While prior research documenting the antimicrobial properties of ɛ-PL does exist, its inhibitory effects against certain bacterium of interest had yet to be reported. Researchers at the Qilu University of Technology in the Shandong Province of China conducted a study to test for the antibacterial mechanisms of ɛ-PL, specifically relating to two dangerous bacteria: Escherichia coli and Staphylococcus aureus.
To test the effects of ɛ-PL on E. coli and S. aureus, Li et al. treated cultures of each bacterium with ɛ-PL in comparison to a control treated with sterile water. An Oxford cup test was used to evaluate antimicrobial activity, and a minimum inhibition concentration (MIC) was determined by measuring the lowest concentration of ɛ-PL that visibly inhibited bacterial growth. SEM analysis was then used to observe morphology changes of the bacterial cells when treated with ɛ-PL, and electrical conductivity tests were used to determine if cellular leakage was occurring. Finally, SDS-PAGE of the bacterial proteins was carried out to test for any damage caused to them by ɛ-PL. Every experiment was performed three times, and averages reported.
The Oxford cup tests and MIC measurements found that ɛ-PL had significant antibacterial effects against bacteria at very low dosages, and it became even more effective as the concentration of ɛ-PL was increased. S. aureus, a Gram-positive bacterium, was found to be more susceptible to ɛ-PL than E. coli, a Gram-negative bacterium, indicating that different concentrations of ɛ-PL are necessary to effectively inhibit different bacteria.
SEM analysis revealed damage to the surface of bacterial cells that had been treated with ɛ-PL in comparison to the control. Electrical conductivity tests showed an increase in the conductivity of samples as the concentration of ɛ-PL was increased, suggesting that the cytoplasmic membranes of the bacterial cells were damaged and cellular leaking was occurring. And SDS-PAGE profiles of the bacterial proteins revealed destruction of those cellular proteins when ɛ-PL was introduced. The researchers noted that the destruction could have been due to either the interference of ɛ-PL with the synthesis of those proteins or to leakage of the proteins from the cells, but they were unable to conclude which scenario was taking place.
Based on their experiments, the researchers were able to conclude that ɛ-PL does indeed have strong antibacterial activity against E. coli and S. aureus, and that activity increases as the concentration of the treatment is increased. ɛ-PL not only reacted with the bacteria but damaged the very structure of the bacterial cells, proving that it can serve as an effective natural preservative in food.
1 Li, Y. et al. (2014) ‘Antibacterial characteristics and mechanisms of ɛ-poly-lysine against Escherichia coli and Staphylococcus aureus.’ Food Control 43(1) 22-27.