In Vitro Evaluation of the Antimicrobial Activity of Several Short and Medium Chain Fatty Acid Salts and Their Combinations

Organic acid salts are feed additives used as alternatives to antibiotics for health and growth promotion in animal production. The aim of this study was to investigate the Minimum Inhibitory Concentration (MIC) of organic acid salts against Salmonella enterica spp. enterica (7 strains), E. coli (9 strains) and Cl. perfringens (6 strains). The products tested were sodium butyrate (BUT), sodium heptanoate (HEP), sodium salt of coconut fatty acid distillate (DIC) and their combinations BUT+DIC (1:1 ratio) and HEP+DIC (1:1 ratio). For each product and each strain, MIC was determined per triplicate using a broth micro-dilution method at pH 4.8 and 6 and the value which inhibits 50% of the tested isolates was determined (MIC₅₀). The results at pH 4.8 of MIC₅₀ against E. coli showed that HEP was the most effective (32 ppm) followed by HEP+DIC (128 ppm), BUT (512 ppm) and BUT+DIC (1024 ppm), DIC could not solubilize at more than 5000 ppm. Regarding to Salmonella, HEP was also the most effective (32 ppm) followed by HEP+DIC and BUT (256 ppm), and DIC and BUT+DIC (512 ppm). Cl. perfringens was unable to grow at pH 4.8. The results at pH 6 of MIC₅₀ against E. coli showed that HEP was again the most effective (3125 ppm) followed by HEP+DIC (3500 ppm), and BUT (62500 ppm). DIC could not solubilize at more than 5000 ppm and BUT+DIC at more than 9000 ppm. Regarding to Salmonella at pH 6, HEP was also the most effective (1562 ppm) followed by BUT (125000 ppm), however DIC, BUT+DIC and HEP+DIC could not solubilize at more than 5000, 9000 and 14000 ppm, respectively. Finally, the results against Cl. perfringens showed that DIC was the most effective (15.4 ppm) followed by HEP+DIC (21.5 ppm), BUT+DIC (27.6 ppm), HEP (1562 ppm) and BUT (31250 ppm). In summary, the results showed that HEP was the most effective against gram negative bacteria, while DIC was the most effective against gram positive bacteria. The combination HEP+DIC showed an increased effect of DIC against Salmonella and also an increased effect of HEP against Cl. perfringens. The combination of BUT with DIC potentiated the effect of BUT against Cl. perfringens. Further studies would be required to better understand these interactions as well as in vivo studies to demonstrate the effects on microbial populations.