identifying anaerobic bacteria by comparing the microscopic morphology and number of organisms to culture results. Significant numbers of characteristic bacteria on Gram stain not appearing on aerobic culture often signals the presence of an anaerobic infection.
The Gram stain will identify male patients with Neisseria gonorrhoeae genital infections with a specficity approaching 100% and a sensitivity above 90%. In female patients, the sensitivity and specificity are lower owing to the presence of other genital flora, but the test is still sufficiently specific to justify immediate antibiotic therapy when symptoms of pelvic inflammatory disease are present. The presence of bacteria on Gram stain of concentrated CSF is presumptive evidence of bacterial meningitis and reason to begin antibiotic therapy. The Gram stain is positive in the majority of bacterial meningitis cases. Recovery of bacteria from other normally sterile fluids including exudative plueral, pericardial, and abdominal fluid and inflammatory joint fluid is also presumptive
Robert Harr, The Gale Group Inc., Gale, Detroit,