Alterations in the composition of commensal bacterial communities are associated with enhanced susceptibility to multiple inflammatory, allergic, metabolic and infectious diseases in humans. In the context of infection, commensal bacteria-derived signals can influence the host immune response to invasive pathogens by acting as an adjuvant to boost the immune response to infection or by providing tonic stimulation to induce basal expression of factors required for host defense. Conversely, some pathogens have evolved mechanisms that can utilize commensal bacteria to establish a replicative advantage within the host. Thus, examining the dynamic relationship that exists between the mammalian host, commensal bacteria and invasive pathogens can provide insights into the etiology of pathogenesis from an infection.
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