Interactions between Streptococcus pneumoniae and influenza virus: a mutually beneficial relationship?

Historically, most research on infectious diseases has focused on infections with single pathogens. However, infections with pathogens often occur in the context of pre-existing viral and bacterial infections. Clinically, this is of particular relevance for coinfections with Streptococcus pneumoniae and influenza virus, which together are an important cause of global morbidity and mortality. In recent years new evidence has emerged regarding the underlying mechanisms of influenza virus-induced susceptibility to secondary pneumococcal infections, in particular regarding the sustained suppression of innate recognition of S. pneumoniae. Conversely, it is also increasingly being recognized that there is not a unidirectional effect of the virus on S. pneumoniae, but that asymptomatic pneumococcal carriage may also affect subsequent influenza virus infection and the clinical outcome. Here, we will review both aspects of pneumococcal influenza virus infection, with a particular focus on the age-related differences in pneumococcal colonization rates and invasive pneumococcal disease.