Natural drug resistance is an innate properties of the bacterium and is unrelated to previous exposure to the drug. An entire bacterial species may be resistant to an antibiotic even before the introduction of the drug. For example, Strep. Pyogenses is resistant to gentamicin, staphylococcus aureus is resistant to polymixins, Proteus mirabilis is resistant to colistin and pseudomonas aeruginosa is resistant to cloxacillin. The reason for this natural resistance include lack of penetration of the drugs through the cell wall, lack of suitable cell wall or other target receptors, naturally produced enzyme that may have existed before the introduction of the drug which may be lethal to the drug.