Much of the recent research in asthma has focused on delineating the causative factors that contribute to the pathobiology of both airway eosinophilia and neutrophilia, and steroid-refractory disease. The distal airway, once thought to be a sterile environment, is now known to be a complex arena of microorganisms, termed the lung microbiome. Complementary and counterbalancing mechanisms are essential to maintaining homeostasis in this environment. An altered lung microbiome has been well described in asthma, with a higher burden of bacteria and differing species seen in asthmatic lungs compared with normal lungs, and these differences are more marked in severe asthma. There are likely several different mechanisms that account for these differences, including the usage of inhaled or oral corticosteroids and the higher prevalence of antimicrobial prescription in patients with more severe asthma.