@ERSpublications Small differences in the capsule of causative bacteria may determine the exact pattern of clinical illness in pneumonia http://ow.ly/qlQbA When considering the microbial aetiology of community-acquired pneumonia (CAP) we seldom think beyond the name (genus and species) of the causative organism. In a recent study of CAP aetiology in outpatients, Streptococcus pneumoniae was reported to be the most frequent pathogen [1]; unchanged from previous studies. The knowledge that S. pneumoniae is the commonest cause is sufficient to assist the choice of appropriate antibiotics; the most important management decision required for most patients.However, there is a lot more to this bacterial species than the name might imply. In fact, there are now known to be .90 distinct pneumococcal serotypes, each characterised by a slightly different structure in the pneumococcal capsular polysaccharide. These small molecular differences are an important determinant of immune response; therefore, immunity is serotype specific. Studies in the pre-antibiotic era dealt in depth with serotype distributions, as at that time serotype specific serum was the only available therapy. With the advent of antibiotics, pneumococcal serotypes have received much less attention. Now with the development of pneumococcal vaccines, and the importance of matching vaccines to causative serotypes, this subject is having a renaissance. From this we are starting to understand that there is more to the different serotypes than just a difference in polysaccharide structure. Indeed, the serotype is increasingly being linked, not just with the immune response, but also with the clinical phenotype in individual patients.In this issue of the European Respiratory Journal, BURGOS et al.[2] describe an investigation into the risk factors for respiratory failure (defined as an oxygen saturation of ,90% on room air or an arterial oxygen tension/inspiratory oxygen fraction ratio ,250) in a prospectively collected cohort of 1258 adults admitted to hospital with invasive (i.e. blood culture positive) pneumococcal pneumonia. Patients with respiratory failure (present in 48.9% of cases) were perhaps not surprisingly older, were more likely to have had chronic heart or lung disease, were more severely ill (as judged by the presence of septic shock and pneumonia severity index score), had more extensive radiographic change, were more likely to have received assisted ventilation and intensive care unit admission, and were more likely to have died. After multivariate analysis, age, chronic heart or lung disease, and pneumococcal serotypes 3, 19A and 19F remained associated with respiratory failure, while type 1 remained associated with its absence. This is the first study to find an association between specific serotypes and the presence or absence of respiratory failure. Previous studies have found associations between patterns of serotypes and other clinical phenotypes including nasopharyngeal carriage [3], invasive disease [4], septic shock [5], empyema and pa...