grampositivenegative

Gram-positive and Gram-negative bacteria are classifications based on the Gram stain, a differential staining method introduced by Hans Christian Gram. The results reflect differences in cell wall structure that influence staining properties, identification, and, in clinical settings, antibiotic choice.

Gram-positive bacteria have a thick peptidoglycan layer that consists largely of sugar chains cross-linked by peptides, and they often contain teichoic and lipoteichoic acids. This dense layer retains the crystal violet-iodine complex during the decolorization step, so Gram-positive organisms appear purple under a light microscope. In contrast, Gram-negative bacteria possess a much thinner peptidoglycan layer and an outer membrane containing lipopolysaccharide. The outer membrane and the periplasmic space render Gram-negative cells susceptible to decolorization, causing them to take up the counterstain and appear pink or red.

During the standard staining procedure—crystal violet, iodine, decolorization with alcohol or acetone, then counterstaining—Gram-positive and Gram-negative bacteria typically show the described colors. Results can vary with age, growth conditions, or unusual cell envelopes, and some species may appear Gram-variable or require alternative methods for classification.

Clinically, the distinction helps guide empirical therapy and interpretation of results. Gram-negative organisms often have intrinsic resistance barriers due to their outer membrane and may produce endotoxins (lipopolysaccharide). Common Gram-positive examples include Staphylococcus aureus and Streptococcus pneumoniae, while common Gram-negative examples include Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa.

Not all bacteria conform to the Gram dichotomy. Mycobacteria and Nocardia are acid-fast, and Mycoplasma species lack a cell wall and do not reliably stain by Gram methods. The Gram phenotype is a practical, not a definitive measure of phylogeny.