Bacillus-based probiotics affect gut barrier integrity in different ways in chickens subjected to optimal or challenge conditions

•Probiotic action depends upon rearing conditions in chickens.•Probiotics modulate gut barrier function of chickens.•Multistrain probiotic is more effective than single-strain in supporting gut barrier function.•Multi-strain probiotic ameliorate consequences of C. perfringens challenge. Dietary supplementation with spore-forming Bacillus-based probiotics represents an efficient means to improve gut health while maintaining good broiler performance. This study investigated the potential of two probiotic products in chickens subjected to optimal (Experiment 1) and Clostridium perfringens-challenged (Experiment 2) conditions. The treatments in Experiment 1 were as follows: (i) CON (no probiotic additive), (ii) One-strain Pro (supplemented with Bacillus licheniformis) or (iii) Multi-strain Pro (supplemented with a multistrain Bacillus-based probiotic). The treatment groups in Experiment 2 received the same diets as those in Experiment 1 but were subjected to C. perfringens challenge. Both experiments lasted 35 days. Both products marginally affected broiler performance in the optimal or challenge conditions. In Experiment 1, Multi-strain Pro upregulated the mRNA expression level of 11 out of 15 selected genes, whereas in Experiment 2, this was less evident, and One-strain Pro was more effective. The multistrain probiotic was effective in maintaining gut morphostructure indices and increasing gut wall thickness, which was particularly evident in challenged birds. Neither additive induced bacterial activity (assessed by measuring enzymatic activity and short-chain fatty acid production) in the cecum, and Multi-strain Pro maintained the cecal butyrate concentration in challenged birds as in the challenged CON treatment, in which butyrate concentration was significantly higher than in the One-strain Pro treatment. Our findings indicated that the activity of these single- and multistrain probiotic products varies depending on rearing conditions, and the effect is highly strain- and product-specific. However, the multistrain probiotic apparently had more beneficial effects than the one-strain probiotic in the maintenance of gut functional status under optimal and challenge conditions.