3.3 Antimicrobial study
The antimicrobial efficacy of each product was determined and can be found in Table 1. Antimicrobial tests indicate that the precursors of ethyl fatty acid arginate (i.e. L-arginine, L-arginine ethyl hydrochloride), oleic acid, and iso-oleic acid had MICs and MBCs greater than 232.7 µg mL-1, indicating that those compounds have little or low antimicrobial activities. All synthesized ethyl fatty acid arginate compounds were effective against Listeria with MICs and MBCs ranging from 1.8 to 29.1 µg mL-1, indicating the conjugation of the fatty acids with arginine resulted in the increase in antimicrobial potency, probably due to the positively charged polar arginate head and long fatty acid hydrocarbon tail which facilitate the interaction with cell membranes. However, not all these fatty acid arginate compounds were effective against E. coli . Ethyl n-oleoyl arginate hydrochloride had some antimicrobial activity against E. coliwhile ethyl lauroyl arginate hydrochloride was the most effective. Similar to the precursors, ethyl iso-oleoyl arginate hydrochloride, ethyl iso-stearoyl arginate hydrochloride and ethyl n-stearoyl arginate hydrochloride did not have antimicrobial activity against E. coli . Compared to the commercial LAE, ethyl lauroyl arginate hydrochloride synthesized in the present study had similar or slightly higher MICs and MBCs against Listeria and E. coli . An earlier study reported that the MIC and MBC of commercial LAE againstL. innocua and E. coli were 25 µg mL-1(Becerril et al ., 2013), levels higher than those of ethyl lauroyl arginate hydrochloride synthesized in the present study.
The difference in the response of the two bacteria to the fatty acid arginates may be a result of variation in membrane structures ofListeria and E. coli . L. innocua , a Gram-positive bacterium, has a thick peptidoglycan layer without outer membrane whileE. coli , being Gram-negative, has a thick peptidoglycan cell wall surrounded by an outer membrane. There were also some differences in MICs and MBCs against Listeria among the various ethyl fatty acid arginates. For example, ethyl iso-oleoyl arginate hydrochloride, ethyl iso-stearoyl arginate hydrochloride, and ethyl n-oleoyl arginate hydrochloride had lower MIC/MBC than ethyl lauroyl arginate hydrochloride. It appears that the length of fatty acid chains may affect antimicrobial activity of the arginate compounds againstListeria .