Gram stain

Gram-positive bacteria are those that are stained dark blue or violet by the staining. Organisms that are Gram-positive have cell walls containing multiple layers of peptidoglycan bound together by amino acid bridges.

Especially when antibiotics were new, a given antibiotic, such as penicillin , of the beta-lactam class, tended to attack only Gram-positive bacteria. Other classes, such as streptomycin, a member of the aminoglycoside class, tended to attack Gram-negative bacteria. Broad-spectrum antibiotics attack both Gram positive and negative bacteria.