What makes Allium species effective against pathogenic microbes?

The antimicrobial activity of garlic (Allium sativum L.) has been known since ancient times. The first citation dates back to the Egyptian period of 15th century B.C. when garlic was reported to be used in folk medicine as a remedy for microbial infections. Scientific investigations on garlic started in 1858 with the work of Pasteur who first noted antibacterial properties of garlic extracts. From that date to the discovery of antibiotics, garlic has been used against amoebic dysentery and epidemic diseases such as typhus, cholera, diphtheria, and tuberculosis. But what makes garlic and Allium species effective against pathogenic microbes? The volatile sulphur compound allicin and other thiosulfinates, giving pungency to Allium plants, are well-studied and are the main element responsible for garlic activity against microbes. The thiosulfinates or alkane(ene) thial-S-oxide are formed by the action of the enzyme alliinase (EC 4.4.1.4) from their respective S-alk(en)yl cysteine sulfoxides when the bulbs are cut. However, depending on the Allium species, and under differing conditions, thiosulfinates can decompose to form additional sulfur constituents, including diallyl, methyl allyl, and dipropyl mono-, di-, tri- e tetra-sulfides, and (E)- and (Z)-ajoene without losing antimicrobial activity. Besides these apolar compounds, onion and garlic are characterized by more polar compounds of steroidal and phenolic origin, often glycosilated, showing also interesting antimicrobial activity. These latter compounds, compared to the more studied thiosulfinates, have the advantages of not being pungent an are more stable during cooking. Recently, there has been increasing scientific attention given to such compounds. Compounds possessing nitrogen atoms, like alkaloids and polypeptides, have also been isolated from these plants and have shown antimicrobial activity. In this paper, the literature about the major volatile and non-volatile organic compounds of garlic and other Allium plants has been reviewed. Particular attention is given to the compounds possessing antimicrobial activity and to the correlation between the observed activity and the chemical structure of the tested compound