Pneumococcal capsular serotypes and lung infection
- Correspondence to Centre for Respiratory Research, Department of Medicine, University College Medical School, Rayne Institute, London WC1E 6JJ, UK;
Contributors Written entirely by JSB.
- Aspergillus lung disease
- bacterial infection
- innate immunity
- opportunist lung infections
Streptococcus pneumoniae is second only to Mycobacterium tuberculosis as a bacterial cause of worldwide mortality. Unlike M tuberculosis, S pneumoniae remains a major cause of death in the developed world, with a standardised mortality rate of 25 per 100 000 in the UK. S pneumoniae is the commonest pathogen causing community acquired pneumonia (CAP),1 and the majority of serious S pneumoniae infections are cases of CAP in infants and older people. S pneumoniae is also an important cause of septicaemia, meningitis and infective exacerbations of chronic obstructive pulmonary disease and bronchiectasis. S pneumoniae is surrounded by an extracellular layer of polysaccharide called the capsule which promotes immune evasion and is an essential virulence factor.2 The structure of the capsule differs between S pneumoniae strains, with 93 variants identifiable by serotyping. Serotype prevalence is unequal, with the majority of disease caused by around 20 common serotypes. Which serotypes are predominant varies with age, geography and site of infection (eg, the nasopharynx, pleural space or the blood). In the paper by Bewick et al, technical advances in microbiology have been used to identify which S pneumoniae capsular serotypes are the commonest causes of CAP in a UK centre.3 Why do these rather technical data matter to a clinician, and how could they affect future developments in the management of S pneumoniae lung infection?
First, the ability to identify which serotypes are causing CAP is in itself a major advance. Previously, capsular serotyping required cultured bacteria and was therefore biased towards serotypes that cause septicaemia as respiratory cultures have such low sensitivity in CAP. For example, in Bewick et al's paper only 58 of 366 cases of S pneumoniae CAP had positive cultures, mainly from blood.3 By using urine samples for a …