Forty per cent of today’s antibiotics are derived from natural substances. Antibiotics can be produced from fungi or bacteria, in order to kill off other fungi or bacteria. How an antibiotic operates against bacteria depends on the construction of the cell, the cell walls and the different metabolisms of the bacteria. Bacteria are found almost everywhere and differ according to their environments. Researchers have discovered bacteria living about a mile beneath the ocean’s surface that have completely different defence strategies to those on land. Because of this they could be used to produce new antibiotics.