Antibiotic activity of an isocyanide metabolite of Trichoderma hamatum against rumen bacteria

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Abstract
A metabolite of Trichoderma hamatum, 3-(3-isocyanocyclopent-2-enylidene)propionic acid, was tested for its effects on growth of and carbohydrate metabolism in 11 strains of functionally important rumen bacteria. To standardize the biological activity of this unstable metabolite, a rapid, aerobic disc diffusion assay was developed using Escherichia coli ATCC 11775. In an anaerobic broth dilution assay using a medium lacking rumen fluid and containing a soluble carbohydrate, the minimum inhibitory concentration of the metabolite which completely inhibited growth of the rumen bacteria for 18 h at 39 °C was generally < 10 μg∙mL−1; however, the minimum inhibitory concentrations for Megasphaera elsdenii B159 and Streptococcus bovis Pe18 were 10–25 and 25–64 μg∙mL−1, respectively. In general, the Gram-negative strains were more sensitive than the Gram positive. The minimum inhibitory concentration for Bacteroides ruminicola 23 grown with glucose was 1 μg∙mL−1; for B. ruminicola GA33 (glucose), B. succinogenes S85 (cellobiose), and Succinivibrio dextrinosolvens 24 (maltose), it was 2 μg∙mL−1. When added to a cellulose-containing rumen fluid medium, 1–4 μg∙mL−1 of the metabolite delayed cellulose hydrolysis by B. succinogenes S85, Ruminococcus albus 7, and R. flavefaciens FD1 for up to 4 days, and 6–7 μg∙mL−1 prevented hydrolysis for at least 1 month. In the presence of the metabolite, the proportion of acetate produced from soluble carbohydrate by the majority of strains increased, but with some strains net production of acetate decreased relative to production of other acidic fermentation products. If the metabolite gained entrance to the rumen, a concentration of as little as 1 μg∙mL−1 would probably cause a significant depression of the fermentation and result in nutritional deprivation of the animal.