Gram-positive bacteria are a diverse group of bacteria that share the same type of cell wall structure, allowing them to be identified and differentiated from gram-negative bacteria using a Gram stain (a lab technique). Following a Gram stain, gram-positive bacteria will appear blue/purple under a microscope, while gram-negative bacteria will appear pink/red.^[1]

The cell walls of gram-positive bacteria consist of a thick peptidoglycan layer composed primarily of sugars and amino acids. Unlike gram-negative bacteria, there is no outer lipid membrane.^[1]

Knowing whether the bacteria causing a given infection are gram-negative or gram-positive is an important initial step for guiding antibiotic choice and predicting pathogenicity. Gram-positive bacteria tend to be more susceptible to antibiotics that work by targeting the cell wall (e.g., penicillins), although this can still vary between genera.^[2]

Many different bacterial genera and species are gram-positive. Some examples include Staphylococcus, Streptococcus, Clostridia, and Listeria.^[2]