Antibacterial effects of guava tannins and related polyphenols on Vibrio and Aeromonas species

An extract obtained from a bottled tea drink of Psidium guajava L. (Myrtaceae) showed antibacterial effects on Vibrio vulnificus, V. mimicus, V. parahaemolyticus, and Aeromonas sobria. HPLC-diode array detector (DAD) analysis of an effective fraction obtained from the extract revealed the presence of several tannins and related polyphenols. To verify these results and to estimate the antibacterial effects of the polyphenols, we isolated the polyphenols from the leaves of P. guajava. Among the polyphenols isolated, pedunculagin, castalagin, casuarinin, and stenophyllanin A were effective against Vibrio and Aeromonas species. Studies of structurally related compounds revealed that penta-O-galloyl-β-D-glucose (PGG), (-)-epigallocatechin gallate (EGCG), and alkyl gallates such as isoamyl gallate (IG) and n-octyl gallate exhibited potent antibacterial activities. The minimum bactericidal concentrations (MBCs) of three polyphenols (i.e., PGG, EGCG, and IG) that exhibited low minimum inhibitory concentrations (MICs) were then determined. Comparisons of the MBCs and MICs indicated that PGG was not bactericidal at the MIC, whereas EGCG and IG were. The effect of combinations of the three polyphenols with several antibiotics was also examined. The combination of IG and kanamycin (KM) effectively reduced the MIC of KM against V. vulnificus and V. mimicus; the combination of EGCG and tetracycline (TC) also reduced the MIC of TC against V. parahaemolyticus. Thus, polyphenols may be useful in the development of antibacterial agents against Vibrio bacteria.