Assessment of the anti-pathogenic effects of condensed tannin extracts using scanning electron microscopy

Two different types of condensed tannins (CTs), which were extracted and purified from tilia (Tilia L.) and black locust (Robinia pseudoacacia), were studied and tested against two kinds of bacteria, including Gram-negative and Gram-positive, avian pathogenic E. coli (APEC) and Staphylococcus epidermidis (S. epidermidis) respectively, by minimal bactericidal concentrations (MBCs) and scanning electron microscopy (SEM). Both CT extracts were significantly effective (p ≤ 0.05) at MBCs of 5-10 mg CT/ml against APEC (Gram-negative), and at 1.25-5 mg CT/ml on S. epidermidis (Gram-positive). This indicated that the CTs were more potent against the Gram-positive than the Gram-negative bacteria. Further, SEM revealed that CTs caused mainly morphological deformations of the bacterial cells and some conjoined cell growth.