ANTIMICROBIAL ACTIVITY OF SHORT- AND MEDIUM-CHAIN FATTY ACID SALTS AND THEIR COMBINATIONS AGAINST STREPTOCOCCUS SUIS

In the last ASAS Virtual Meeting 2020, it was presented a study 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 medium-chain fatty acids distillate (DICOSAN, DIC) alone and two combinations with sodium butyrate (NaBut): DIC70:30, 70% of NaBut protected with 30% of DIC; and DIC50:50, 50% of NaBut protected with 50% of DIC. The MIC was assessed for each product and strain by broth microdilution method at pH 6.0. MBC values were 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 showed 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 has strong antimicrobial activity against Gram- bacteria. In contrast, medium-chain fatty acids source (MCFA) rich in lauric acid have strong antimicrobial activity against Gram+ bacteria. In this study, DIC was the most effective against the Gram+ bacteria, followed by DIC50:50 and DIC70:30, the sodium butyrate based products. As expected, higher concentration of MCFA in the tested product was associated with 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.