Penicillin
Penicillin can be used to enter and kill two types of bacteria, Gram-positive and Gram-negative bacteria. Gram-positive bacteria have no outer cell membrane (protective layer around the bacteria cell) and have large pores (holes) which allows penicillin to enter easily and kill the cell from the inside. Gram-negative bacteria are harder for penicillin to enter as they have smaller pores and an outer cell membrane so penicillin has to enter through smaller aqueous pores (holes in the membrane where water enters and leaves the cell). This means that killing Gram-negative bacteria takes a longer amount of time and some penicillin drugs may not work against them.