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SUMMARY : Many sparingly water-soluble neutral substances, including several indole derivatives, will quickly kill and often disintegrate washed living rumen ciliates at 85-39', 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 Isotricha, Dasytricha and Ophryoscolex, in particular, exhibit characteristic modes of disintegration when acted upon by such substances. Prominent among these are the terpene alcohols menthol, borneol and isoborneol, and their more soluble glucosides (both a- and p-) and their acid succinates. Menthoxyacetic acid is also similarly toxic. On the other hand, menthol and borneol /$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 Streptococcus bovis and Sarcina bakeri and quite high concentrations (0.1-0.4 yo) 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 grass- fed sheep was of the order of 5OO,OOO/g. rumen contents. Bacillus licheniformis was the chief anaerobic lipolytic species isolated. Although much is now known about carbohydrate fermentations by rumen ciliate protozoa (see Hungate, 1955, for summary), very little success has attended efforts to cultivate individual species in chemically defined media. Consequently, their nitrogen metabolism is still a closed book, and we do not know whether the conversion of part of the fodder organic-N into protozoan protein of higher nutritional value, which undoubtedly takes place in the rumen (see McNaught, Owen, Henry & Kon, 1954), is accompanied by an appreciable loss of fodder protein nitrogen as ammonia, or by destruction of B-vitamins synthesized by rumen bacteria. To obtain evidence on these points it might help greatly to be able to maintain individual protozoan species in the rumen of living sheep. The simplest way to do this might be to defaunate a sheep under conditions bringing about the minimum of harm to the rumen bacteria and to the host and then to introduce the single ciliate species under investigation in the hope that it would multiply and maintain itself in the rumen. Something along these lines has already been attempted by Becker, Schulz & Emmerson (1930) with goats, but they used a relatively drastic method of defaunation, viz. long-continued starvation followed by dosing with copper sulphate solution at defini tely bactericidal concentrations. New infusori- cidal agents, which have little toxic action on the host or its rumen bacteria,

journal of general microbiology/journal of general microbiology

A Survey of Physically Active Organic Infusoricidal Compounds and their Soluble Derivatives with Special Reference to their Action on the Rumen Microbial System