Lamarche D, et al. - Authors sought to profile the microbial composition of critically ill mechanically ventilated patients and ascertain whether microbial diversity is realated to illness severity and mortality. They conducted a prospective, observational study of mechanically ventilated critically ill patients with a high incidence of pneumonia in 2 intensive care units (ICUs) in Hamilton, Canada, nested within a randomized trial for the prevention of healthcare-associated infections. During critical illness, the composition of the host-associated microbial communities is severely perturbed. High illness severity was reflected by reduced microbial diversity and it was seen to be associated with mortality. In mechanically ventilated patients, microbial diversity may be a biomarker of prognostic value.