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.