Date of Graduation


Document Type


Degree Name

Doctor of Philosophy in Poultry Science (PhD)

Degree Level



Poultry Science


Billy M. Hargis

Committee Member

Michael T. Kidd

Second Committee Member

Luc R. Berghman

Third Committee Member

Guillermo Tellez


Biological sciences, Bacillus, Bone parameters, Digesta viscosity, Direct-fed microbials, Enzymes, Gut health


During the last decade, the increasing interest in renewable energy sources has been changing the distribution of corn utilization from human and animal consumption to biofuel production, leading to a continuous rise in feed costs of livestock diets. Therefore, alternative feed ingredients such as distillers dried grains with solubles (DDGS), as well as cereals like wheat, barley, and sorghum have become part of the feed matrix to maintain or reduce production costs. However, these raw materials often contain a higher concentration of antinutritional factors in comparison to corn, including non-starch polysaccharides (NSP) which increase digesta viscosity and reduce nutrient absorption in monogastric animals. As a result, the addition of exogenous enzymes in poultry feed has steadily increased to maximize nutrient utilization and maintain performance parameters with diets containing less digestible ingredients. On the other hand, the poultry industry is also facing Social concerns regarding the use of antibiotic growth promoters (AGP) and the development of antibiotic resistant microorganisms. One alternative among others is the utilization of direct-fed microbials (DFM) as substitutes of AGP and also as a prophylactic practice to reduce the incidence of bacterial gastrointestinal diseases. Therefore, the objectives of the present dissertation were to evaluate and select different Bacillus spp. strains as DFM candidates based on enzyme production profiles to improve nutrient absorption and intestinal integrity, as well as, maintain a healthy microflora balance in poultry consuming commercial and alternative diets. Due to Bacillus endospores are in a dormant state when delivered into the feed, it was crucial to evaluate the spores’ germination rate, distribution and persistence in the gastrointestinal tract (GIT) of chickens to understand the probable mechanism of action of this remarkable beneficial microorganism. It was observed that some full life-cycle development occurred and around 90 % of the spores germinated in the GIT, suggesting that a continuous administration is advisable for consistent improvement. Additionally, in a series of in vitro experiments, three Bacillus spp. strains were selected based on their enzyme production activity profile of amylase, cellulase, protease, lipase, xylanase and phytase. Analysis of the 16S rRNA sequence classified two strains as B. amyloliquefaciens and one of the strains as B. subtilis. The three isolates were combined in an equal ratio (1:1:1) and showed to reduce viscosity and Clostridium perfringens proliferation in an in vitro digestive model simulating different compartments of the GIT of poultry. For in vivo trials in broilers and turkeys a rye-based diet was used as a source of high soluble NSP. Inclusion of the Bacillus-DFM candidate significantly reduce digesta viscosity and bacterial translocation to the liver, resulting in an increase of performance and bone quality parameters, along with maintenance of the beneficial microflora in the GIT. Moreover, due to the wide availability of DDGS from the ethanol industry, a different set of experiments including 8% of DDGS in the grower diet were developed. Supplementation with the Bacillus-DFM candidate improve growth performance, bone mineralization, and intestinal morphology in comparison to the control group (P < 0.05), suggesting that the dietary inclusion of selected Bacillus spp. spores is a viable alternative in commercial diets, having a positive impact in gut health and production parameters.