ely 90%, therefore directly impacting host physiology. The ruminal fluids 
from the same animals display a similar inhibition of gastrin secretion, suggesting the gastrin inhibitors might be produced by rumen microorganisms that survive in the abomasum, which may account for the bulk of observed increases in anaerobic populations in the abomasal ecosystem when the luminal pH is elevated to 3.5 or above. Gastrin inhibitory activities is pH-dependent, suggesting normal pH environment in the abomasum may not favor production of the inhibitor. Our results indicated that the abomasal microbiota can produce an abundant level of serpins, a large class of protease inhibitors involved in regulation of a wide spectrum of physiological processes. Many gut bacteria, especially those commensal bacteria, are facing constant attack in their native habitat by proteases secreted by host cells, such as neutrophils. Serpins produced by the abomasal microbiota prevent attachment of host proteases, playing an important role in the interaction between the abomasal microbiota and the host. In addition, it has been known that intestinal parasite infection tends to increase dietary lysine requirements by up to 50%. One possible mechanism is that parasite infection alters gut microbiota composition and thus reduces the production of essential amino acids by the microbiota. Our results suggest that unlike in naive animals, Ostertagia infection in immune cattle induced a minimal disruption in the abomasal microbiota, which may contribute equally to the development of long-term protective immunity. While immune animals may develop abilities to maintain proper stability of their abomasal microbiota, metagenome plasticity in the bovine abomasum was evident. Approximately 1.3% of Pfam protein families identified in the microbial communities displayed a significant difference in abundance upon reinfection. The microbiota was responsive to external changes in the environment, which allows certain species to fluctuate in the population, to explore newly available niche space, and yet maintain overall function of the entire community by expressing a stable level of various proteins. It is foreseeable that a holistic understanding of the complex three-way interactions between the host, its microflora, and parasites will lead to the improvement of animal and human health. Materials and Methods Animals and Parasitology Animals and standard parasitology protocols were previously described. “1533424 Briefly, Holstein bull calves obtained shortly after September 2011 | Volume 6 | Issue 9 | e24417 The Bovine Abomasal Microbiota birth were maintained on concrete throughout the experiment and fed ad lib a standard calf ration after weaning. All animal work has been KU55933 site conducted according to Institutional Animal Care and Use Committees guidelines and approved by the Beltsville United States Department of Agriculture – Agricultural Research Service Animal Care and Use Committee. Oral infection of Ostertagia ostertagi ” infective L3 larvae was initiated after calves reached 34 months of age. All 6 calves were initially orally infected with a single dose of 105 L3 infective larvae for 14 days and then treated with a 26 labeled dose of fenbendazole to remove existing parasites. The calves were allowed to rest for 30 days on concrete before a 2nd round of infection with the same number of L3 larvae for 14 days. This infection-drug treatment-resting cycle was repeated 4 times on all calves used. These