Synergistic effects of allicin and β-lactamase antibiotics on Acinetobacter baumannii

Abstract

Objective To investigate the synergistic antibacterial mechanisms of allicin and β-lactamase antibiotics on Acinetobacter baumannii (A.baumannii).Methods The micro-broth dilution method was employed to determine the 50％ and 90％ minimal inhibitory concentration (MIC50 and MIC90) of allicin and cefoperazone and the fractional inhibitory index (FIC) of their combination by using 16 isolates of A.baumannii derived from clinics between November 2011 and June 2012.Changes in reactive oxygen species (ROS) of bacteria in vivo or in vitro were detected by using nitroblue tetrazolium (NBT) approach following incubation with allicin at different concentrations (512,64 and 4 mg/L).The cefoperazone absorption rate was assessed following incubation with different concentrations (128,32 and 1 mg/L) of allicin,and the changes in bacterial membrane protein expression was determined following incubation with 128 mg/L allicin.Results Allicin yielded an MIC50 of 256 mg/L and MIC90 of 512 mg/L for both antibioticsensitive and-resistant bacteria.Cefoperazone yielded an MIC50 of 64 mg/L and MIC90 of 128 mg/L for antibiotic-resistant bacteria,and an MIC50 of 16 mg/L and MIC90 of 32 mg/L for antibiotic-sensitive bacteria.The combination of cefoperazone and allicin resulted in an FIC index of ＜1.Treatment with allicin of different concentrations (512,64 and 4 mg/L) yielded the in vitro ROS concentration of (0.33 ± 0.10),(0.21 ± 0.07) and (0.10±0.05) and in vivo ROS concentration of (0.66±0.06),(0.59±0.08) and (0.56±0.01) in antibioticresistant bacteria.Treatment of allicin was also linked to the in vitro ROS concentration of (0.31 ±0.08),(0.22±0.09) and (0.11±0.06) and in vivo ROS concentration of (0.65±0.08),(0.59±0.07) and (0.53±0.06) in antibiotic-sensitive bacteria.There was a significant difference between in vivo and in vitro ROS concentration (both P＜0.05).Compared with control group,treatment with allicin led to reduced concentration of cefoperazone in the bacterial supernatant.The concentration of cefoperazone in bacterial supernatant in 128 mg/L group was markedly higher (both P＜0.05) than that in 32 mg/L group and 1 mg/L group.However,the difference did not reach statistical significance in the latter two groups (P＞0.05).Incubation with 128 mg/L allicin suppressed the membrane proteins (molecular weight:34 000 and 25 000) expression.Conclusion The substantially increased in vivo and in vitro ROS concentration and altered cell membrane structures following incubation with allicin may be related to the synergistic antibacterial effects of allicin with cefoperazone on A.baumannii. Key words: Allicin; Beta-lactams ; Anti-bacterial agents ; Acinetobacter baumannii; Drug synergism