2.1. Source of Lactobacillus acidophilus
Dried L. acidophilus fermentation product was kindly provided by a commercial company (Diamond V, Cedar Rapids, IA, USA). The L. acidophilus fermentation product derived from L. acidophilus in an anaerobic fermentation technology platform to produce beneficial microbial metabolites. L. acidophilus was grown in solid state fermentation, and traditional feed ingredients was used as the substrate. This product is composed of a mixture of spray-dried L. acidophilus and other metabolites, such as organic acids and peptidoglycans, which help promote the activity of the beneficial bacteria in the gastrointestinal tract. There are at least 5 × 1010 cfu/g L. acidophilus in spray-dried powder. The bacterial concentration was determined by an ultraviolet spectrophotometer (Nano Drop, Thermo Fisher, America) at an OD of 550 nm.