Bacterial carriage in infancy: Risk factors and consequences - The Generation R Study Labout, J.A.M. 2010-12-22 Doctoral Thesis

In this thesis various studies on the prevalence, risk factors and consequences of carriage of Streptococcus pneumoniae (pneumococcus), Haemophilus influenzae, Moraxella catarrhalis and Staphylococcus aureus in young children are presented. These bacteria are considered important (airway) pathogens in this age group. The definition of pathogen needs further explanation, since the boundary between commensal organisms and pathogenic organisms is not always obvious. According to Casadevall and Pirofski, defining a pathogen as an organism that causes disease in a host is inadequate because some pathogens do not cause disease in all human hosts. The adjective opportunistic is used for pathogens that cause disease only in hosts that are immunocompromised or whose pathogenesis is facilitated by traumatic breaching of an epithelial barrier. For example, about 50% of children two years of age are colonized by S. pneumoniae. This does not lead to disease in most cases, and thus, the pneumococcus can be considered a commensal organism. Weiser et al. have shown that colonization is the natural state of the pneumococcus, and invasive disease, also from the perspective of the pneumococcus, is not favourable. However, S. pneumoniae is the main cause of bacterial otitis media and bacterial pneumonia in young children worldwide. It is also an important cause of life threatening sepsis and meningitis. Moreover, pneumonia is “the leading killer in children”, and since S. pneumoniae is considered to cause more than half the cases of bacterial pneumonia worldwide, it is without any doubt a fearful pathogen.
http://repub.eur.nl/res/pub/22193/101222_Labout%2C%20Joost%20Adriaan%20Maria_Bewerkt.pdf