3.3. Antimicrobial activity of PBS/thymol films
Fig. 3 illustrates the antimicrobial activity performed against foodborne pathogens namely S. aureus and E. coli. The antimicrobial evaluation was performed by placing the PBS/thymol film in direct contact with pathogenic bacteria. Further investigation was made by measuring clear zone produced by a film containing thymol and clearly shown in Table 4. Under the test duration, thymol was expected to release from PBS into the agar and then produced the inhibition zone around the active films. The minimum inhibition concentration (MIC) value of thymol against E. coli and S. aureus was 10 and 6 wt% respectively. As illustrated in Fig. 3(a)–(f), PBS containing 10 wt% thymol was the most effective in the E. coli (gram-negative) growth inhibition. At this thymol content, the clear zone was estimate by only 2 cm. Lower than this concentration, no inhibition zone was observed. Another antimicrobial activity was performed against S. aureus (gram-positive). The clear zone was observed by around 1.8 cm when thymol was blended at 6 wt%. Beyond this concentration, the growth inhibition zone gradually increased by 4.4 and 5.5 cm for 8 and 10 wt% thymol respectively. In comparison, the film was found to have lower antimicrobial performance against E. coli. This was expected because of the outer membrane of the gram-negative bacteria helped them stronger. To overcome this feature, larger amounts of antimicrobial agent is required. In PP, thymol was also found to inhibit the S. aureus growth. The inhibition clear zone diameter of 3.7 cm was observed when 8 wt% thymol was added [9]. Thymol was also found to inhibit the E. coli, S. aureus, Listeria, B. subtilis and Salmonella. However, the inhibition was directly depended on thymol concentration [14].