Beneath our noses, yet unseen by the naked eye, there is a war going on. And the enemy is crafty in its response. Daily, humans wage chemical warfare against an overwhelming population–bacteria. In normal circumstances, humans and other animals coexist with these abundant life forms. And importantly, commensal (one species benefits while the other is unaffected) bacteria carry out work for us and our horses, helping with food digestion, keeping skin invaders in check, and controlling overgrowth of pathogens. But not all bacteria are good players; some can take hold of any organ system, creating infection and disease.

The arsenal of our chemical warfare relies on a mighty weapon–antimicrobial drugs (AMD), which are drugs or chemicals capable of killing or inhibiting growth of microorganisms. Of these substances, antibiotics have been a formidable weapon for over a century in the war against pathogenic bacteria. But antibiotics alone cannot vanquish bacterial invaders entirely; for complete resolution of infection, an animal's native immune system plays a crucial defensive role