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Isolation and Characteristics of a Ureolytic Strain of Selenomonas ruminantium

Departments of Dairy Science, Animal Science and Microbiology, University of Illinois, Urbana 61801

ABSTRACT

A ureolytic strain (strain D) of Selenomonas ruminantium was isolated from rumen fluid in numbers of 2 X 10⁷ per ml or more. Isolation was expedited by a chemically defined culture medium containing urea as the main source of nitrogen. Morphologically similar anaerobic ureolytic strains as well as other ureolytic types were isolated on several occasions, but results were erratic. Studies on the nutrition of strain D showed that it could utilize ammonia, nitrate, urea, and a number of amino acids and purines as sole sources of nitrogen, and it requires one or more B-vitamins for rapid growth in glucose-containing defined media but does not require volatile acids. None of five previously described strains of S. ruminantium utilized either urea or nitrate, although two of these reduce nitrate to nitrite. When strain D was grown in medium containing a small amount of urea and nitrate, diauxic growth occurred, suggesting that urea was used before nitrate and that the system involved in reduction of nitrate to ammonia is repressed by urea or ammonia. Preliminary studies on urease production by strain D indicate that it is strongly repressed by ammonia and may be induced by urea. Urease production was also repressed by inclusion of high urea in the growth medium and was increased after ammonia became very low in a medium containing a growth-limiting concentration of ammonia-nitrogen.