Impact of Fatty Acid Chain Length of Rosmarinate Esters on Their Antimicrobial Activity against Staphylococcus carnosus LTH1502 and Escherichia coli K-12 LTH4263

Abstract

The effect of the addition of a newly synthesized series of rosmarinic acid (RA) esters (REs) and alcohols with chain lengths of 1,4, 8, 10, 12,16, and 18 carbons (REI to 18) on the growth behavior of Staphylococcus carnosus LTH1502 and Esclierichia coli K-12 LTH4263 was investigated. An initial microtiter dilution assay indicated activity of compounds against S. carnosus LTH1502 only, but not E. coli K-12 LTH4263, and a strong dependence of growth inhibition of S. carnosus LTH1502 on the esters' alkyl acid chain lengths. Esters with chains shorter than 8 and longer than 12 had little effect against S. carnosus LTH1502, whereas esters with chain lengths between 8 and 12 inhibited growth. To better understand the dependence of the antimicrobial activity of the REs on alkyl chain lengths, RA, n-methyl rosmarinate (RE1), n-dodecyl rosmarinate (RE12), and n- octadecyl rosmarinate (RE18) were used in a time-kill assay against S. carnosus LTH1502. Compounds were added at 0.75 mM in the lag phase, 5 mM in the exponential phase, and 10 mM in the stationary phase. RA had no effect in the lag and exponential phases but decreased cell counts during the stationary phase. In contrast, RE1 and RE12 decreased cell numbers in all three phases, with RE12 reducing counts most rapidly. Addition of RE18 did not affect growth regardless of the growth phase. Appearance and physiological State of S. carnosus LTH1502 cells treated with 200 mM RA after 48 h and with 10 mM RE12 after4 h were observed by fluorescence microscopy. Images indicated differenees in the way the compounds interacted with and damaged cells. Results were attributed to the different physicochemical properties of RA and its esters.