Natural products against gram-negative bacteria: promising antimicrobials in future complementary medicine

Gram-negative bacteria are nonfermentative and opportunistic and are among the most important causes of nosocomial infections. These bacteria, especially Pseudomonas aeruginosa, Acinetobacter baumannii and Escherichia coli, are resistant to various antibiotics worldwide. This level of antibiotic resistance as well as the overuse of drugs can be among the most important reasons for the tendency toward naturally occurring products. Although the majority of natural products have much greater antibacterial activity against gram-positive bacteria than gram-negative ones, in this review, we presented natural products with anti-gram-negative bacteria and their mechanisms of action. Phenolic compounds, terpenoids, alkaloids and organosulfur compounds are among the most important classes of antimicrobial natural products and are briefly discussed.