Feed Matrix Effect on the Antimicrobial Efficacy of a Commercial Essential-Oil Premix Against Three Major Animal Pathogens

Functional feeds that include phytobiotics, organic acids or other antibiotic alternatives are widely used in terrestrial and aquatic production, but their activity is measured almost exclusively in clear laboratory broth. This study compared a concentrated commercial essential-oil premix (SFF IG Premix), citric acid and chloramphenicol in tryptic soy broth (TSB) versus a 1:1 corn meal: soybean meal (CM: SBM) slurry at 5, 15 and 30 g L−1. Assays were run in shaken flasks against Salmonella enterica, Vibrio parahaemolyticus and Streptococcus agalactiae and read by plate count after 24 h. The slurry changed both the EC50 and the shape of the dose–response. The direction depended on strain for the premix, and on compound only in V. parahaemolyticus. Against S. enterica, the premix EC50 rose from 283 to 404 mg L−1 at 5 g L−1 and from 252 to 660 mg L−1 at 30 g L−1. Against V. parahaemolyticus, it rose from 176 to 471 mg L−1 at 30 g L−1. Against S. agalactiae, the EC50 fell, from 89 to 18 mg L−1 at 5 g L−1, while maximum kill remained smaller than in TSB. In the same 30 g L−1 matrix, the chloramphenicol EC50 fell from 1.00 to 0.68 mg L−1 and the citric acid EC50 from 493 to 183 mg L−1. Chloramphenicol and citric acid were tested only against V. parahaemolyticus at 30 g L−1, so compound and strain were not crossed. The citric acid shift coincided with a lower endpoint pH in the slurry. All three pathogens grew in CM: SBM without added TSB. The results do not establish in vivo efficacy. They show that a particulate feed matrix already alters laboratory potency at dry-matter levels far below those of digesta. A TSB EC50 should not be copied into a formula as the inclusion rate. This assay shows that the feed matrix changes potency, and that the change depends on the solids load, but it does not identify an optimal solids concentration. That must be tested for each feed and each strain.