SUMMARY: Many sparingly water-soluble neutral substances, including several indole derivatives, will quickly kill and often disintegrate washed living rumen ciliates at 35-39°, when acting at or near the saturation point (0·002-0·1 M) in a phosphate + acetate buffer at pH 7. The toxic compounds are mostly readily soluble in light petroleum. Organisms of the genera and , in particular, exhibit characteristic modes of disintegration when acted upon by such substances. Prominent among these are the terpene alcohols menthol, borneol and borneol, and their more soluble glucosides (both α- and β-) and their acid succinates. Menthoxyacetic acid is also similarly toxic. On the other hand, menthol and borneol β-glucuronides are not toxic to the rumen ciliates unless added to rumen contents containing bacteria also. Unlike menthol, borneol, in nearly saturated solution, is not particularly toxic to the rumen bacterial species and and quite high concentrations (0·1-0·4%) of the terpenoid succinates and glucosides are required for even partial inhibition of the growth of true rumen saccharolytic bacteria under otherwise favourable conditions. The population of lipolytic (esterase producing) bacteria in rumen contents from a hay and dried grassfed sheep was of the order of 500,000/g. rumen contents. was the chief anaerobic lipolytic species isolated.


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