Antimicrobial activity of oil extracted from Rana tigerina skin and its impact on gelatin-based film

Oil from the Asian bullfrog ( Rana tigerina ) skin (FSO) was extracted and evaluated for its antibacterial activities against food-borne pathogenic bacteria. The FSO showed inhibitory activities on both Gram +ve ( Bacillus cereus , Staphylococcus aureus , Listeria monocytogenes ) and Gram -ve ( Escherichia coli , Salmonella Typhimurium and Vibrio cholera ) bacteria with minimum inhibitory concentrations (MIC) ranged from 0.059-0.469 mg/mL; on the other hand, those bacteria could not be inhibited by palm oil. Gelatin film solutions emulsified with FSO (FSOS) or palm oil (POS) were tested for antimicrobial activity. The result showed that emulsion system of FSOS had impact on inhibitory activity. FSOS exhibited the higher activity with the lower MIC (p<0.05), compared to that of FSO. However, MIC obtained from FSOS increased for S. aureus and S. Typhimurium. This might indicate the loss of antimicrobial availability of FSO when incorporated with gelatin film solution. Characteristics of the FSO emulsion gelatin-based film (FSOF) were evaluated in comparison with those of palm oil emulsion film (POF). FSOF showed the higher intense of yellow color, compared with those of POF (p<0.05). Two different assays, suspension culture medium (SCM) and disk-diffusion (DD) assays, were conducted to determine the antimicrobial activity of FSOF and POF. Based on SCM method, FSOF showed the potential on microbial growth inhibition against all bacterial tested by decreasing the bacterial count number. Nevertheless, there was no inhibition zone detected, when DD was applied. These could be accounted that the oil might be trapped in the film matrix and limited the release of the oil into agar. Therefore, the oil extracted from R. tigerina skin could be an excellent natural alternative antimicrobial agent against both Gram +ve and Gram -ve bacteria, which could be develop and use as antimicrobial film packaging.