PSVII-32 In vitro evaluation of antimicrobial activity of short- and medium-chain fatty acid salts and their combinations against Streptococcus suis

The aim of this study was to investigate the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of organic acid salts against six field isolates of Streptococcus suis. The three products evaluated were sodium salt of coconut fatty acids distillate (DIC) alone and two combinations with sodium butyrate (NaBut): DIC70:30, being 70% of NaBut protected with 30% of DIC; and DIC50:50, being 50% of NaBut protected with 50% of DIC. Antimicrobial susceptibility testing was performed to estimate the MIC values for each product and strain by the broth microdilution method at pH 6.0. MBC values were also determined by sub-culturing supernatant from wells without evident bacterial growth. The values of MIC50/MBC50 were calculated as the concentration which inhibited/killed 50% of the isolates tested. The MIC50 showed DIC as the most effective (8 ppm) against S. suis followed by DIC50:50 (32 ppm) and DIC70:30 (64 ppm). The MBC50 demonstrated a similar trend, DIC being the most effective (16 ppm) followed by DIC50:50 (64 ppm) and DIC70:30 (64 ppm). It is well known that butyric acid is a short-chain fatty acid which has strong antimicrobial activity against Gram-negative bacteria. In contrast, coconut fatty acids distillate is a medium-chain fatty acid source (MCFA) rich in lauric acid which has strong antimicrobial activity against Gram-positive bacteria. Both products are generally available as salts to facilitate their application in feed. In this study, the results showed that DIC was the most effective against the Gram-positive bacteria tested, followed by DIC50:50 and DIC70:30, the sodium butyrate-based products. As expected, a higher concentration of MCFA in the tested product was associated with a higher inhibitory and bactericidal activity. Further studies would be required to better understand these interactions as well as in vivo studies to demonstrate the effects on microbial populations.