*Review* **Antimicrobial** *Bacillus***: Metabolites and Their Mode of Action**

**Charlie Tran <sup>1</sup> , Ian E. Cock <sup>2</sup> , Xiaojing Chen <sup>3</sup> and Yunjiang Feng 1,\***


**Abstract:** The agricultural industry utilizes antibiotic growth promoters to promote livestock growth and health. However, the World Health Organization has raised concerns over the ongoing spread of antibiotic resistance transmission in the populace, leading to its subsequent ban in several countries, especially in the European Union. These restrictions have translated into an increase in pathogenic outbreaks in the agricultural industry, highlighting the need for an economically viable, non-toxic, and renewable alternative to antibiotics in livestock. Probiotics inhibit pathogen growth, promote a beneficial microbiota, regulate the immune response of its host, enhance feed conversion to nutrients, and form biofilms that block further infection. Commonly used lactic acid bacteria probiotics are vulnerable to the harsh conditions of the upper gastrointestinal system, leading to novel research using spore-forming bacteria from the genus *Bacillus*. However, the exact mechanisms behind *Bacillus* probiotics remain unexplored. This review tackles this issue, by reporting antimicrobial compounds produced from *Bacillus* strains, their proposed mechanisms of action, and any gaps in the mechanism studies of these compounds. Lastly, this paper explores omics approaches to clarify the mechanisms behind *Bacillus* probiotics.

**Keywords:** antimicrobials; *Bacillus*; probiotic; animal feed; omics
