“In the last 40 years, there have been only two new classes of antibiotic drugs discovered and brought to the market,” said graduate student Christopher Loose, lead author of a paper on the work that appears in the Oct. 19 issue of Nature. “There is an incredible need to come up with new medicines.”
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focusing their attention on antimicrobial peptides, or short strings of amino acids. Such peptides are naturally found in multicellular organisms, where they play a role in defense against infectious bacteria.

Because there are 20 naturally occurring amino acids, there are about 1026 possible peptide sequences of length 20. Some of those kill microbes with varying levels of effectiveness; the overwhelming majority have no effect.
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To design their new peptides, the researchers first came up with all possible 20-amino acid sequences in which each overlapping string of 10 amino acids conformed to one of the grammars. They then removed any peptides that had six or more amino acids in a row in common with naturally occurring peptides. Then, they threw out sequences that were very similar to each other and chose 42 peptides to test.

About half of the peptides displayed significant antimicrobial activity against two common strains of bacteria — Escherichia coli and Bacillus cereus. That is a much higher success rate than one would expect from testing randomly generated sequences