The global movement to phase out antibiotic growth promoters (AGPs) from livestock production demands the identification and validation of effective alternative strategies. Bacillus-based probiotics represent a particularly promising option owing to their intrinsic resistance to environmental stressors and pronounced antagonistic activity against pathogenic microorganisms. This study investigated the probiotic potential of a selected Bacillus (B.) clausii isolate (DM 11) as a dietary supplement in broiler chickens through in vitro and in vivo trials. B. clausii DM 11 was selected from various Bacillus isolates for its pronounced antimicrobial activity and high tolerance to acidic pH and bile salts. In vitro assays demonstrated that B. clausii DM 11 exhibited superior antimicrobial activity and stress tolerance compared with other tested strains, with a broad, dose-dependent inhibitory spectrum. Minimum inhibitory concentrations were as low as 15 µg/mL against pathogens, including Clostridium perfringens. A 42-day feeding trial was conducted in which broilers were randomly assigned to six dietary treatments: a basal diet (negative control), a diet supplemented with virginiamycin (50mg/kg; AGP group), and four diets containing graded levels of B. clausii DM 11 (60, 120, 240, and 480mg/kg). The broilers supplemented with 240mg/kg B. clausii DM 11 yielded the most favorable growth response, comparable to the AGP group; broilers in this treatment attained a final average body weight of 2,720g with a feed conversion ratio of 1.86. B. clausii DM 11 (240mg/kg) significantly enhanced serum protein and lipid profiles, maintained balanced leukocyte populations, and showed no evidence of hepatotoxicity or nephrotoxicity. Furthermore, supplementation with B. clausii DM 11 upregulated the ileal expression of tight junction proteins (OCLN, ZO-1), nutrient transporters (SGLT1, PepT1), and antimicrobial peptides (AvBD1), while downregulating pro-inflammatory cytokines (IL-1β, TNF-α). Cecal microbiological analysis characterized by an increase in lactic acid bacteria (+0.95 log CFU/g) and a reduction in yeast and mold counts (−0.8 log CFU/g) relative to the control group. Bacillus clausii DM 11 (240mg/kg) may exhibit characteristics for a potential probiotic alternative to antibiotics in broilers. It enhances growth and feed efficiency by suppressing pathogens, modulating immune responses, and optimizing gut microbiota and intestinal barrier integrity.

To Cite This Article: Al-Quwaie DAH, 2026. Potential probiotic effect of Bacillus clausii DM11 isolate on biological parameters in broiler chickens. Pak Vet J, 46(3): 628-637. http://dx.doi.org/10.29261/pakvetj/2026.051