Briefly, blood-agar plates were seeded using a swab with a suspension of the type strain CCUG 17874 or the strain C/M-R2, whose density corresponded to McFarland no. 4 opacity standard. After the surface was dried, three paper discs were deposited on each plate, one disc was charged with the antibiotic (amoxicillin 2 μg, clarithromycin 15 μg, metronidazole and levofloxacin 5 μg each and tetracycline 10 μg), one with polysorbate 80 (0.4 mg) and the third one with both drugs, polysorbate 80 and antibiotic,

at the same concentration present in the discs charged with single antibiotics. After a 3-day incubation in microaerobic environment at 37°C, plates were inspected and the halos of growth inhibition measured. The broth dilution test was carried

out as follows: MDV3100 datasheet after the first drug was diluted, the second drug was added to each well of the first row containing different concentrations of the first compound; afterwards, the dilution of the second compound PP2 mw was carried out. Concurrently, we determined the MBC of the single substances. Tests were performed in triplicate. Ultrastructural analysis of H pylori with transmission electron microscopy (TEM) For the ultrastructural analysis two strains of H. pylori were used: CCUG 17874 (metronidazole resistant type strain, isolated from a chronic gastritis case) and C/M-R2 (clarithromycin resistant clinical Org 27569 strain isolated

from a chronic gastritis case). These two strains were treated with: 1-polysorbate 80, 2-clarithromycin, 3- metronidazole, 4- polysorbate 80 and clarithromycin, 5- polysorbate 80 and metronidazole. The other antibiotics were not tested because they did not exert any synergistic effect when examined in association with polysorbate 80. The bacterial suspensions, after overnight incubation with the drugs at the concentrations corresponding to the respective MBCs and MBCs of their associations, were washed in phosphate-buffered saline (PBS), fixed in cold Karnovsky fixative and maintained at 4°C for 2 h. Fixed organisms were washed in 0.1 mol/L cacodylate buffer (pH 7.2) for 12 h at 4°C and postfixed in 1% buffered osmium tetroxide at 4°C for 1 h. Then the samples were washed in 0.1 mol/L cacodylate buffer (pH 7.2) for at least 2 h at 4°C, dehydrated in a series of ethanol (50%, 75%, 95%, 100%), exchanged through propylene oxide and embedded in Epon Araldite. Ultra-thin sections were obtained with a Supernova ultramicrotome (Reickert Jung, Vienna, Austria) with diamond knife, mounted on copper grids, stained with uranyl acetate and lead citrate and observed and photographed with a Philips EM208 TEM (Philips Scientifics, Eindhoven, The Netherlands).