Probiotic Bacillus amyloliquefaciens C-1 Improves Growth Performance, Stimulates GH/IGF-1, and Regulates the Gut Microbiota of Growth-Retarded Beef Calves

Growth retardation of calves is defined as a symptom of impaired growth and development, probably due to growth hormone disorder as well as natural and environmental factors in livestock. The growth-promoting effects of probiotics were determined in 50 growth-retarded growth calves. They were supplied with <i>Bacillus amyloliquefaciens</i> C-1 (Ba, 4 × 10¹⁰CFU/d, <i>n</i> = 16), <i>B. subtilis</i> (Bs, 4 × 10¹⁰CFU/d, <i>n</i> = 18), and negative control (NC, <i>n</i> = 16) for 30 days. Pre- and post-intervention, the growth performance (weight gain rate, feed intake and feed conversion rate) was analyzed, the serum GH, IGH-1 and immunoglobulin levels were assayed, and the fecal microbiota was detected. Calves in Ba and Bs groups demonstrated increased body weight gain, feed intake and GH/IGF-1 levels, as well as a more efficient feed conversion rate, compared with NC group (<i>P</i> < 0.05). Additionally, the abundances of bacteria contributing to the production of energy and SCFAs (short chain fatty acids), including <i>Proteobacteria, Rhodospirillaceae</i>, <i>Campylobacterales</i>, and <i>Butyricimonas</i> were increased compared with NC group (<i>P</i> < 0.05, <i>FDR</i> < 0.1); and the suspected pathogens, which included <i>Anaeroplasma</i> and <i>Acholeplasma</i> were decreased (<i>P</i> < 0.05, <i>FDR</i> < 0.1) in both the Bs and Ba groups. <i>Akkermansia</i>, which is involved in the intestinal mucosal immune response, was increased in Bs group after intervention (<i>P</i> < 0.05, <i>FDR</i> < 0.1), but exhibited no obvious difference in Ba group. The increased bacterial genera in Ba group were <i>Sphaerochaeta</i> and <i>Treponema</i> (<i>P</i> < 0.05, <i>FDR</i> < 0.1). These results indicate that the probiotics <i>B. amyloliquefaciens</i> and <i>B. subtilis</i> exhibited similar therapeutic potential in terms of growth performance by regulating hormones, and improving the intestinal and rumen development in growth-retarded animals.