Why Should Gram Staining Be Done on Young Cultures?

Gram staining separates bacteria into two classes, Gram-positive and Gram-negative, allowing medical providers to determine an appropriate antibiotic to prescribe a patient, according to Reference.com. The cell wall, which is the protective coating of the bacteria, determines the outcome of this test.

Determining if a bacterium is Gram-positive or Gram-negative is the first step in identifying a species. In the medical field, workers perform a Gram stain on body fluids and biopsies when the provider suspects an infection. Laboratory personnel gather results much more quickly from this test than when they wait for a culture to grow. While there are exceptions, Gram-negative bacteria are more pathogenic than Gram-positive ones.

Gram stain is made of a chemical known as crystal violet. Adding the stain to a sample, along with acetone or alcohol, causes the bacteria to dehydrate and retain the stain in its peptidoglycan layer or cell wall. In Gram-negative bacteria, there is an additional outer membrane, which resists the dehydration, so crystal violet is lost. However, the addition of a fuchsin counterstain turns these bacteria pink or red, according to About.com.

Streptococcus is a Gram-positive bacterium. It has a thin, single-layered cell wall that is easily permeable by antibiotics. A thicker, two-layer cell wall that is less penetrable surrounds E. coli, a Gram-negative bacterium.