Evaluation of the best method to assess antibiotic potentiation by phytochemicals against Staphylococcus aureus

Abstract

The increasing occurrence of bacterial resistance to antibiotics has now reached a critical level. Finding antibiotic coadjuvants capable to inhibit the bacterial resistance mechanisms would be a valuable mid-term solution, until new classes of antibiotics are discovered. Selected plant alkaloids were combined with 5 antibiotics against 10 Staphylococcus aureus strains, including strains expressing distinct efflux pumps and methicillin-resistant S. aureus strains. The efficacy of each combination was assessed using the microdilution checkerboard, time-kill, Etest, and disc diffusion methods. The cytotoxicity of the alkaloids was evaluated in a mouse fibroblast cell line. Potentiation was obtained in 6% of all 190 combinations, especially with the combination of: ciprofloxacin with reserpine (RES), pyrrolidine (PYR), and quinine (QUIN); tetracycline with RES; and erythromycin with PYR. The highest cytotoxicity values were found for QUIN (half maximal inhibitory concentration [IC50] = 25 ± 2.2 mg/L) and theophylline (IC50 = 100 ± 4.7 mg/L).

Looking to nature for a new concept in antimicrobial treatments: isoflavonoids from Cytisus striatus as antibiotic adjuvants against MRSA

“…The Laboratory strains S . aureus RN6390 (a natural respiratory epithelial cell line that is a sigB -deficient mutant) and CECT 976 (from the Spanish collection), already used as a model microorganism for antimicrobial tests with phytochemical compounds 35, 36 , were included in the experiments. S .…”

“…striatus leaves, flowers and twigs were applied at 0.5 mg/mL. Inhibition zones obtained with the combinations (IZD c ) over antibiotic-single activity (IZD a ) are given and the combinations are classified as: (—) indifferent (IZD c – IZD a  < 4 mm); (A) additive (4 ≤ IZD c – IZD a  < 6 mm); and (P) potentiation (IZD c – IZD a  ≥ 6 mm) 20, 35, 36 . Data are means and SD from at least three independent experiments.…”

“…The antibiotic-potentiating activity of C . striatus extracts was categorized, as described in previous studies 20, 35, 36 , into three classes according to the ratio of the increase of the IZD produced by the antibiotic in the presence of the plant extracts to that obtained with the antibiotic alone (on simple MH agar plates): 1 – indifferent (IZD combination – IZD antibiotic < 4 mm); 2 – additive (4 ≤ IZD combination – IZD antibiotic < 6 mm); and 3 – potentiation (IZD combination – IZD antibiotic ≥ 6 mm) of inhibition of growth of S . aureus .…”

“…The checkerboard assay was performed according to CLSI guidelines 32 as described in previous studies 20, 35, 36 . The MICs of ciprofloxacin and erythromycin were determined in the presence of increasing amounts of the tested compounds against S .…”

Combinatorial Activity of Flavonoids with Antibiotics Against Drug-ResistantStaphylococcus aureus

“…Other researchers have already developed some comparisons on the agreement between these methods, which varies a lot between studies. 3,13,38,44,48,64 QUERC is the most abundant flavonol found in the human diet. 17 MOR is the isomer of QUERC and it has also been found to possess antibacterial activity.…”