Bacteria that ferment amino acids to ammonia can be categorized as generalists or specialist hyper-ammonia-producing bacteria. In the rumens of ruminant animals, most of the ammonia produced is eventually excreted as urea in urine. This process can be controlled with off-label use of antibiotics, but the practice can lead to antibiotic resistance
therefore, discovery of antibiotic alternatives is pertinent. Plant-derived phenolic compounds have demonstrated antimicrobial efficacy for such purposes. This study investigated the antimicrobial and metabolic suppressive potential of six phenolic compounds on five amino acid fermenting bacteria: Clostridium sporogenes MD1, C. aminophilum F, Acetoanaerobium sticklandii SR, Peptostreptococcus sp. BG1, and Prevotella bryantii B14. Inhibitory action of the compounds was determined using a 10% v/v serial dilution method in basal media. Carvacrol (1 mM), thymol (1 mM), and eugenol (10 mM) demonstrated the greatest antimicrobial potential, where carvacrol and eugenol inhibited growth of all five species and thymol four species except BG1. The cinnamic acids (trans and hydro) demonstrated variable activity against all organisms. Suppression of metabolic activity was determined via colorimetric assay quantifying ammonia in washed stationary phase culture supernatant after 24 h of metabolism on fresh substrate. Carvacrol and eugenol yielded the greatest reduction of ammonia by all organisms except B14, which produced no ammonia under the growth conditions. Thymol greatly reduced ammonia production of four organisms except F. These data demonstrate that eugenol, carvacrol, and thymol may be worthy antimicrobial candidates for the control of ammonia-producing organisms.