Enhancement of Antimicrobial Activity of Four Classes of Antibiotics Combined with Garlic

The increased resistance to antimicrobial agents among clinical isolates is a serious problem that dramatically raises the cost of health care worldwide. Seeking alternative approaches to enhance the susceptibility of these microorganisms to killing is a major concern to researchers. One such approach is the combination of some adjuncts with antibiotics. Garlic (Allium sativum) is an herbal product traditionally used for many health-related purposes, including protection against microbial infections. This work investigates the effect of combining garlic, in sub-inhibitory concentrations, with four different antibiotics against sixteen selected multidrug resistant Gram-negative clinical isolates belonging to Pseudomonas and Acinetobacter genera. A combination of 5 and 10 mg mL-1 of garlic (for Acinetobacter and Pseudomonas, respectively) with levofloxacin, gentamicin, azithromycin and doxycycline resulted in a decrease in the antibiotics' Minimum Inhibitory Concentrations (MICs) against the isolates in the range of 4-≥32, 4-≥2048, 2-≥2048 and 2-≥128 fold, respectively. The kinetics of killing of the garlic-gentamicin combination were subsequently followed in four Pseudomonads for 24 h and a significant effect ranging between 2 and 5 log reduction in bacterial count, compared to the control, was obtained. The results show a great potential for the use of garlic as an adjunct to antibiotics for the treatment of infections caused by resistant Gram-negative strains and warrant further investigation.