Determination of the Minimum Inhibitory Concentration of Various Medium Chain Fatty Acid-Based Products in E. coli, Enterotoxigenic E. coli, and Campylobacter coli

Research has demonstrated medium chain fatty acids (MCFA) are bactericidal and potential antibiotic alternatives. However, it is unknown how the type or level of MCFA impact bacteria growth. This can be tested through a minimum inhibitory concentration (MIC) benchtop assay, which identifies the lowest concentration of a chemical that prevents visible growth of a bacterium. Our objective was to determine the MIC of pure MCFA (Experiment 1) and products containing MCFA (Experiment 2) against generic E. coli, enterotoxigenic E. coli (ETEC), or Campylobacter coli (campy). Experiment 1 used a 4 × 3 factorial with four types of pure MCFA (C6:0, C8:0, C10:0, or 1:1:1 blend of C6:C8:C10) against the three bacteria and was repeated 3 times. All interactions and main effects were significant (P<0.05). The most effective (P<0.05) pure MCFA tested against generic E coli was the 1:1:1 blend of C6:C8:C10, and against ETEC was C6:0. All tested pure MCFA had similar (P>0.05) MIC against campy. This demonstrated that MCFA efficacy varies with MCFA type and bacteria. Pure MCFA are expensive and not easily available to producers. Thus, 24 commercially-available and developmental products were analyzed for MCFA concentration, with 5 selected based on their C6:0, C8:0, and C10:0 concentrations. In Experiment 2, these products were used in a 5 × 3 factorial to determine their MIC using the same procedures in Experiment 1. There were 5 MCFA products (4 developmental products + coconut oil) tested against three bacteria (E. coli, ETEC, and campy). Only the main effect of treatment was significant, resulting in products 1 and 2 having a lower (P<0.05) MIC than products 3, 4, and coconut oil. In summary, pure MCFA were bactericidal to E. coli, ETEC, and campy. However, their efficacy varied between bacteria. The efficacy of potential commercial products can be predicted based on their MCFA concentration, with the shorter chain MCFA having greater efficacy in the tested bacteria.

Experiment 1. Mean Minimum Inhibitory Concentration of Synthetic MCFA, %
Item	E. coli	ETEC	Campy	Pooled SEM		P =
C6:0	0.70c	0.53ef	0.50fg	0.0316		<.0001
C8:0	0.85b	0.67cd	0.47fg
C10:0	1.00a	1.00a	0.90b
C6:C8:C10 Blend	0.60de	1.00a	0.43g
Experiment 2. Mean Minimum Inhibitory Concentration of Developmental MCFA Products, %
Item	E. coli	ETEC	Campy		Pooled SEM	P =
Product 1	0.37	0.33	1.20		0.4730	0.486
Product 2	1.20	1.30	0.33
Product 3	3.33	3.83	2.75
Product 4	4.17	4.33	3.33
Coconut oil	>5.00	>5.00	>5.00