Major Classes of Antibiotics Summary. Bacteria themselves can be divided into two broad classes – Gram-positive and Gram-negative. The classes derive these names from the Gram test, which involves the addition of a violet dye to the bacteria. Gram-positive bacteria retain the colour of the dye, whilst Gram-negative bacteria do not, and are instead coloured red or pink. Gram-negative bacteria are more resistant to antibodies and antibiotics, because they have a largely impermeable cell wall.