Nitrogen utilization in bacterial isolates from the equine cecum

A total of 114 bacterial isolates were obtained from the cecal contents of two mature cecally fistulated horses on a habitat-simulating medium containing 40% energy-depleted cecal fluid. Of these isolates, 108 were maintained in pure cultures and were tentatively grouped on the basis of cell morphology and physiological characteristics. Gram-negative rods (50.9%), gram-positive rods (22.8%), and gram-positive cocci (21.9%) represented the largest groups isolated from these animals. Fifty isolates were tested for their ability to grow in media containing urea, ammonia, peptones, or amino acids as sole nitrogen sources. None of the isolates had a unique requirement for urea or ammonia since nitrogen derived from peptones, amino acids, or both supported growth as well as did ammonia or urea in a low nitrogen medium. Of the cecal isolates, 18% were able to use urea for growth, and 20.5% were able to grow with ammonia as the sole nitrogen source. All organisms grew in the experimental media containing peptones as the sole nitrogen source. Urease activity was detected in only 2 of 114 isolates tested. The inability of isolates to use urea or ammonia as nitrogen sources may have been a reflection of growth conditions in the habitat-stimulating medium used for isolation, but it could also suggest that many cecal bacteria require nitrogen sources other then ammonia or urea for growth.