Recently, the spread of drug-resistant bacteria has become a serious global concern in the therapy of infectious diseases. A number of antibiotics have been developed and used to treat infectious diseases; however, the increased frequency in the use of such antibiotics has led to changes in bacterial characteristics, with bacteria acquiring drug-resistant ability through the mutations of drug-target molecules, the overexpression of efflux pumps, changes in the composition of the cell membrane, the production of metabolizing enzymes and biofilm formation (1). Among these, the biofilm comprises a large community and aggregation of bacteria and its formation protects microbial cells from antibiotics and immune cells. The composition of biofilm is mainly water and extracellular polymeric substances (EPS), such as proteins, DNA, RNA and polysaccharides (1,2). The production of EPS is regulated by quorum sensing (QS), which represents cell-to-cell communication in bacteria and is controlled by chemical signaling molecules, such as N-acyl-L-homoserine lactones (AHLs) (3,4). Thus, QS plays a crucial role in biofilm formation. The biofilm formation of bacteria has been reported to be associated with chronic infections; therefore, an increase in biofilm-forming bacteria is a serious issue, not only in the medical field, but also in a number of industrial fields and facilities. Therefore, the discovery and development of drugs to combat the formation of biofilm is an important approach for the fight against drug-resistant infections.

Garlic (Allium sativum L.) has been used as not only a food, but also as a remedy for several diseases, such as cardiovascular diseases and cancer (5–7). In addition, garlic has long been used in the treatment of infectious diseases, as described in the 9th century literary book entitled ure ‘Bald's Leechbook’. A remedy termed Bald's eye salve for stye that is caused by Staphylococcus aureus (S. aureus) infection was prepared by the alcoholic extraction of garlic, onion or leek in a brass pot (7,8). Fuchs et al demonstrated the antimicrobial activity of Bald's eye salve against S. aureus and Pseudomonas aeruginosa, including the multidrug-resistant phenotype and identified allicin as the principal antimicrobial compound in Bald's eye salve formulation. Allicin may greatly contributed to the treatment of stye in that era (7). On the other hand, allicin is chemically unstable and rapidly disappears when it comes into contact with body fluids (9,10). Therefore, it is difficult for allicin to reach the infected sites of the body as an intact form. Recent studies have demonstrated that sulfur-containing compounds derived from garlic, such as diallyl disulfide (DAS₂) and ajoene, inhibit biofilm formation and the QS of bacteria, even though the antimicrobial activities of these compounds are lower than those in medical antibiotics used in clinical settings (11–16). Furthermore, Slachmuylders et al and others have demonstrated that some natural products, which exert an inhibitory effect on biofilm formation, have antibiotic-potentiating activity (17–20).

In this review, we focus on the antimicrobial activity of sulfur-containig compounds derived from garlic and describe their chemical and biological properties, including their inhibitory effect on bacterial biofilm formation.

Various hydrophobic antimicrobial compounds have been isolated from garlic and their structures are illustrated in Fig. 1. Among these compounds, allicin is considered to play a central role in the biological activity of garlic. However, allicin is unstable and tends to be converted into various compounds, such as ajoenes and diallyl polysulfides (DAS_n), which have been reported to exhibit antimicrobial activity. In this section, we describe the chemical and biological properties of hydrophobic compounds in garlic and its preparation, exhibiting antimicrobial activity.

Bacteria have a barrier system, biofilm formation, which inhibits the entry of disinfectants, antibiotics and host immune molecules into the bacterial cells and is a major cause of the drug-resistance of bacteria (44). In addition, QS molecules, such as AHL regulate biofilm formation, intercellular communication, bacterial population and other processes (3,4,11,46). The inhibition of biofilm formation and QS has been studied in various scientific and technological fields. Certain natural products have been reported to provide effective resources for the inhibition of biofilm formation. Rasmussen et al performed screening to identify QS inhibitors (QSIs) by using a novel genetic system and found that toluene extract of garlic inhibited biofilm formation (13). This result suggests that hydrophobic compounds extracted from garlic might have activity as a QSI (47). Allicin prevents biofilm formation by inhibiting early bacterial adhesion and EPS secretion (42,48,49). In addition, allicin inhibits the secretion of virulence factors by regulating QS (50). Ajoene regulates biofilm formation by blocking the QS-induced production of virulence factors (51,52). DAS₂ inhibits the biofilm formation of Pseudomonas spp by inhibiting the production of virulence factors via the regulation of QS at the concentration of 0.16–1.28 mg/ml with no effects on microbial growth (44). In addition, DAS₂ inhibits the formation of biofilm by suppressing the expression of key QS-related genes (44). Moreover, in the S. aureus QS system, a peptidic compound having a thioester group acts as an autoinducer (53). It is expected that DAS₂ may also inhibit QS through the reaction with the thioester group of the autoinducer. These hydrophobic compounds may contribute to the reduction of undesirable impacts of microorganisms on humans and they can be expected to suppress the development of drug resistance due to biofilm formation of bacteria.

Various hydrophobic compounds derived from garlic and its preparations have broad-spectrum antimicrobial activities. In particular, allicin and its derivatives have been studied extensively as antimicrobial active ingredients and have shown the inhibitory activity of biofilm formation by inhibiting QS. However, these compounds are unstable and could not be used against systemic infections. Therefore, the development of allicin derivatives with sufficient stability may lead to the development of superior compounds with greater antimicrobial activity and more potent inhibitory activity against biofilm formation for the treatment of drug-resistant bacteria.