Antibacterial action of several tannins against Staphylococcus aureus

We examined the antibacterial action of several tannins on plasma coagulati on by Staphylococcus aureus and the effect of conventional chemotherapy com bined with tannic acid below the MIC. Coagulation was inhibited in plasma c ontaining tannic acid (100 mg/L), gallic acid (5000 mg/L), ellagic acid (50 00 mg/L), (-)-epicatechin (1500 mg/L), (-)-epicatechin gallate (500 mg/L) o r (-)-epigallocatechin gallate (200 mg/L) after incubation for 24 h. All ta nnins inhibited coagulation at a concentration below the MIC. The MICs of o xacillin and cefdinir for S. aureus were reduced to less than or equal to0. 06 mg/L in Mueller-Hinton agar plates with tannic acid (100 mg/L) at a conc entration below the MIC. The antistaphylococcal activity of tannic acid was reduced in plates with 10% rabbit blood, but not in those with 10% rabbit plasma. Membranous structures formed in a culture medium containing equal p roportions of plasma and tryptic soy broth after incubation for 24 h. The c olony counts of S. aureus in membranous structures in the medium containing oxacillin (40 mg/L) and tannic acid (100 mg/L) were c. 10-fold lower than those in medium containing oxacillin (40 mg/L) alone (P < 0.01). Tannic aci d merits further investigation as a possible adjuvant agent against S. aure us skin infections treated with beta -lactam antibiotics.