Multidrug Pump Inhibitors Uncover Remarkable Activity of Plant Antimicrobials

Abstract

Plant antimicrobials are not used as systemic antibiotics at present. The main reason for this is their low level of activity, especially against gram-negative bacteria. The reported MIC is often in the range of 100 to 1,000 g/ml, orders of magnitude higher than those of common broad-spectrum antibiotics from bacteria or fungi. Major plant pathogens belong to the gram-negative bacteria, which makes the low level of activity of plant antimicrobials against this group of microorganisms puzzling. Gram-negative bacteria have an effective permeability barrier, comprised of the outer membrane, which restricts the penetration of amphipathic compounds, and multidrug resistance pumps (MDRs), which extrude toxins across this barrier. It is possible that the apparent ineffectiveness of plant antimicrobials is largely due to the permeability barrier. We tested this hypothesis in the present study by applying a combination of MDR mutants and MDR inhibitors. A panel of plant antimicrobials was tested by using a set of bacteria representing the main groups of plant pathogens. The human pathogens Pseudomonas aeruginosa, Escherichia coli, and Salmonella enterica serovar Typhimurium were also tested. The results show that the activities of the majority of plant antimicrobials were considerably greater against the gram-positive bacteria Staphylococcus aureus and Bacillus megaterium and that disabling of the MDRs in gram-negative species leads to a striking increase in antimicrobial activity. Thus, the activity of rhein, the principal antimicrobial from rhubarb, was potentiated 100-to 2,000-fold (depending on the bacterial species) by disabling the MDRs. Comparable potentiation of activity was observed with plumbagin, resveratrol, gossypol, coumestrol, and berberine. Direct measurement of the uptake of berberine, a model plant antimicrobial, confirmed that disabling of the MDRs strongly increases the level of penetration of berberine into the cells of gram-negative bacteria. These results suggest that plants might have developed means of delivering their antimicrobials into bacterial cells. These findings also suggest that plant antimicrobials might be developed into effective, broad-spectrum antibiotics in combination with inhibitors of MDRs.

Chemical composition and antimicrobial activity of propolis collected from some localities of Western Algeria

“…Even though all propolis samples showed activity only against Grampositive bacteria, no activity was observed against Gram-negative bacteria (P. aeruginosa and E. coli) strains. The mechanisms of action are not fully understood, the most possible explanation for the low sensitivity of Gram-negative bacteria is that their outer membrane (phospholipids, proteins, and lipopolysaccharides structure) inhibits and/or retards the penetration of propolis (TegoS et al, 2002). These results are in agreement with previous reports (GonSALeS et al, 2006).…”

“…Moreover, it has been reported that propolis exhibits bacteriostatic activity against different bacterial strains and can be bactericidal in a high concentration (MirzoevA et al, 1997). Another possible reason is their possession of multi drug resistance (MDR) pumps, which prevents intracellular accumulation of propolis constituents (TegoS et al, 2002).…”

Effector-Repressor Interactions, Binding of a Single Effector Molecule to the Operator-bound TtgR Homodimer Mediates Derepression

“…Plant defense metabolites such as certain flavonoids (quercetin, genistein) were previously shown to possess antibacterial activity (45)(46)(47), which is because of the inhibition of DNA gyrases (52). In the present work, we demonstrate that this system is involved in the resistance to plant antimicrobials like phloretin, quercetin, naringenin, or coumestrol.…”

“…Based on the fact that the plant rhizosphere is one of the natural habitats of P. putida, and considering that the AcrB transporter from the plant pathogen Erwinia amylovora, involved in the resistance of this strain to various plant defense metabolites (27), is 60% identical to the TtgB transporter from P. putida DOT-T1E, we investigated the potential role of TtgABC in the resistance to plant-derived antimicrobials. To verify this hypothesis, the MICs of five plant secondary metabolites known to have antibacterial activities (45)(46)(47) were determined for both, P. putida DOT-T1E and its derivative mutant strain DOT-T1E18, deficient in the TtgABC efflux pump (Table 2). To different extents, the wild type strain was susceptible to all of the compounds tested.…”

Atromentin and Leucomelone, the First Inhibitors Specific to Enoyl-ACP Reductase (FabK) of Streptococcus pneumoniae

“…Some polyphenol compounds have been reported to be pumped out in bacteria [16]. For example, phenolic compounds such as rhein, gossypol, or resveratrol have been reported to be extruded by a multidrug resistance (MDR) pump in P. aeruginosa, E. coli, or Salmonella enterica, since their antibacterial activity was uncovered by the treatment with MDR inhibitors, MC 207110 or INF 271 , or in an MDR mutant.…”