When calculating the FICI a CCM MIC one dilution above the maximum concentration tested was used (Table 2). MICs of EGCG ranged from 128–1024 μg/mL. The antimicrobial activity of CCM was much lower against A. baumannii than those

reported for S. aureus (MIC = 125-250 μg/mL) [6] and H. pylori (5-50 μg/mL) [5]. This could reflect variations in the growth media, differences in lipopolysaccharide (LPS) or cell wall architecture as well as penetration HMPL-504 solubility dmso and transport of CCM across the Gram-negative outer membrane, issues well known to mediate resistance in A. baumannii [25]. Table 2 Minimum inhibitory concentrations (MICs) of curcumin, epigallocatechin gallate and combinations of both compounds and fractional inhibitory concentration indexes

(FICIs) versus Acinetobacter baumannii Isolate MICs in monotherapy (μg/mL) MICs in combination (μg/mL) FICIs CCM EGCG CCM EGCG AB 19606 >256 1024 4 256 0.258 (S) AB 14 >256 1024 4 512 0.508 (Ad) AB 16 >256 1024 32 512 0.56 (Ad) AB 186 >256 512 64 128 0.38 (S) AB 202 >256 1024 64 512 0.63 (Ad) AB 205 >256 1024 https://www.selleckchem.com/products/pexidartinib-plx3397.html 4 512 0.508 (Ad) AB 292 >256 1024 4 256 0.258 (S) AB 306 >256 128 4 32 0.258 (S) AB 308 >256 256 4 64 0.258 (S) MICs were within +/-1 dilution on replicate tests. CCM = curcumin, EGCG = epigallocatechin gallate, S = synergy, Ad = additive effect. Several mechanisms for the antibacterial activity of CCM have been proposed including disruption of core metabolic pathways involved in folic acid metabolism (shikimate dehydrogenase) [5] and bacterial cell division (FtsZ) [26].The MICs of EGCG against the A. baumannii isolates used in our study were also higher than those previously reported [10] although it should be noted that the isolates tested in our study belonged to extensively resistant clones. In combination tests, increased

antibacterial activity was Molecular motor observed, with MICs for the combination being significantly lower than those for individual compounds. The addition of EGCG reduced the MIC of CCM by up to 3 -7 fold and was as low as 4 μg/mL for several isolates. Synergy between the two polyphenols was observed against five isolates (FICI ≤ 0.5) including one of the OXA-23 clone 1 isolates and the two NDM producers. An additive effect was observed with the remaining 4 isolates (Table 2). These results indicate that combinations of CCM and EGCG synergistically inhibit the growth of A. baumannii and that no antagonism occurs. This adds to previous research which showed synergy between natural compounds including tea polyphenols [12], where the addition of epicatechin, a compound with no antimicrobial activity against A. baumannii potentiated the activity of theaflavin. The FICI as a measure of synergistic activity has limitations and more conservative limits of interpretation have been suggested [27]. The susceptibility breakpoint index (SBPI) may be a more useful parameter to assess positive interactions and the clinical usefulness of antimicrobial combinations [28].