By dawn or dusk-how circadian timing rewrites bacterial infection outcomes

The immune system exists in perpetual co-evolution with pathogens, and microbial pathogenesis is inexorably linked to the cyclical interactions between the pathogen and the host. Because pathogens exploit the immune system in unique ways, the antimicrobial efficacy of any given immune process varies between pathogens, and the consequences of activation or inhibition of antimicrobial programs must be interpreted in the context of the given pathogen. An increasing body of literature shows that numerous facets of the immune system are tightly regulated by the circadian clock, with multiple immune processes demonstrating increased activity during certain times of the day. However, the field of circadian immunology has generally given its attention to unraveling the mechanism of circadian regulation and comparatively little attention to how these circadian oscillations may influence the ultimate outcome of diseases. Therefore, this review aims to interpret these findings in the context of a select number of clinically relevant pathogens: Salmonella enterica, Listeria monocytogenes, and Streptococcus pneumoniae. In this way, we hope to discuss the complex factors that determine how the circadian clock regulates disease progression.