Gram staining is a method for identifying different classes of bacteria. Originally developed to make bacteria easier to view in a blood culture, it was discovered that some bacteria reacted one way to the stain and others reacted in a different way. As antibiotics have a mechanism of action, some antibiotics will not work on gram negative bacteria. For example, penicillin works by preventing the formation of a new cell wall, so only works on gram positive bacteria. Others, such as streptomycin, will work on either type, but have more severe side effects.