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 .