Antibacterial effect of caprylic acid and potassium sorbate in combination against listeria monocytogenes ATCC 7644

Abstract

Listeria monocytogenes is a common foodborne pathogen that cause life-threatening infection with high mortality rates. Biofilm development of L. monocytogenes decreases its sensitivity to antibiotics, which has long attracted attention globally. Caprylic acid (CA) and potassium sorbate (PS) are both widely used food preservatives, but their synergistic effect against L. monocytogenes has not been described. This study explored the antibacterial activities of the CA-PS combination against L. monocytogenes ATCC 7644 grown in planktonic or biofilm cultures. The fractional inhibitory concentration index values, determined by the checkerboard microdilution method, were 0.37 ± 0.03 and 0.31 ± 0.04, showing their synergistic antimicrobial effects against L. monocytogenes ATCC 7644 in planktonic and biofilm cultures, respectively. CA-PS effectively eradicated the biofilm biomass to 10.8% by crystal violet assay and to 8.63% by fluorescence microscopic analysis compared with the control. The apoptosis rates of microbial cells embedded within biofilm significantly increased to 51.4%. Subsequent analysis revealed that the combination inhibited biofilm formation by affecting extracellular DNA release and polysaccharide intercellular adhesion expression, which was decreased from 8.93 to 1.04 ng of extracellular DNA per relative biomass and to 54.7% of the control, respectively. In addition, the combination inhibited the growth of L. monocytogenes ATCC 7644 by up to 0.67 ± 0.05 and 0.30 ± 0.03 log CFU/cm2 in planktonic and biofilm modes on a carrot surface, respectively. The synergistic antibacterial effects of CA-PS against L. monocytogenes ATCC 7644 were statistically significant, and the combination is an excellent candidate to be a novel food preservative.